we / crates / js / src / compiler.rs

//! AST → register-based bytecode compiler.
//!
//! Walks the AST produced by the parser and emits bytecode instructions.
//! Uses a simple greedy register allocator: each new temporary gets the next
//! available register, and registers are freed when no longer needed.

use crate::ast::*;
use crate::bytecode::*;
use crate::JsError;
use std::collections::HashSet;

/// Compiler state for a single function scope.
struct FunctionCompiler {
    builder: BytecodeBuilder,
    /// Maps local variable names to their register slots.
    locals: Vec<Local>,
    /// Next free register index.
    next_reg: u8,
    /// Stack of loop contexts for break/continue.
    loop_stack: Vec<LoopCtx>,
    /// Upvalues captured from the parent scope (used in inner functions).
    upvalues: Vec<UpvalueEntry>,
    /// Set of local variable names that are captured by inner functions.
    /// Pre-populated before compilation by scanning inner function bodies.
    captured_names: HashSet<String>,
    /// True for the top-level script scope. Top-level `var` declarations
    /// are also stored as globals so they persist across `execute()` calls.
    is_top_level: bool,
}

#[derive(Debug, Clone)]
struct Local {
    name: String,
    reg: Reg,
    /// Whether this variable is captured by an inner function (stored in a cell).
    is_captured: bool,
    /// Whether this variable was declared with `const`.
    is_const: bool,
}

/// An upvalue entry tracking how this function captures an outer variable.
struct UpvalueEntry {
    /// Name of the captured variable (for dedup during resolution).
    name: String,
    /// The resolved upvalue definition.
    def: UpvalueDef,
    /// Whether the original declaration was `const`.
    is_const: bool,
}

struct LoopCtx {
    /// Label, if this is a labeled loop.
    label: Option<String>,
    /// Patch positions for break jumps.
    break_patches: Vec<usize>,
    /// Patch positions for continue jumps (patched after body compilation).
    continue_patches: Vec<usize>,
}

impl FunctionCompiler {
    fn new(name: String, param_count: u8) -> Self {
        Self {
            builder: BytecodeBuilder::new(name, param_count),
            locals: Vec::new(),
            next_reg: 0,
            loop_stack: Vec::new(),
            upvalues: Vec::new(),
            captured_names: HashSet::new(),
            is_top_level: false,
        }
    }

    /// Allocate a register, updating the high-water mark.
    fn alloc_reg(&mut self) -> Reg {
        let r = self.next_reg;
        self.next_reg = self.next_reg.checked_add(1).expect("register overflow");
        if self.next_reg > self.builder.func.register_count {
            self.builder.func.register_count = self.next_reg;
        }
        r
    }

    /// Free the last allocated register (must be called in reverse order).
    fn free_reg(&mut self, r: Reg) {
        debug_assert_eq!(
            r,
            self.next_reg - 1,
            "registers must be freed in reverse order"
        );
        self.next_reg -= 1;
    }

    /// Look up a local variable by name, returning full info.
    fn find_local_info(&self, name: &str) -> Option<&Local> {
        self.locals.iter().rev().find(|l| l.name == name)
    }

    /// Look up an upvalue by name, returning its index.
    fn find_upvalue(&self, name: &str) -> Option<u8> {
        self.upvalues
            .iter()
            .position(|u| u.name == name)
            .map(|i| i as u8)
    }

    /// Check if an upvalue is const.
    fn is_upvalue_const(&self, idx: u8) -> bool {
        self.upvalues
            .get(idx as usize)
            .map(|u| u.is_const)
            .unwrap_or(false)
    }

    /// Define a local variable with capture and const flags.
    fn define_local_ext(&mut self, name: &str, is_captured: bool, is_const: bool) -> Reg {
        let reg = self.alloc_reg();
        self.locals.push(Local {
            name: name.to_string(),
            reg,
            is_captured,
            is_const,
        });
        reg
    }
}

// ── Free variable analysis ──────────────────────────────────

/// Collect identifiers referenced inside `body` that are not declared locally
/// (params or variable declarations within the body). This set represents the
/// "free variables" of a function body — variables that must be captured.
/// This includes transitive free variables from nested functions.
fn collect_free_vars(params: &[Pattern], body: &[Stmt]) -> HashSet<String> {
    let mut declared = HashSet::new();
    let mut referenced = HashSet::new();

    // Params are local declarations.
    for p in params {
        collect_pattern_names(p, &mut declared);
    }

    // Collect declarations and references from the body.
    for stmt in body {
        collect_stmt_decls(stmt, &mut declared);
    }
    for stmt in body {
        collect_stmt_refs(stmt, &declared, &mut referenced);
    }

    // Also include transitive free variables from nested inner functions.
    // If an inner function references `x` and `x` is not declared in THIS scope,
    // then `x` is also a free variable of THIS function.
    let inner_caps = collect_inner_captures(body);
    for name in inner_caps {
        if !declared.contains(&name) {
            referenced.insert(name);
        }
    }

    referenced
}

/// Collect free variables from an arrow function body.
fn collect_free_vars_arrow(params: &[Pattern], body: &ArrowBody) -> HashSet<String> {
    let mut declared = HashSet::new();
    let mut referenced = HashSet::new();

    for p in params {
        collect_pattern_names(p, &mut declared);
    }

    match body {
        ArrowBody::Expr(expr) => {
            collect_expr_refs(expr, &declared, &mut referenced);
        }
        ArrowBody::Block(stmts) => {
            for stmt in stmts {
                collect_stmt_decls(stmt, &mut declared);
            }
            for stmt in stmts {
                collect_stmt_refs(stmt, &declared, &mut referenced);
            }
            // Transitive free variables from nested functions.
            let inner_caps = collect_inner_captures(stmts);
            for name in inner_caps {
                if !declared.contains(&name) {
                    referenced.insert(name);
                }
            }
        }
    }

    referenced
}

fn collect_pattern_names(pat: &Pattern, names: &mut HashSet<String>) {
    match &pat.kind {
        PatternKind::Identifier(name) => {
            names.insert(name.clone());
        }
        PatternKind::Array { elements, rest } => {
            for elem in elements.iter().flatten() {
                collect_pattern_names(elem, names);
            }
            if let Some(rest) = rest {
                collect_pattern_names(rest, names);
            }
        }
        PatternKind::Object { properties, rest } => {
            for prop in properties {
                collect_pattern_names(&prop.value, names);
            }
            if let Some(rest) = rest {
                collect_pattern_names(rest, names);
            }
        }
        PatternKind::Assign { left, .. } => {
            collect_pattern_names(left, names);
        }
    }
}

/// Collect all variable/function declarations in a statement (not recursing into
/// inner functions — those form their own scope).
fn collect_stmt_decls(stmt: &Stmt, declared: &mut HashSet<String>) {
    match &stmt.kind {
        StmtKind::VarDecl { declarators, .. } => {
            for d in declarators {
                collect_pattern_names(&d.pattern, declared);
            }
        }
        StmtKind::FunctionDecl(f) => {
            if let Some(name) = &f.id {
                declared.insert(name.clone());
            }
        }
        StmtKind::ClassDecl(c) => {
            if let Some(name) = &c.id {
                declared.insert(name.clone());
            }
        }
        StmtKind::Block(stmts) => {
            for s in stmts {
                collect_stmt_decls(s, declared);
            }
        }
        StmtKind::If {
            consequent,
            alternate,
            ..
        } => {
            collect_stmt_decls(consequent, declared);
            if let Some(alt) = alternate {
                collect_stmt_decls(alt, declared);
            }
        }
        StmtKind::While { body, .. }
        | StmtKind::DoWhile { body, .. }
        | StmtKind::Labeled { body, .. } => {
            collect_stmt_decls(body, declared);
        }
        StmtKind::For { init, body, .. } => {
            if let Some(ForInit::VarDecl { declarators, .. }) = init {
                for d in declarators {
                    collect_pattern_names(&d.pattern, declared);
                }
            }
            collect_stmt_decls(body, declared);
        }
        StmtKind::ForIn { left, body, .. } | StmtKind::ForOf { left, body, .. } => {
            if let ForInOfLeft::VarDecl { pattern, .. } = left {
                collect_pattern_names(pattern, declared);
            }
            collect_stmt_decls(body, declared);
        }
        StmtKind::Try {
            block,
            handler,
            finalizer,
        } => {
            for s in block {
                collect_stmt_decls(s, declared);
            }
            if let Some(h) = handler {
                if let Some(param) = &h.param {
                    collect_pattern_names(param, declared);
                }
                for s in &h.body {
                    collect_stmt_decls(s, declared);
                }
            }
            if let Some(fin) = finalizer {
                for s in fin {
                    collect_stmt_decls(s, declared);
                }
            }
        }
        StmtKind::Switch { cases, .. } => {
            for case in cases {
                for s in &case.consequent {
                    collect_stmt_decls(s, declared);
                }
            }
        }
        _ => {}
    }
}

/// Collect all identifier references in a statement, excluding inner function scopes.
/// Identifiers that are in `declared` are local and skipped.
fn collect_stmt_refs(stmt: &Stmt, declared: &HashSet<String>, refs: &mut HashSet<String>) {
    match &stmt.kind {
        StmtKind::Expr(expr) => collect_expr_refs(expr, declared, refs),
        StmtKind::Block(stmts) => {
            for s in stmts {
                collect_stmt_refs(s, declared, refs);
            }
        }
        StmtKind::VarDecl { declarators, .. } => {
            for d in declarators {
                if let Some(init) = &d.init {
                    collect_expr_refs(init, declared, refs);
                }
            }
        }
        StmtKind::FunctionDecl(_) => {
            // Don't recurse into inner functions — they have their own scope.
        }
        StmtKind::If {
            test,
            consequent,
            alternate,
        } => {
            collect_expr_refs(test, declared, refs);
            collect_stmt_refs(consequent, declared, refs);
            if let Some(alt) = alternate {
                collect_stmt_refs(alt, declared, refs);
            }
        }
        StmtKind::While { test, body } => {
            collect_expr_refs(test, declared, refs);
            collect_stmt_refs(body, declared, refs);
        }
        StmtKind::DoWhile { body, test } => {
            collect_stmt_refs(body, declared, refs);
            collect_expr_refs(test, declared, refs);
        }
        StmtKind::For {
            init,
            test,
            update,
            body,
        } => {
            if let Some(init) = init {
                match init {
                    ForInit::VarDecl { declarators, .. } => {
                        for d in declarators {
                            if let Some(init) = &d.init {
                                collect_expr_refs(init, declared, refs);
                            }
                        }
                    }
                    ForInit::Expr(e) => collect_expr_refs(e, declared, refs),
                }
            }
            if let Some(t) = test {
                collect_expr_refs(t, declared, refs);
            }
            if let Some(u) = update {
                collect_expr_refs(u, declared, refs);
            }
            collect_stmt_refs(body, declared, refs);
        }
        StmtKind::ForIn { right, body, .. } | StmtKind::ForOf { right, body, .. } => {
            collect_expr_refs(right, declared, refs);
            collect_stmt_refs(body, declared, refs);
        }
        StmtKind::Return(Some(expr)) | StmtKind::Throw(expr) => {
            collect_expr_refs(expr, declared, refs);
        }
        StmtKind::Try {
            block,
            handler,
            finalizer,
        } => {
            for s in block {
                collect_stmt_refs(s, declared, refs);
            }
            if let Some(h) = handler {
                for s in &h.body {
                    collect_stmt_refs(s, declared, refs);
                }
            }
            if let Some(fin) = finalizer {
                for s in fin {
                    collect_stmt_refs(s, declared, refs);
                }
            }
        }
        StmtKind::Switch {
            discriminant,
            cases,
        } => {
            collect_expr_refs(discriminant, declared, refs);
            for case in cases {
                if let Some(test) = &case.test {
                    collect_expr_refs(test, declared, refs);
                }
                for s in &case.consequent {
                    collect_stmt_refs(s, declared, refs);
                }
            }
        }
        StmtKind::Labeled { body, .. } => {
            collect_stmt_refs(body, declared, refs);
        }
        _ => {}
    }
}

/// Collect identifier references in an expression. Does NOT recurse into
/// inner function/arrow bodies (those form their own scope).
fn collect_expr_refs(expr: &Expr, declared: &HashSet<String>, refs: &mut HashSet<String>) {
    match &expr.kind {
        ExprKind::Identifier(name) => {
            if !declared.contains(name) {
                refs.insert(name.clone());
            }
        }
        ExprKind::Binary { left, right, .. }
        | ExprKind::Logical { left, right, .. }
        | ExprKind::Assignment { left, right, .. } => {
            collect_expr_refs(left, declared, refs);
            collect_expr_refs(right, declared, refs);
        }
        ExprKind::Unary { argument, .. } | ExprKind::Update { argument, .. } => {
            collect_expr_refs(argument, declared, refs);
        }
        ExprKind::Conditional {
            test,
            consequent,
            alternate,
        } => {
            collect_expr_refs(test, declared, refs);
            collect_expr_refs(consequent, declared, refs);
            collect_expr_refs(alternate, declared, refs);
        }
        ExprKind::Call { callee, arguments } | ExprKind::New { callee, arguments } => {
            collect_expr_refs(callee, declared, refs);
            for arg in arguments {
                collect_expr_refs(arg, declared, refs);
            }
        }
        ExprKind::Member {
            object,
            property,
            computed,
            ..
        } => {
            collect_expr_refs(object, declared, refs);
            if *computed {
                collect_expr_refs(property, declared, refs);
            }
        }
        ExprKind::Array(elements) => {
            for elem in elements.iter().flatten() {
                match elem {
                    ArrayElement::Expr(e) | ArrayElement::Spread(e) => {
                        collect_expr_refs(e, declared, refs);
                    }
                }
            }
        }
        ExprKind::Object(props) => {
            for prop in props {
                if let PropertyKey::Computed(e) = &prop.key {
                    collect_expr_refs(e, declared, refs);
                }
                if let Some(val) = &prop.value {
                    collect_expr_refs(val, declared, refs);
                }
            }
        }
        ExprKind::Sequence(exprs) => {
            for e in exprs {
                collect_expr_refs(e, declared, refs);
            }
        }
        ExprKind::Spread(inner) => {
            collect_expr_refs(inner, declared, refs);
        }
        ExprKind::TemplateLiteral { expressions, .. } => {
            for e in expressions {
                collect_expr_refs(e, declared, refs);
            }
        }
        // Function/Arrow/Class bodies are new scopes — don't recurse.
        ExprKind::Function(_) | ExprKind::Arrow { .. } | ExprKind::Class(_) => {}
        _ => {}
    }
}

/// Collect the free variables of ALL inner functions/arrows within a list of
/// statements. Returns the set of outer-scope names they reference.
fn collect_inner_captures(stmts: &[Stmt]) -> HashSet<String> {
    let mut captures = HashSet::new();
    for stmt in stmts {
        collect_inner_captures_stmt(stmt, &mut captures);
    }
    captures
}

fn collect_inner_captures_stmt(stmt: &Stmt, caps: &mut HashSet<String>) {
    match &stmt.kind {
        StmtKind::FunctionDecl(f) => {
            let fv = collect_free_vars(&f.params, &f.body);
            caps.extend(fv);
        }
        StmtKind::Expr(expr) => collect_inner_captures_expr(expr, caps),
        StmtKind::VarDecl { declarators, .. } => {
            for d in declarators {
                if let Some(init) = &d.init {
                    collect_inner_captures_expr(init, caps);
                }
            }
        }
        StmtKind::Block(stmts) => {
            for s in stmts {
                collect_inner_captures_stmt(s, caps);
            }
        }
        StmtKind::If {
            test,
            consequent,
            alternate,
        } => {
            collect_inner_captures_expr(test, caps);
            collect_inner_captures_stmt(consequent, caps);
            if let Some(alt) = alternate {
                collect_inner_captures_stmt(alt, caps);
            }
        }
        StmtKind::While { test, body } => {
            collect_inner_captures_expr(test, caps);
            collect_inner_captures_stmt(body, caps);
        }
        StmtKind::DoWhile { body, test } => {
            collect_inner_captures_stmt(body, caps);
            collect_inner_captures_expr(test, caps);
        }
        StmtKind::For {
            init,
            test,
            update,
            body,
        } => {
            if let Some(ForInit::Expr(e)) = init {
                collect_inner_captures_expr(e, caps);
            }
            if let Some(ForInit::VarDecl { declarators, .. }) = init {
                for d in declarators {
                    if let Some(init) = &d.init {
                        collect_inner_captures_expr(init, caps);
                    }
                }
            }
            if let Some(t) = test {
                collect_inner_captures_expr(t, caps);
            }
            if let Some(u) = update {
                collect_inner_captures_expr(u, caps);
            }
            collect_inner_captures_stmt(body, caps);
        }
        StmtKind::ForIn { right, body, .. } | StmtKind::ForOf { right, body, .. } => {
            collect_inner_captures_expr(right, caps);
            collect_inner_captures_stmt(body, caps);
        }
        StmtKind::Return(Some(expr)) | StmtKind::Throw(expr) => {
            collect_inner_captures_expr(expr, caps);
        }
        StmtKind::Try {
            block,
            handler,
            finalizer,
        } => {
            for s in block {
                collect_inner_captures_stmt(s, caps);
            }
            if let Some(h) = handler {
                for s in &h.body {
                    collect_inner_captures_stmt(s, caps);
                }
            }
            if let Some(fin) = finalizer {
                for s in fin {
                    collect_inner_captures_stmt(s, caps);
                }
            }
        }
        StmtKind::Switch {
            discriminant,
            cases,
        } => {
            collect_inner_captures_expr(discriminant, caps);
            for case in cases {
                if let Some(test) = &case.test {
                    collect_inner_captures_expr(test, caps);
                }
                for s in &case.consequent {
                    collect_inner_captures_stmt(s, caps);
                }
            }
        }
        StmtKind::Labeled { body, .. } => {
            collect_inner_captures_stmt(body, caps);
        }
        _ => {}
    }
}

fn collect_inner_captures_expr(expr: &Expr, caps: &mut HashSet<String>) {
    match &expr.kind {
        ExprKind::Function(f) => {
            let fv = collect_free_vars(&f.params, &f.body);
            caps.extend(fv);
        }
        ExprKind::Arrow { params, body, .. } => {
            let fv = collect_free_vars_arrow(params, body);
            caps.extend(fv);
        }
        ExprKind::Binary { left, right, .. }
        | ExprKind::Logical { left, right, .. }
        | ExprKind::Assignment { left, right, .. } => {
            collect_inner_captures_expr(left, caps);
            collect_inner_captures_expr(right, caps);
        }
        ExprKind::Unary { argument, .. } | ExprKind::Update { argument, .. } => {
            collect_inner_captures_expr(argument, caps);
        }
        ExprKind::Conditional {
            test,
            consequent,
            alternate,
        } => {
            collect_inner_captures_expr(test, caps);
            collect_inner_captures_expr(consequent, caps);
            collect_inner_captures_expr(alternate, caps);
        }
        ExprKind::Call { callee, arguments } | ExprKind::New { callee, arguments } => {
            collect_inner_captures_expr(callee, caps);
            for arg in arguments {
                collect_inner_captures_expr(arg, caps);
            }
        }
        ExprKind::Member {
            object,
            property,
            computed,
            ..
        } => {
            collect_inner_captures_expr(object, caps);
            if *computed {
                collect_inner_captures_expr(property, caps);
            }
        }
        ExprKind::Array(elements) => {
            for elem in elements.iter().flatten() {
                match elem {
                    ArrayElement::Expr(e) | ArrayElement::Spread(e) => {
                        collect_inner_captures_expr(e, caps);
                    }
                }
            }
        }
        ExprKind::Object(props) => {
            for prop in props {
                if let PropertyKey::Computed(e) = &prop.key {
                    collect_inner_captures_expr(e, caps);
                }
                if let Some(val) = &prop.value {
                    collect_inner_captures_expr(val, caps);
                }
            }
        }
        ExprKind::Sequence(exprs) => {
            for e in exprs {
                collect_inner_captures_expr(e, caps);
            }
        }
        ExprKind::Spread(inner) => {
            collect_inner_captures_expr(inner, caps);
        }
        ExprKind::TemplateLiteral { expressions, .. } => {
            for e in expressions {
                collect_inner_captures_expr(e, caps);
            }
        }
        ExprKind::Class(c) => {
            for member in &c.body {
                if let ClassMemberKind::Method { value, .. } = &member.kind {
                    let fv = collect_free_vars(&value.params, &value.body);
                    caps.extend(fv);
                }
            }
        }
        _ => {}
    }
}

/// Compile a parsed program into a top-level bytecode function.
pub fn compile(program: &Program) -> Result<Function, JsError> {
    let mut fc = FunctionCompiler::new("<main>".into(), 0);
    fc.is_top_level = true;

    // Pre-scan to find which top-level locals are captured by inner functions.
    fc.captured_names = collect_inner_captures(&program.body);

    // Reserve r0 for the implicit return value.
    let result_reg = fc.alloc_reg();
    fc.builder.emit_reg(Op::LoadUndefined, result_reg);

    compile_stmts(&mut fc, &program.body, result_reg)?;

    fc.builder.emit_reg(Op::Return, result_reg);
    Ok(fc.builder.finish())
}

fn compile_stmts(
    fc: &mut FunctionCompiler,
    stmts: &[Stmt],
    result_reg: Reg,
) -> Result<(), JsError> {
    for stmt in stmts {
        compile_stmt(fc, stmt, result_reg)?;
    }
    Ok(())
}

fn compile_stmt(fc: &mut FunctionCompiler, stmt: &Stmt, result_reg: Reg) -> Result<(), JsError> {
    match &stmt.kind {
        StmtKind::Expr(expr) => {
            // Expression statement: compile and store result in result_reg.
            compile_expr(fc, expr, result_reg)?;
        }

        StmtKind::Block(stmts) => {
            let saved_locals = fc.locals.len();
            let saved_next = fc.next_reg;
            compile_stmts(fc, stmts, result_reg)?;
            // Pop locals from this block.
            fc.locals.truncate(saved_locals);
            fc.next_reg = saved_next;
        }

        StmtKind::VarDecl { kind, declarators } => {
            for decl in declarators {
                compile_var_declarator(fc, decl, *kind)?;
            }
        }

        StmtKind::FunctionDecl(func_def) => {
            compile_function_decl(fc, func_def)?;
        }

        StmtKind::If {
            test,
            consequent,
            alternate,
        } => {
            compile_if(fc, test, consequent, alternate.as_deref(), result_reg)?;
        }

        StmtKind::While { test, body } => {
            compile_while(fc, test, body, None, result_reg)?;
        }

        StmtKind::DoWhile { body, test } => {
            compile_do_while(fc, body, test, None, result_reg)?;
        }

        StmtKind::For {
            init,
            test,
            update,
            body,
        } => {
            compile_for(
                fc,
                init.as_ref(),
                test.as_ref(),
                update.as_ref(),
                body,
                None,
                result_reg,
            )?;
        }

        StmtKind::ForIn { left, right, body } => {
            let saved_locals = fc.locals.len();
            let saved_next = fc.next_reg;

            // Evaluate the RHS object.
            let obj_r = fc.alloc_reg();
            compile_expr(fc, right, obj_r)?;

            // ForInInit: collect enumerable keys into an array.
            let keys_r = fc.alloc_reg();
            fc.builder.emit_reg_reg(Op::ForInInit, keys_r, obj_r);
            // obj_r is no longer needed but we don't free it (LIFO constraint).

            // Index register (starts at 0).
            let idx_r = fc.alloc_reg();
            fc.builder.emit_load_int8(idx_r, 0);

            // Key and done registers (reused each iteration).
            let key_r = fc.alloc_reg();
            let done_r = fc.alloc_reg();

            let loop_start = fc.builder.offset();

            // ForInNext: get next key or done flag.
            fc.builder
                .emit_reg4(Op::ForInNext, key_r, done_r, keys_r, idx_r);

            // Exit loop if done.
            let exit_patch = fc.builder.emit_cond_jump(Op::JumpIfTrue, done_r);

            // Bind the loop variable.
            match left {
                ForInOfLeft::VarDecl { kind, pattern } => {
                    if let PatternKind::Identifier(name) = &pattern.kind {
                        let is_captured = fc.captured_names.contains(name.as_str());
                        let is_const = *kind == VarKind::Const;
                        let var_r = fc.define_local_ext(name, is_captured, is_const);
                        if is_captured {
                            fc.builder.emit_reg(Op::NewCell, var_r);
                            fc.builder.emit_reg_reg(Op::CellStore, var_r, key_r);
                        } else {
                            fc.builder.emit_reg_reg(Op::Move, var_r, key_r);
                        }
                    }
                }
                ForInOfLeft::Pattern(pattern) => {
                    if let PatternKind::Identifier(name) = &pattern.kind {
                        if let Some(local) = fc.find_local_info(name) {
                            let reg = local.reg;
                            let captured = local.is_captured;
                            if captured {
                                fc.builder.emit_reg_reg(Op::CellStore, reg, key_r);
                            } else {
                                fc.builder.emit_reg_reg(Op::Move, reg, key_r);
                            }
                        } else if let Some(uv_idx) = fc.find_upvalue(name) {
                            fc.builder.emit_store_upvalue(uv_idx, key_r);
                        } else {
                            let ni = fc.builder.add_name(name);
                            fc.builder.emit_store_global(ni, key_r);
                        }
                    }
                }
            }

            // Compile the body.
            fc.loop_stack.push(LoopCtx {
                label: None,
                break_patches: Vec::new(),
                continue_patches: Vec::new(),
            });
            compile_stmt(fc, body, result_reg)?;

            // Increment index: idx = idx + 1.
            // Use a temp register for the constant 1. Since we allocate it
            // after the loop body, we can't free it with LIFO either — the
            // saved_next restoration handles cleanup.
            let one_r = fc.alloc_reg();
            fc.builder.emit_load_int8(one_r, 1);
            fc.builder.emit_reg3(Op::Add, idx_r, idx_r, one_r);

            // Jump back to loop start.
            fc.builder.emit_jump_to(loop_start);

            // Patch exit and continue jumps.
            fc.builder.patch_jump(exit_patch);
            let ctx = fc.loop_stack.pop().unwrap();
            for patch in ctx.break_patches {
                fc.builder.patch_jump(patch);
            }
            for patch in ctx.continue_patches {
                fc.builder.patch_jump_to(patch, loop_start);
            }

            // Restore locals/regs — frees all temporaries at once.
            fc.locals.truncate(saved_locals);
            fc.next_reg = saved_next;
        }

        StmtKind::ForOf {
            left,
            right,
            body,
            is_await,
        } => {
            let saved_locals = fc.locals.len();
            let saved_next = fc.next_reg;

            // Evaluate the iterable.
            let iterable_r = fc.alloc_reg();
            compile_expr(fc, right, iterable_r)?;

            // Get the iterator: call iterable[@@iterator]() or @@asyncIterator().
            let iter_method_r = fc.alloc_reg();
            let sym_name = if *is_await {
                "@@asyncIterator"
            } else {
                "@@iterator"
            };
            let sym_iter_ni = fc.builder.add_name(sym_name);
            fc.builder
                .emit_get_prop_name(iter_method_r, iterable_r, sym_iter_ni);

            // Set `this` = iterable for the call.
            let this_ni = fc.builder.add_name("this");
            fc.builder.emit_store_global(this_ni, iterable_r);

            // Call [@@iterator/@@asyncIterator]() with 0 args.
            let iterator_r = fc.alloc_reg();
            let args_start = fc.next_reg;
            fc.builder
                .emit_call(iterator_r, iter_method_r, args_start, 0);

            // Temp registers for next method, result, done, value.
            let next_method_r = fc.alloc_reg();
            let next_ni = fc.builder.add_name("next");
            fc.builder
                .emit_get_prop_name(next_method_r, iterator_r, next_ni);

            let result_obj_r = fc.alloc_reg();
            let done_r = fc.alloc_reg();
            let value_r = fc.alloc_reg();

            // Loop start.
            let loop_start = fc.builder.offset();

            // Set `this` = iterator for the .next() call.
            fc.builder.emit_store_global(this_ni, iterator_r);

            // Call iterator.next().
            fc.builder
                .emit_call(result_obj_r, next_method_r, args_start, 0);

            // For await: await the result of .next() (which returns a Promise).
            if *is_await {
                let awaited_r = fc.alloc_reg();
                fc.builder.emit_await(awaited_r, result_obj_r);
                fc.builder.emit_reg_reg(Op::Move, result_obj_r, awaited_r);
                fc.free_reg(awaited_r);
            }

            // Extract done and value.
            let done_ni = fc.builder.add_name("done");
            let value_ni = fc.builder.add_name("value");
            fc.builder.emit_get_prop_name(done_r, result_obj_r, done_ni);

            // Exit if done.
            let exit_patch = fc.builder.emit_cond_jump(Op::JumpIfTrue, done_r);

            // Extract value.
            fc.builder
                .emit_get_prop_name(value_r, result_obj_r, value_ni);

            // Bind the loop variable.
            match left {
                ForInOfLeft::VarDecl { kind, pattern } => match &pattern.kind {
                    PatternKind::Identifier(name) => {
                        let is_captured = fc.captured_names.contains(name.as_str());
                        let is_const = *kind == VarKind::Const;
                        let var_r = fc.define_local_ext(name, is_captured, is_const);
                        if is_captured {
                            fc.builder.emit_reg(Op::NewCell, var_r);
                            fc.builder.emit_reg_reg(Op::CellStore, var_r, value_r);
                        } else {
                            fc.builder.emit_reg_reg(Op::Move, var_r, value_r);
                        }
                    }
                    _ => {
                        // Destructuring pattern in for...of.
                        compile_destructuring_pattern(fc, pattern, value_r)?;
                    }
                },
                ForInOfLeft::Pattern(pattern) => match &pattern.kind {
                    PatternKind::Identifier(name) => {
                        if let Some(local) = fc.find_local_info(name) {
                            let reg = local.reg;
                            let captured = local.is_captured;
                            if captured {
                                fc.builder.emit_reg_reg(Op::CellStore, reg, value_r);
                            } else {
                                fc.builder.emit_reg_reg(Op::Move, reg, value_r);
                            }
                        } else if let Some(uv_idx) = fc.find_upvalue(name) {
                            fc.builder.emit_store_upvalue(uv_idx, value_r);
                        } else {
                            let ni = fc.builder.add_name(name);
                            fc.builder.emit_store_global(ni, value_r);
                        }
                    }
                    _ => {
                        compile_destructuring_pattern(fc, pattern, value_r)?;
                    }
                },
            }

            // Push loop context for break/continue.
            fc.loop_stack.push(LoopCtx {
                label: None,
                break_patches: Vec::new(),
                continue_patches: Vec::new(),
            });

            // Compile body.
            compile_stmt(fc, body, result_reg)?;

            // Jump back to loop start.
            fc.builder.emit_jump_to(loop_start);

            // Patch exit.
            fc.builder.patch_jump(exit_patch);
            let ctx = fc.loop_stack.pop().unwrap();
            for patch in ctx.break_patches {
                fc.builder.patch_jump(patch);
            }
            for patch in ctx.continue_patches {
                fc.builder.patch_jump_to(patch, loop_start);
            }

            // Restore locals/regs.
            fc.locals.truncate(saved_locals);
            fc.next_reg = saved_next;
        }

        StmtKind::Return(expr) => {
            let ret_reg = fc.alloc_reg();
            if let Some(e) = expr {
                compile_expr(fc, e, ret_reg)?;
            } else {
                fc.builder.emit_reg(Op::LoadUndefined, ret_reg);
            }
            fc.builder.emit_reg(Op::Return, ret_reg);
            fc.free_reg(ret_reg);
        }

        StmtKind::Throw(expr) => {
            let tmp = fc.alloc_reg();
            compile_expr(fc, expr, tmp)?;
            fc.builder.emit_reg(Op::Throw, tmp);
            fc.free_reg(tmp);
        }

        StmtKind::Break(label) => {
            // Find the matching loop context.
            let idx = find_loop_ctx(&fc.loop_stack, label.as_deref())
                .ok_or_else(|| JsError::SyntaxError("break outside of loop".into()))?;
            let patch = fc.builder.emit_jump(Op::Jump);
            fc.loop_stack[idx].break_patches.push(patch);
        }

        StmtKind::Continue(label) => {
            let idx = find_loop_ctx(&fc.loop_stack, label.as_deref())
                .ok_or_else(|| JsError::SyntaxError("continue outside of loop".into()))?;
            let patch = fc.builder.emit_jump(Op::Jump);
            fc.loop_stack[idx].continue_patches.push(patch);
        }

        StmtKind::Labeled { label, body } => {
            // If body is a loop, propagate the label.
            match &body.kind {
                StmtKind::While { test, body: inner } => {
                    compile_while(fc, test, inner, Some(label.clone()), result_reg)?;
                }
                StmtKind::DoWhile { body: inner, test } => {
                    compile_do_while(fc, inner, test, Some(label.clone()), result_reg)?;
                }
                StmtKind::For {
                    init,
                    test,
                    update,
                    body: inner,
                } => {
                    compile_for(
                        fc,
                        init.as_ref(),
                        test.as_ref(),
                        update.as_ref(),
                        inner,
                        Some(label.clone()),
                        result_reg,
                    )?;
                }
                _ => {
                    compile_stmt(fc, body, result_reg)?;
                }
            }
        }

        StmtKind::Switch {
            discriminant,
            cases,
        } => {
            compile_switch(fc, discriminant, cases, result_reg)?;
        }

        StmtKind::Try {
            block,
            handler,
            finalizer,
        } => {
            if let Some(catch) = handler {
                // The catch register will receive the exception value. Use the
                // current next_reg so it doesn't conflict with temporaries
                // allocated inside the try block.
                let saved_next = fc.next_reg;
                let catch_reg = fc.alloc_reg();
                // Immediately "release" it so the try block can reuse registers
                // from this point. We remember catch_reg for PushExceptionHandler.
                fc.next_reg = saved_next;

                // Emit PushExceptionHandler with placeholder offset to catch block.
                let catch_patch = fc.builder.emit_push_exception_handler(catch_reg);

                let locals_len = fc.locals.len();

                // Compile the try block.
                compile_stmts(fc, block, result_reg)?;

                // If we reach here, no exception was thrown. Pop handler and
                // jump past the catch block.
                fc.builder.emit_pop_exception_handler();
                let end_patch = fc.builder.emit_jump(Op::Jump);

                // Reset register state for catch block — locals declared in
                // the try block are out of scope.
                fc.locals.truncate(locals_len);
                fc.next_reg = saved_next;

                // Patch the exception handler to jump here (catch block start).
                fc.builder.patch_jump(catch_patch);

                // Bind the catch parameter if present.
                if let Some(param) = &catch.param {
                    if let PatternKind::Identifier(name) = &param.kind {
                        let is_captured = fc.captured_names.contains(name.as_str());
                        let local = fc.define_local_ext(name, is_captured, false);
                        if is_captured {
                            fc.builder.emit_reg(Op::NewCell, local);
                            fc.builder.emit_reg_reg(Op::CellStore, local, catch_reg);
                        } else {
                            fc.builder.emit_reg_reg(Op::Move, local, catch_reg);
                        }
                    }
                }

                // Compile the catch body.
                compile_stmts(fc, &catch.body, result_reg)?;

                // End of catch — restore state.
                fc.locals.truncate(locals_len);
                fc.next_reg = saved_next;

                // Jump target from the try block.
                fc.builder.patch_jump(end_patch);
            } else {
                // No catch handler: just compile the try block.
                compile_stmts(fc, block, result_reg)?;
            }

            // Compile the finally block (always runs after try or catch).
            if let Some(fin) = finalizer {
                compile_stmts(fc, fin, result_reg)?;
            }
        }

        StmtKind::Empty | StmtKind::Debugger => {
            // No-op.
        }

        StmtKind::With { object, body } => {
            // Compile `with` as: evaluate object (discard), then run body.
            // Proper `with` scope requires VM support.
            let tmp = fc.alloc_reg();
            compile_expr(fc, object, tmp)?;
            fc.free_reg(tmp);
            compile_stmt(fc, body, result_reg)?;
        }

        StmtKind::Import { .. } => {
            // Module imports are resolved before execution; no bytecode needed.
        }

        StmtKind::Export(export) => {
            compile_export(fc, export, result_reg)?;
        }

        StmtKind::ClassDecl(class_def) => {
            compile_class_decl(fc, class_def)?;
        }
    }
    Ok(())
}

// ── Variable declarations ───────────────────────────────────

fn compile_var_declarator(
    fc: &mut FunctionCompiler,
    decl: &VarDeclarator,
    kind: VarKind,
) -> Result<(), JsError> {
    match &decl.pattern.kind {
        PatternKind::Identifier(name) => {
            let is_const = kind == VarKind::Const;
            let is_captured = fc.captured_names.contains(name.as_str());

            if is_const && decl.init.is_none() {
                return Err(JsError::SyntaxError(
                    "Missing initializer in const declaration".into(),
                ));
            }

            let reg = fc.define_local_ext(name, is_captured, is_const);

            if is_captured {
                // Allocate a cell for this variable.
                fc.builder.emit_reg(Op::NewCell, reg);
                if let Some(init) = &decl.init {
                    let tmp = fc.alloc_reg();
                    compile_expr(fc, init, tmp)?;
                    fc.builder.emit_reg_reg(Op::CellStore, reg, tmp);
                    // Top-level var: also store as global for cross-script access.
                    if fc.is_top_level && kind == VarKind::Var {
                        let name_idx = fc.builder.add_name(name);
                        fc.builder.emit_store_global(name_idx, tmp);
                    }
                    fc.free_reg(tmp);
                }
                // No init => cell stays undefined (already the default).
            } else if let Some(init) = &decl.init {
                compile_expr(fc, init, reg)?;
            } else {
                fc.builder.emit_reg(Op::LoadUndefined, reg);
            }

            // Top-level var/let/const: also store as global so the value
            // persists across separate `execute()` calls (multiple scripts
            // sharing the same global scope).
            if fc.is_top_level && !is_captured {
                let name_idx = fc.builder.add_name(name);
                fc.builder.emit_store_global(name_idx, reg);
            }
        }
        _ => {
            // Destructuring: evaluate init, then bind patterns.
            // Note: don't free tmp — destructuring pattern allocates permanent
            // local registers above it. The tmp register slot is reused via
            // next_reg restoration by the parent scope.
            let tmp = fc.alloc_reg();
            if let Some(init) = &decl.init {
                compile_expr(fc, init, tmp)?;
            } else {
                fc.builder.emit_reg(Op::LoadUndefined, tmp);
            }
            compile_destructuring_pattern(fc, &decl.pattern, tmp)?;
        }
    }
    Ok(())
}

fn compile_destructuring_pattern(
    fc: &mut FunctionCompiler,
    pattern: &Pattern,
    src: Reg,
) -> Result<(), JsError> {
    match &pattern.kind {
        PatternKind::Identifier(name) => {
            let is_captured = fc.captured_names.contains(name.as_str());
            let reg = fc.define_local_ext(name, is_captured, false);
            if is_captured {
                fc.builder.emit_reg(Op::NewCell, reg);
                fc.builder.emit_reg_reg(Op::CellStore, reg, src);
            } else {
                fc.builder.emit_reg_reg(Op::Move, reg, src);
            }
        }
        PatternKind::Object { properties, rest } => {
            // For each property, extract the value and bind it.
            // We use a single temp register that we reuse for each property
            // by resetting next_reg after each binding.
            for prop in properties {
                let key_name = match &prop.key {
                    PropertyKey::Identifier(s) | PropertyKey::String(s) => s.clone(),
                    PropertyKey::Computed(expr) => {
                        let saved = fc.next_reg;
                        let key_reg = fc.alloc_reg();
                        compile_expr(fc, expr, key_reg)?;
                        let val_reg = fc.alloc_reg();
                        fc.builder.emit_reg3(Op::GetProperty, val_reg, src, key_reg);
                        compile_destructuring_pattern(fc, &prop.value, val_reg)?;
                        // Temp regs are buried; just let them be.
                        let _ = saved;
                        continue;
                    }
                    PropertyKey::Number(n) => {
                        if n.fract() == 0.0 && n.abs() < 1e15 {
                            format!("{}", *n as i64)
                        } else {
                            format!("{n}")
                        }
                    }
                };
                // For simple identifier patterns, load property directly into
                // the target register to avoid LIFO register allocation issues.
                if let PatternKind::Identifier(name) = &prop.value.kind {
                    let is_captured = fc.captured_names.contains(name.as_str());
                    let reg = fc.define_local_ext(name, is_captured, false);
                    let name_idx = fc.builder.add_name(&key_name);
                    if is_captured {
                        let tmp = fc.alloc_reg();
                        fc.builder.emit_get_prop_name(tmp, src, name_idx);
                        fc.builder.emit_reg(Op::NewCell, reg);
                        fc.builder.emit_reg_reg(Op::CellStore, reg, tmp);
                        fc.free_reg(tmp);
                    } else {
                        fc.builder.emit_get_prop_name(reg, src, name_idx);
                    }
                } else {
                    // Complex inner pattern (nested, default, etc.)
                    // Allocate temp, extract value, then recurse.
                    // Temp register won't be freed (LIFO constraint with inner locals).
                    let val_reg = fc.alloc_reg();
                    let name_idx = fc.builder.add_name(&key_name);
                    fc.builder.emit_get_prop_name(val_reg, src, name_idx);
                    compile_destructuring_pattern(fc, &prop.value, val_reg)?;
                }
            }

            // Handle rest: collect remaining own enumerable properties.
            if let Some(rest_pat) = rest {
                // Collect extracted key names for exclusion.
                let extracted_keys: Vec<String> = properties
                    .iter()
                    .filter_map(|prop| match &prop.key {
                        PropertyKey::Identifier(s) | PropertyKey::String(s) => Some(s.clone()),
                        _ => None,
                    })
                    .collect();

                let rest_obj = fc.alloc_reg();
                fc.builder.emit_reg(Op::CreateObject, rest_obj);

                let keys_r = fc.alloc_reg();
                fc.builder.emit_reg_reg(Op::ForInInit, keys_r, src);
                let idx_r = fc.alloc_reg();
                fc.builder.emit_load_int8(idx_r, 0);
                let key_r = fc.alloc_reg();
                let done_r = fc.alloc_reg();

                let loop_start = fc.builder.offset();
                fc.builder
                    .emit_reg4(Op::ForInNext, key_r, done_r, keys_r, idx_r);
                let exit_patch = fc.builder.emit_cond_jump(Op::JumpIfTrue, done_r);

                let mut skip_patches = Vec::new();
                for excluded in &extracted_keys {
                    let excluded_r = fc.alloc_reg();
                    let ci = fc.builder.add_constant(Constant::String(excluded.clone()));
                    fc.builder.emit_reg_u16(Op::LoadConst, excluded_r, ci);
                    let cmp_r = fc.alloc_reg();
                    fc.builder.emit_reg3(Op::StrictEq, cmp_r, key_r, excluded_r);
                    let skip = fc.builder.emit_cond_jump(Op::JumpIfTrue, cmp_r);
                    skip_patches.push(skip);
                    fc.free_reg(cmp_r);
                    fc.free_reg(excluded_r);
                }

                let val_r = fc.alloc_reg();
                fc.builder.emit_reg3(Op::GetProperty, val_r, src, key_r);
                fc.builder
                    .emit_reg3(Op::SetProperty, rest_obj, key_r, val_r);
                fc.free_reg(val_r);

                for patch in skip_patches {
                    fc.builder.patch_jump(patch);
                }

                let one_r = fc.alloc_reg();
                fc.builder.emit_load_int8(one_r, 1);
                fc.builder.emit_reg3(Op::Add, idx_r, idx_r, one_r);
                fc.free_reg(one_r);

                fc.builder.emit_jump_to(loop_start);
                fc.builder.patch_jump(exit_patch);

                fc.free_reg(done_r);
                fc.free_reg(key_r);
                fc.free_reg(idx_r);
                fc.free_reg(keys_r);

                compile_destructuring_pattern(fc, rest_pat, rest_obj)?;
            }
        }
        PatternKind::Array { elements, rest } => {
            for (i, elem) in elements.iter().enumerate() {
                if let Some(pat) = elem {
                    // For simple identifier patterns, load directly into local.
                    if let PatternKind::Identifier(name) = &pat.kind {
                        let is_captured = fc.captured_names.contains(name.as_str());
                        let reg = fc.define_local_ext(name, is_captured, false);
                        let idx_reg = fc.alloc_reg();
                        if i <= 127 {
                            fc.builder.emit_load_int8(idx_reg, i as i8);
                        } else {
                            let ci = fc.builder.add_constant(Constant::Number(i as f64));
                            fc.builder.emit_reg_u16(Op::LoadConst, idx_reg, ci);
                        }
                        if is_captured {
                            let tmp = fc.alloc_reg();
                            fc.builder.emit_reg3(Op::GetProperty, tmp, src, idx_reg);
                            fc.builder.emit_reg(Op::NewCell, reg);
                            fc.builder.emit_reg_reg(Op::CellStore, reg, tmp);
                            fc.free_reg(tmp);
                        } else {
                            fc.builder.emit_reg3(Op::GetProperty, reg, src, idx_reg);
                        }
                        fc.free_reg(idx_reg);
                    } else {
                        // Complex inner pattern (nested, default, etc.)
                        let idx_reg = fc.alloc_reg();
                        if i <= 127 {
                            fc.builder.emit_load_int8(idx_reg, i as i8);
                        } else {
                            let ci = fc.builder.add_constant(Constant::Number(i as f64));
                            fc.builder.emit_reg_u16(Op::LoadConst, idx_reg, ci);
                        }
                        let val_reg = fc.alloc_reg();
                        fc.builder.emit_reg3(Op::GetProperty, val_reg, src, idx_reg);
                        compile_destructuring_pattern(fc, pat, val_reg)?;
                        // Don't free val_reg/idx_reg — inner pattern may have
                        // allocated locals above them.
                    }
                }
            }

            // Handle rest element: ...rest = src.slice(elements.len())
            if let Some(rest_pat) = rest {
                let slice_fn_r = fc.alloc_reg();
                let slice_ni = fc.builder.add_name("slice");
                fc.builder.emit_get_prop_name(slice_fn_r, src, slice_ni);

                let this_ni = fc.builder.add_name("this");
                fc.builder.emit_store_global(this_ni, src);

                let start_r = fc.alloc_reg();
                let elem_count = elements.len();
                if elem_count <= 127 {
                    fc.builder.emit_load_int8(start_r, elem_count as i8);
                } else {
                    let ci = fc.builder.add_constant(Constant::Number(elem_count as f64));
                    fc.builder.emit_reg_u16(Op::LoadConst, start_r, ci);
                }

                let rest_val = fc.alloc_reg();
                fc.builder.emit_call(rest_val, slice_fn_r, start_r, 1);

                compile_destructuring_pattern(fc, rest_pat, rest_val)?;
                // Don't free temps — rest pattern allocates locals.
            }
        }
        PatternKind::Assign { left, right } => {
            // Default value: if src is undefined, use default.
            let val_reg = fc.alloc_reg();
            fc.builder.emit_reg_reg(Op::Move, val_reg, src);
            // Check if undefined, if so use default.
            let check_reg = fc.alloc_reg();
            let undef_reg = fc.alloc_reg();
            fc.builder.emit_reg(Op::LoadUndefined, undef_reg);
            fc.builder
                .emit_reg3(Op::StrictEq, check_reg, val_reg, undef_reg);
            fc.free_reg(undef_reg);
            let patch = fc.builder.emit_cond_jump(Op::JumpIfFalse, check_reg);
            fc.free_reg(check_reg);
            // Is undefined → evaluate default.
            compile_expr(fc, right, val_reg)?;
            fc.builder.patch_jump(patch);
            compile_destructuring_pattern(fc, left, val_reg)?;
            // Don't free val_reg — inner pattern may have allocated locals.
        }
    }
    Ok(())
}

// ── Function declarations ───────────────────────────────────

fn compile_function_decl(fc: &mut FunctionCompiler, func_def: &FunctionDef) -> Result<(), JsError> {
    let name = func_def.id.clone().unwrap_or_default();
    let inner = compile_function_body_with_captures(fc, func_def)?;
    let func_idx = fc.builder.add_function(inner);

    let is_captured = fc.captured_names.contains(name.as_str());
    let reg = fc.define_local_ext(&name, is_captured, false);

    if is_captured {
        // Create a cell, then create the closure into a temp, then store into cell.
        fc.builder.emit_reg(Op::NewCell, reg);
        let tmp = fc.alloc_reg();
        fc.builder.emit_reg_u16(Op::CreateClosure, tmp, func_idx);
        fc.builder.emit_reg_reg(Op::CellStore, reg, tmp);
        // Also store as global.
        if !name.is_empty() {
            let name_idx = fc.builder.add_name(&name);
            fc.builder.emit_store_global(name_idx, tmp);
        }
        fc.free_reg(tmp);
    } else {
        fc.builder.emit_reg_u16(Op::CreateClosure, reg, func_idx);
        // Also store as global so inner/recursive calls via LoadGlobal can find it.
        if !name.is_empty() {
            let name_idx = fc.builder.add_name(&name);
            fc.builder.emit_store_global(name_idx, reg);
        }
    }
    Ok(())
}

/// Compile a function body, resolving upvalue captures from the parent scope.
fn compile_function_body_with_captures(
    parent: &mut FunctionCompiler,
    func_def: &FunctionDef,
) -> Result<Function, JsError> {
    // 1. Collect free variables of this inner function.
    let free_vars = collect_free_vars(&func_def.params, &func_def.body);

    // 2. Build upvalue list by resolving free vars against the parent scope.
    let mut upvalue_entries = Vec::new();
    for name in &free_vars {
        if let Some(local) = parent.find_local_info(name) {
            let reg = local.reg;
            let is_const = local.is_const;
            // Mark the parent's local as captured (if not already).
            // We need to update the parent's local, so find the index and mutate.
            if let Some(l) = parent.locals.iter_mut().rev().find(|l| l.name == *name) {
                l.is_captured = true;
            }
            upvalue_entries.push(UpvalueEntry {
                name: name.clone(),
                def: UpvalueDef {
                    is_local: true,
                    index: reg,
                },
                is_const,
            });
        } else if let Some(parent_uv_idx) = parent.find_upvalue(name) {
            // Transitive capture: the parent captures it from its own parent.
            let is_const = parent.is_upvalue_const(parent_uv_idx);
            upvalue_entries.push(UpvalueEntry {
                name: name.clone(),
                def: UpvalueDef {
                    is_local: false,
                    index: parent_uv_idx,
                },
                is_const,
            });
        }
        // If not found in parent or parent's upvalues, it must be a global — no upvalue needed.
    }

    // 3. Compile the inner function with its own scope.
    let mut inner = compile_function_body_inner(func_def, &upvalue_entries)?;

    // 4. Attach upvalue definitions to the compiled function.
    inner.upvalue_defs = upvalue_entries.iter().map(|e| e.def.clone()).collect();

    Ok(inner)
}

/// Core function body compilation. The `upvalue_entries` tell this function which
/// outer variables it can access via LoadUpvalue/StoreUpvalue.
fn compile_function_body_inner(
    func_def: &FunctionDef,
    upvalue_entries: &[UpvalueEntry],
) -> Result<Function, JsError> {
    let name = func_def.id.clone().unwrap_or_default();
    let param_count = func_def.params.len().min(255) as u8;
    let mut inner = FunctionCompiler::new(name, param_count);

    // Copy upvalue entries into the inner compiler so it can resolve references.
    for entry in upvalue_entries {
        inner.upvalues.push(UpvalueEntry {
            name: entry.name.clone(),
            def: entry.def.clone(),
            is_const: entry.is_const,
        });
    }

    // Pre-scan to find which of this function's locals are captured by ITS inner functions.
    let inner_caps = collect_inner_captures(&func_def.body);
    inner.captured_names = inner_caps;

    // Allocate registers for parameters.
    for p in &func_def.params {
        if let PatternKind::Identifier(pname) = &p.kind {
            let is_captured = inner.captured_names.contains(pname.as_str());
            inner.define_local_ext(pname, is_captured, false);
        } else {
            let _ = inner.alloc_reg();
        }
    }

    // Box captured parameters into cells.
    for p in &func_def.params {
        if let PatternKind::Identifier(pname) = &p.kind {
            if let Some(local) = inner.find_local_info(pname) {
                if local.is_captured {
                    let reg = local.reg;
                    // Move param value to temp, allocate cell, store value into cell.
                    let tmp = inner.alloc_reg();
                    inner.builder.emit_reg_reg(Op::Move, tmp, reg);
                    inner.builder.emit_reg(Op::NewCell, reg);
                    inner.builder.emit_reg_reg(Op::CellStore, reg, tmp);
                    inner.free_reg(tmp);
                }
            }
        }
    }

    // Result register for the function body.
    let result_reg = inner.alloc_reg();
    inner.builder.emit_reg(Op::LoadUndefined, result_reg);

    compile_stmts(&mut inner, &func_def.body, result_reg)?;

    // Implicit return undefined.
    inner.builder.emit_reg(Op::Return, result_reg);
    let mut func = inner.builder.finish();
    func.is_generator = func_def.is_generator;
    func.is_async = func_def.is_async;
    Ok(func)
}

// ── Class declarations ──────────────────────────────────────

fn compile_class_decl(fc: &mut FunctionCompiler, class_def: &ClassDef) -> Result<(), JsError> {
    let name = class_def.id.clone().unwrap_or_default();
    let is_captured = fc.captured_names.contains(name.as_str());
    let reg = fc.define_local_ext(&name, is_captured, false);

    // For captured classes, build the constructor into a temp register so we can
    // set prototype methods on it before wrapping it in a cell.
    let ctor_reg = if is_captured { fc.alloc_reg() } else { reg };

    // Find constructor or create empty one.
    let ctor = class_def.body.iter().find(|m| {
        matches!(
            &m.kind,
            ClassMemberKind::Method {
                kind: MethodKind::Constructor,
                ..
            }
        )
    });

    if let Some(member) = ctor {
        if let ClassMemberKind::Method { value, .. } = &member.kind {
            let inner = compile_function_body_with_captures(fc, value)?;
            let func_idx = fc.builder.add_function(inner);
            fc.builder
                .emit_reg_u16(Op::CreateClosure, ctor_reg, func_idx);
        }
    } else {
        // No constructor: create a minimal function that returns undefined.
        let mut empty = BytecodeBuilder::new(name.clone(), 0);
        let r = 0u8;
        empty.func.register_count = 1;
        empty.emit_reg(Op::LoadUndefined, r);
        empty.emit_reg(Op::Return, r);
        let func_idx = fc.builder.add_function(empty.finish());
        fc.builder
            .emit_reg_u16(Op::CreateClosure, ctor_reg, func_idx);
    }

    // Compile methods: set them as properties on the constructor's prototype.
    // This is simplified — real class compilation needs prototype chain setup.
    for member in &class_def.body {
        match &member.kind {
            ClassMemberKind::Method {
                key,
                value,
                kind,
                is_static: _,
                computed: _,
            } => {
                if matches!(kind, MethodKind::Constructor) {
                    continue;
                }
                let method_name = match key {
                    PropertyKey::Identifier(s) | PropertyKey::String(s) => s.clone(),
                    _ => continue,
                };
                let inner = compile_function_body_with_captures(fc, value)?;
                let func_idx = fc.builder.add_function(inner);
                let method_reg = fc.alloc_reg();
                fc.builder
                    .emit_reg_u16(Op::CreateClosure, method_reg, func_idx);
                let name_idx = fc.builder.add_name(&method_name);
                fc.builder
                    .emit_set_prop_name(ctor_reg, name_idx, method_reg);
                fc.free_reg(method_reg);
            }
            ClassMemberKind::Property { .. } => {
                // Class fields are set in constructor; skip here.
            }
        }
    }

    if is_captured {
        fc.builder.emit_reg(Op::NewCell, reg);
        fc.builder.emit_reg_reg(Op::CellStore, reg, ctor_reg);
        fc.free_reg(ctor_reg);
    }

    Ok(())
}

// ── Export ───────────────────────────────────────────────────

fn compile_export(
    fc: &mut FunctionCompiler,
    export: &ExportDecl,

    result_reg: Reg,
) -> Result<(), JsError> {
    match export {
        ExportDecl::Declaration(stmt) => {
            compile_stmt(fc, stmt, result_reg)?;
        }
        ExportDecl::Default(expr) => {
            compile_expr(fc, expr, result_reg)?;
        }
        ExportDecl::Named { .. } | ExportDecl::AllFrom(_) => {
            // Named re-exports are module-level; no bytecode needed.
        }
    }
    Ok(())
}

// ── Control flow ────────────────────────────────────────────

fn compile_if(
    fc: &mut FunctionCompiler,
    test: &Expr,
    consequent: &Stmt,
    alternate: Option<&Stmt>,

    result_reg: Reg,
) -> Result<(), JsError> {
    let cond = fc.alloc_reg();
    compile_expr(fc, test, cond)?;
    let else_patch = fc.builder.emit_cond_jump(Op::JumpIfFalse, cond);
    fc.free_reg(cond);

    compile_stmt(fc, consequent, result_reg)?;

    if let Some(alt) = alternate {
        let end_patch = fc.builder.emit_jump(Op::Jump);
        fc.builder.patch_jump(else_patch);
        compile_stmt(fc, alt, result_reg)?;
        fc.builder.patch_jump(end_patch);
    } else {
        fc.builder.patch_jump(else_patch);
    }
    Ok(())
}

fn compile_while(
    fc: &mut FunctionCompiler,
    test: &Expr,
    body: &Stmt,
    label: Option<String>,

    result_reg: Reg,
) -> Result<(), JsError> {
    let loop_start = fc.builder.offset();

    let cond = fc.alloc_reg();
    compile_expr(fc, test, cond)?;
    let exit_patch = fc.builder.emit_cond_jump(Op::JumpIfFalse, cond);
    fc.free_reg(cond);

    fc.loop_stack.push(LoopCtx {
        label,
        break_patches: Vec::new(),
        continue_patches: Vec::new(),
    });

    compile_stmt(fc, body, result_reg)?;
    fc.builder.emit_jump_to(loop_start);
    fc.builder.patch_jump(exit_patch);

    let ctx = fc.loop_stack.pop().unwrap();
    for patch in ctx.break_patches {
        fc.builder.patch_jump(patch);
    }
    // In a while loop, continue jumps back to the condition check (loop_start).
    for patch in ctx.continue_patches {
        fc.builder.patch_jump_to(patch, loop_start);
    }
    Ok(())
}

fn compile_do_while(
    fc: &mut FunctionCompiler,
    body: &Stmt,
    test: &Expr,
    label: Option<String>,

    result_reg: Reg,
) -> Result<(), JsError> {
    let loop_start = fc.builder.offset();

    fc.loop_stack.push(LoopCtx {
        label,
        break_patches: Vec::new(),
        continue_patches: Vec::new(),
    });

    compile_stmt(fc, body, result_reg)?;

    // continue in do-while should jump here (the condition check).
    let cond_start = fc.builder.offset();

    let cond = fc.alloc_reg();
    compile_expr(fc, test, cond)?;
    fc.builder
        .emit_cond_jump_to(Op::JumpIfTrue, cond, loop_start);
    fc.free_reg(cond);

    let ctx = fc.loop_stack.pop().unwrap();
    for patch in ctx.break_patches {
        fc.builder.patch_jump(patch);
    }
    for patch in ctx.continue_patches {
        fc.builder.patch_jump_to(patch, cond_start);
    }
    Ok(())
}

fn compile_for(
    fc: &mut FunctionCompiler,
    init: Option<&ForInit>,
    test: Option<&Expr>,
    update: Option<&Expr>,
    body: &Stmt,
    label: Option<String>,

    result_reg: Reg,
) -> Result<(), JsError> {
    let saved_locals = fc.locals.len();
    let saved_next = fc.next_reg;

    // Init.
    if let Some(init) = init {
        match init {
            ForInit::VarDecl { kind, declarators } => {
                for decl in declarators {
                    compile_var_declarator(fc, decl, *kind)?;
                }
            }
            ForInit::Expr(expr) => {
                let tmp = fc.alloc_reg();
                compile_expr(fc, expr, tmp)?;
                fc.free_reg(tmp);
            }
        }
    }

    let loop_start = fc.builder.offset();

    // Test.
    let exit_patch = if let Some(test) = test {
        let cond = fc.alloc_reg();
        compile_expr(fc, test, cond)?;
        let patch = fc.builder.emit_cond_jump(Op::JumpIfFalse, cond);
        fc.free_reg(cond);
        Some(patch)
    } else {
        None
    };

    fc.loop_stack.push(LoopCtx {
        label,
        break_patches: Vec::new(),
        continue_patches: Vec::new(),
    });

    compile_stmt(fc, body, result_reg)?;

    // continue in a for-loop should jump here (the update expression).
    let continue_target = fc.builder.offset();

    // Update.
    if let Some(update) = update {
        let tmp = fc.alloc_reg();
        compile_expr(fc, update, tmp)?;
        fc.free_reg(tmp);
    }

    fc.builder.emit_jump_to(loop_start);

    if let Some(patch) = exit_patch {
        fc.builder.patch_jump(patch);
    }

    let ctx = fc.loop_stack.pop().unwrap();
    for patch in ctx.break_patches {
        fc.builder.patch_jump(patch);
    }
    for patch in ctx.continue_patches {
        fc.builder.patch_jump_to(patch, continue_target);
    }

    fc.locals.truncate(saved_locals);
    fc.next_reg = saved_next;
    Ok(())
}

fn compile_switch(
    fc: &mut FunctionCompiler,
    discriminant: &Expr,
    cases: &[SwitchCase],

    result_reg: Reg,
) -> Result<(), JsError> {
    let disc_reg = fc.alloc_reg();
    compile_expr(fc, discriminant, disc_reg)?;

    // Use a loop context for break statements.
    fc.loop_stack.push(LoopCtx {
        label: None,
        break_patches: Vec::new(),
        continue_patches: Vec::new(),
    });

    // Phase 1: emit comparison jumps for each non-default case.
    // Store (case_index, patch_position) for each case with a test.
    let mut case_jump_patches: Vec<(usize, usize)> = Vec::new();
    let mut default_index: Option<usize> = None;

    for (i, case) in cases.iter().enumerate() {
        if let Some(test) = &case.test {
            let test_reg = fc.alloc_reg();
            compile_expr(fc, test, test_reg)?;
            let cmp_reg = fc.alloc_reg();
            fc.builder
                .emit_reg3(Op::StrictEq, cmp_reg, disc_reg, test_reg);
            let patch = fc.builder.emit_cond_jump(Op::JumpIfTrue, cmp_reg);
            fc.free_reg(cmp_reg);
            fc.free_reg(test_reg);
            case_jump_patches.push((i, patch));
        } else {
            default_index = Some(i);
        }
    }

    // After all comparisons: jump to default body or end.
    let fallthrough_patch = fc.builder.emit_jump(Op::Jump);

    // Phase 2: emit case bodies in order (fall-through semantics).
    let mut body_offsets: Vec<(usize, usize)> = Vec::new();
    for (i, case) in cases.iter().enumerate() {
        body_offsets.push((i, fc.builder.offset()));
        compile_stmts(fc, &case.consequent, result_reg)?;
    }

    let end_offset = fc.builder.offset();

    // Patch case test jumps to their respective body offsets.
    for (case_idx, patch) in &case_jump_patches {
        let body_offset = body_offsets
            .iter()
            .find(|(i, _)| i == case_idx)
            .map(|(_, off)| *off)
            .unwrap();
        fc.builder.patch_jump_to(*patch, body_offset);
    }

    // Patch fallthrough: jump to default body if present, otherwise to end.
    if let Some(def_idx) = default_index {
        let default_offset = body_offsets
            .iter()
            .find(|(i, _)| *i == def_idx)
            .map(|(_, off)| *off)
            .unwrap();
        fc.builder.patch_jump_to(fallthrough_patch, default_offset);
    } else {
        fc.builder.patch_jump_to(fallthrough_patch, end_offset);
    }

    fc.free_reg(disc_reg);

    let ctx = fc.loop_stack.pop().unwrap();
    for patch in ctx.break_patches {
        fc.builder.patch_jump(patch);
    }
    Ok(())
}

fn find_loop_ctx(stack: &[LoopCtx], label: Option<&str>) -> Option<usize> {
    if let Some(label) = label {
        stack
            .iter()
            .rposition(|ctx| ctx.label.as_deref() == Some(label))
    } else {
        if stack.is_empty() {
            None
        } else {
            Some(stack.len() - 1)
        }
    }
}

// ── Expressions ─────────────────────────────────────────────

fn compile_expr(fc: &mut FunctionCompiler, expr: &Expr, dst: Reg) -> Result<(), JsError> {
    match &expr.kind {
        ExprKind::Number(n) => {
            // Optimize small integers.
            let int_val = *n as i64;
            if int_val as f64 == *n && (-128..=127).contains(&int_val) {
                fc.builder.emit_load_int8(dst, int_val as i8);
            } else {
                let ci = fc.builder.add_constant(Constant::Number(*n));
                fc.builder.emit_reg_u16(Op::LoadConst, dst, ci);
            }
        }

        ExprKind::String(s) => {
            let ci = fc.builder.add_constant(Constant::String(s.clone()));
            fc.builder.emit_reg_u16(Op::LoadConst, dst, ci);
        }

        ExprKind::Bool(true) => {
            fc.builder.emit_reg(Op::LoadTrue, dst);
        }

        ExprKind::Bool(false) => {
            fc.builder.emit_reg(Op::LoadFalse, dst);
        }

        ExprKind::Null => {
            fc.builder.emit_reg(Op::LoadNull, dst);
        }

        ExprKind::Identifier(name) => {
            if let Some(local) = fc.find_local_info(name) {
                let reg = local.reg;
                let captured = local.is_captured;
                if captured {
                    fc.builder.emit_reg_reg(Op::CellLoad, dst, reg);
                } else if reg != dst {
                    fc.builder.emit_reg_reg(Op::Move, dst, reg);
                }
            } else if let Some(uv_idx) = fc.find_upvalue(name) {
                fc.builder.emit_load_upvalue(dst, uv_idx);
            } else {
                // Global lookup.
                let ni = fc.builder.add_name(name);
                fc.builder.emit_load_global(dst, ni);
            }
        }

        ExprKind::This => {
            // `this` is loaded as a global named "this" (the VM binds it).
            let ni = fc.builder.add_name("this");
            fc.builder.emit_load_global(dst, ni);
        }

        ExprKind::Binary { op, left, right } => {
            let lhs = fc.alloc_reg();
            compile_expr(fc, left, lhs)?;
            let rhs = fc.alloc_reg();
            compile_expr(fc, right, rhs)?;
            let bytecode_op = binary_op_to_opcode(*op);
            fc.builder.emit_reg3(bytecode_op, dst, lhs, rhs);
            fc.free_reg(rhs);
            fc.free_reg(lhs);
        }

        ExprKind::Unary { op, argument } => {
            if *op == UnaryOp::Delete {
                // Handle delete specially: need object + key form for member expressions.
                match &argument.kind {
                    ExprKind::Member {
                        object,
                        property,
                        computed,
                    } => {
                        let obj_r = fc.alloc_reg();
                        compile_expr(fc, object, obj_r)?;
                        let key_r = fc.alloc_reg();
                        if *computed {
                            compile_expr(fc, property, key_r)?;
                        } else if let ExprKind::Identifier(name) = &property.kind {
                            let ci = fc.builder.add_constant(Constant::String(name.clone()));
                            fc.builder.emit_reg_u16(Op::LoadConst, key_r, ci);
                        } else {
                            compile_expr(fc, property, key_r)?;
                        }
                        fc.builder.emit_reg3(Op::Delete, dst, obj_r, key_r);
                        fc.free_reg(key_r);
                        fc.free_reg(obj_r);
                    }
                    _ => {
                        // `delete x` on a simple identifier: always true in non-strict mode.
                        fc.builder.emit_reg(Op::LoadTrue, dst);
                    }
                }
            } else {
                let src = fc.alloc_reg();
                compile_expr(fc, argument, src)?;
                match op {
                    UnaryOp::Minus => fc.builder.emit_reg_reg(Op::Neg, dst, src),
                    UnaryOp::Plus => {
                        fc.builder.emit_reg_reg(Op::Move, dst, src);
                    }
                    UnaryOp::Not => fc.builder.emit_reg_reg(Op::LogicalNot, dst, src),
                    UnaryOp::BitwiseNot => fc.builder.emit_reg_reg(Op::BitNot, dst, src),
                    UnaryOp::Typeof => fc.builder.emit_reg_reg(Op::TypeOf, dst, src),
                    UnaryOp::Void => fc.builder.emit_reg_reg(Op::Void, dst, src),
                    UnaryOp::Delete => unreachable!(),
                }
                fc.free_reg(src);
            }
        }

        ExprKind::Update {
            op,
            argument,
            prefix,
        } => {
            // Get current value.
            compile_expr(fc, argument, dst)?;

            let one = fc.alloc_reg();
            fc.builder.emit_load_int8(one, 1);

            if *prefix {
                // ++x / --x: modify first, return modified.
                match op {
                    UpdateOp::Increment => fc.builder.emit_reg3(Op::Add, dst, dst, one),
                    UpdateOp::Decrement => fc.builder.emit_reg3(Op::Sub, dst, dst, one),
                }
                // Store back.
                compile_store(fc, argument, dst)?;
            } else {
                // x++ / x--: return original, then modify.
                let tmp = fc.alloc_reg();
                fc.builder.emit_reg_reg(Op::Move, tmp, dst);
                match op {
                    UpdateOp::Increment => fc.builder.emit_reg3(Op::Add, tmp, tmp, one),
                    UpdateOp::Decrement => fc.builder.emit_reg3(Op::Sub, tmp, tmp, one),
                }
                compile_store(fc, argument, tmp)?;
                fc.free_reg(tmp);
            }
            fc.free_reg(one);
        }

        ExprKind::Logical { op, left, right } => {
            compile_expr(fc, left, dst)?;
            match op {
                LogicalOp::And => {
                    // Short-circuit: if falsy, skip right.
                    let skip = fc.builder.emit_cond_jump(Op::JumpIfFalse, dst);
                    compile_expr(fc, right, dst)?;
                    fc.builder.patch_jump(skip);
                }
                LogicalOp::Or => {
                    let skip = fc.builder.emit_cond_jump(Op::JumpIfTrue, dst);
                    compile_expr(fc, right, dst)?;
                    fc.builder.patch_jump(skip);
                }
                LogicalOp::Nullish => {
                    let skip = fc.builder.emit_cond_jump(Op::JumpIfNullish, dst);
                    // If NOT nullish, skip the right side. Wait — JumpIfNullish
                    // should mean "jump if nullish" so we want: evaluate left,
                    // if NOT nullish skip right.
                    // Let's invert: evaluate left, check if nullish → evaluate right.
                    // We need the jump to skip the "evaluate right" if NOT nullish.
                    // Since JumpIfNullish jumps when nullish, we need the inverse.
                    // Instead: use a two-step approach.
                    //
                    // Actually, rethink: for `a ?? b`:
                    //   1. evaluate a → dst
                    //   2. if dst is NOT null/undefined, jump to end
                    //   3. evaluate b → dst
                    //   end:
                    // JumpIfNullish jumps when IS nullish. So we want jump when NOT nullish.
                    // Let's just use a "not nullish" check.
                    // For now: negate and use JumpIfFalse.
                    // Actually simpler: skip right when not nullish.
                    // JumpIfNullish jumps WHEN nullish. We want to jump over right when NOT nullish.
                    // So:
                    //   evaluate a → dst
                    //   JumpIfNullish dst → evaluate_right
                    //   Jump → end
                    //   evaluate_right: evaluate b → dst
                    //   end:
                    // But we already emitted JumpIfNullish. Let's fix this.
                    // The JumpIfNullish we emitted jumps to "after patch", which is where
                    // we'll put the right-side code. We need another jump to skip right.
                    let end_patch = fc.builder.emit_jump(Op::Jump);
                    fc.builder.patch_jump(skip); // nullish → evaluate right
                    compile_expr(fc, right, dst)?;
                    fc.builder.patch_jump(end_patch);
                }
            }
        }

        ExprKind::Assignment { op, left, right } => {
            if *op == AssignOp::Assign {
                compile_expr(fc, right, dst)?;
                compile_store(fc, left, dst)?;
            } else {
                // Compound assignment: load current, operate, store.
                compile_expr(fc, left, dst)?;
                let rhs = fc.alloc_reg();
                compile_expr(fc, right, rhs)?;
                let arith_op = compound_assign_op(*op);
                fc.builder.emit_reg3(arith_op, dst, dst, rhs);
                fc.free_reg(rhs);
                compile_store(fc, left, dst)?;
            }
        }

        ExprKind::Conditional {
            test,
            consequent,
            alternate,
        } => {
            let cond = fc.alloc_reg();
            compile_expr(fc, test, cond)?;
            let else_patch = fc.builder.emit_cond_jump(Op::JumpIfFalse, cond);
            fc.free_reg(cond);
            compile_expr(fc, consequent, dst)?;
            let end_patch = fc.builder.emit_jump(Op::Jump);
            fc.builder.patch_jump(else_patch);
            compile_expr(fc, alternate, dst)?;
            fc.builder.patch_jump(end_patch);
        }

        ExprKind::Call { callee, arguments } => {
            // Detect method calls (obj.method()) to set `this`.
            if let ExprKind::Member {
                object,
                property,
                computed,
            } = &callee.kind
            {
                // Layout: [obj_reg] [func_reg] [arg0] [arg1] ...
                // We keep obj_reg alive so we can set `this` before the call.
                let obj_reg = fc.alloc_reg();
                compile_expr(fc, object, obj_reg)?;
                let func_reg = fc.alloc_reg();
                if !computed {
                    if let ExprKind::Identifier(name) = &property.kind {
                        let ni = fc.builder.add_name(name);
                        fc.builder.emit_get_prop_name(func_reg, obj_reg, ni);
                    } else {
                        let key_reg = fc.alloc_reg();
                        compile_expr(fc, property, key_reg)?;
                        fc.builder
                            .emit_reg3(Op::GetProperty, func_reg, obj_reg, key_reg);
                        fc.free_reg(key_reg);
                    }
                } else {
                    let key_reg = fc.alloc_reg();
                    compile_expr(fc, property, key_reg)?;
                    fc.builder
                        .emit_reg3(Op::GetProperty, func_reg, obj_reg, key_reg);
                    fc.free_reg(key_reg);
                }

                // Set `this` to the receiver object before calling.
                let this_ni = fc.builder.add_name("this");
                fc.builder.emit_store_global(this_ni, obj_reg);

                let args_start = fc.next_reg;
                let arg_count = arguments.len().min(255) as u8;
                for arg in arguments {
                    let arg_reg = fc.alloc_reg();
                    compile_expr(fc, arg, arg_reg)?;
                }

                fc.builder.emit_call(dst, func_reg, args_start, arg_count);

                // Free in LIFO order: args, func_reg, obj_reg.
                for _ in 0..arg_count {
                    fc.next_reg -= 1;
                }
                fc.free_reg(func_reg);
                fc.free_reg(obj_reg);
            } else {
                let func_reg = fc.alloc_reg();
                compile_expr(fc, callee, func_reg)?;

                let args_start = fc.next_reg;
                let arg_count = arguments.len().min(255) as u8;
                for arg in arguments {
                    let arg_reg = fc.alloc_reg();
                    compile_expr(fc, arg, arg_reg)?;
                }

                fc.builder.emit_call(dst, func_reg, args_start, arg_count);

                for _ in 0..arg_count {
                    fc.next_reg -= 1;
                }
                fc.free_reg(func_reg);
            }
        }

        ExprKind::New { callee, arguments } => {
            // For now, compile like a regular call. The VM will differentiate
            // based on the `New` vs `Call` distinction (TODO: add NewCall opcode).
            let func_reg = fc.alloc_reg();
            compile_expr(fc, callee, func_reg)?;

            let args_start = fc.next_reg;
            let arg_count = arguments.len().min(255) as u8;
            for arg in arguments {
                let arg_reg = fc.alloc_reg();
                compile_expr(fc, arg, arg_reg)?;
            }

            fc.builder.emit_call(dst, func_reg, args_start, arg_count);

            for _ in 0..arg_count {
                fc.next_reg -= 1;
            }
            fc.free_reg(func_reg);
        }

        ExprKind::Member {
            object,
            property,
            computed,
        } => {
            let obj_reg = fc.alloc_reg();
            compile_expr(fc, object, obj_reg)?;

            if !computed {
                // Static member: obj.prop → GetPropertyByName.
                if let ExprKind::Identifier(name) = &property.kind {
                    let ni = fc.builder.add_name(name);
                    fc.builder.emit_get_prop_name(dst, obj_reg, ni);
                } else {
                    let key_reg = fc.alloc_reg();
                    compile_expr(fc, property, key_reg)?;
                    fc.builder.emit_reg3(Op::GetProperty, dst, obj_reg, key_reg);
                    fc.free_reg(key_reg);
                }
            } else {
                // Computed member: obj[expr].
                let key_reg = fc.alloc_reg();
                compile_expr(fc, property, key_reg)?;
                fc.builder.emit_reg3(Op::GetProperty, dst, obj_reg, key_reg);
                fc.free_reg(key_reg);
            }
            fc.free_reg(obj_reg);
        }

        ExprKind::Array(elements) => {
            let has_spread = elements
                .iter()
                .any(|e| matches!(e, Some(ArrayElement::Spread(_))));

            fc.builder.emit_reg(Op::CreateArray, dst);

            if has_spread {
                // When spreads are present, we track the index dynamically.
                // For each normal element, push at current length.
                // For spread elements, use the Spread opcode.
                for el in elements.iter().flatten() {
                    match el {
                        ArrayElement::Expr(e) => {
                            let val_reg = fc.alloc_reg();
                            compile_expr(fc, e, val_reg)?;
                            // Get current length as index.
                            let idx_reg = fc.alloc_reg();
                            let len_ni = fc.builder.add_name("length");
                            fc.builder.emit_get_prop_name(idx_reg, dst, len_ni);
                            fc.builder.emit_reg3(Op::SetProperty, dst, idx_reg, val_reg);
                            // Increment length.
                            let one_r = fc.alloc_reg();
                            fc.builder.emit_load_int8(one_r, 1);
                            fc.builder.emit_reg3(Op::Add, idx_reg, idx_reg, one_r);
                            fc.builder.emit_set_prop_name(dst, len_ni, idx_reg);
                            fc.free_reg(one_r);
                            fc.free_reg(idx_reg);
                            fc.free_reg(val_reg);
                        }
                        ArrayElement::Spread(e) => {
                            let spread_src = fc.alloc_reg();
                            compile_expr(fc, e, spread_src)?;
                            fc.builder.emit_spread(dst, spread_src);
                            fc.free_reg(spread_src);
                        }
                    }
                }
            } else {
                // No spreads: use simple indexed assignment.
                for (i, elem) in elements.iter().enumerate() {
                    if let Some(ArrayElement::Expr(e)) = elem {
                        let val_reg = fc.alloc_reg();
                        compile_expr(fc, e, val_reg)?;
                        let idx_reg = fc.alloc_reg();
                        if i <= 127 {
                            fc.builder.emit_load_int8(idx_reg, i as i8);
                        } else {
                            let ci = fc.builder.add_constant(Constant::Number(i as f64));
                            fc.builder.emit_reg_u16(Op::LoadConst, idx_reg, ci);
                        }
                        fc.builder.emit_reg3(Op::SetProperty, dst, idx_reg, val_reg);
                        fc.free_reg(idx_reg);
                        fc.free_reg(val_reg);
                    }
                }
                // Set length.
                if !elements.is_empty() {
                    let len_name = fc.builder.add_name("length");
                    let len_reg = fc.alloc_reg();
                    if elements.len() <= 127 {
                        fc.builder.emit_load_int8(len_reg, elements.len() as i8);
                    } else {
                        let ci = fc
                            .builder
                            .add_constant(Constant::Number(elements.len() as f64));
                        fc.builder.emit_reg_u16(Op::LoadConst, len_reg, ci);
                    }
                    fc.builder.emit_set_prop_name(dst, len_name, len_reg);
                    fc.free_reg(len_reg);
                }
            }
        }

        ExprKind::Object(properties) => {
            fc.builder.emit_reg(Op::CreateObject, dst);
            for prop in properties {
                let val_reg = fc.alloc_reg();
                if let Some(value) = &prop.value {
                    compile_expr(fc, value, val_reg)?;
                } else {
                    // Shorthand: `{ x }` means `{ x: x }`.
                    if let PropertyKey::Identifier(name) = &prop.key {
                        if let Some(local) = fc.find_local_info(name) {
                            let reg = local.reg;
                            let captured = local.is_captured;
                            if captured {
                                fc.builder.emit_reg_reg(Op::CellLoad, val_reg, reg);
                            } else {
                                fc.builder.emit_reg_reg(Op::Move, val_reg, reg);
                            }
                        } else if let Some(uv_idx) = fc.find_upvalue(name) {
                            fc.builder.emit_load_upvalue(val_reg, uv_idx);
                        } else {
                            let ni = fc.builder.add_name(name);
                            fc.builder.emit_load_global(val_reg, ni);
                        }
                    } else {
                        fc.builder.emit_reg(Op::LoadUndefined, val_reg);
                    }
                }

                match &prop.key {
                    PropertyKey::Identifier(name) | PropertyKey::String(name) => {
                        let ni = fc.builder.add_name(name);
                        fc.builder.emit_set_prop_name(dst, ni, val_reg);
                    }
                    PropertyKey::Number(n) => {
                        let key_reg = fc.alloc_reg();
                        let ci = fc.builder.add_constant(Constant::Number(*n));
                        fc.builder.emit_reg_u16(Op::LoadConst, key_reg, ci);
                        fc.builder.emit_reg3(Op::SetProperty, dst, key_reg, val_reg);
                        fc.free_reg(key_reg);
                    }
                    PropertyKey::Computed(expr) => {
                        let key_reg = fc.alloc_reg();
                        compile_expr(fc, expr, key_reg)?;
                        fc.builder.emit_reg3(Op::SetProperty, dst, key_reg, val_reg);
                        fc.free_reg(key_reg);
                    }
                }
                fc.free_reg(val_reg);
            }
        }

        ExprKind::Function(func_def) => {
            let inner = compile_function_body_with_captures(fc, func_def)?;
            let func_idx = fc.builder.add_function(inner);
            fc.builder.emit_reg_u16(Op::CreateClosure, dst, func_idx);
        }

        ExprKind::Arrow {
            params,
            body,
            is_async,
        } => {
            // Collect free variables from the arrow body.
            let free_vars = collect_free_vars_arrow(params, body);

            // Resolve upvalues against the parent scope.
            let mut upvalue_entries = Vec::new();
            for name in &free_vars {
                if let Some(local) = fc.find_local_info(name) {
                    let reg = local.reg;
                    let is_const = local.is_const;
                    if let Some(l) = fc.locals.iter_mut().rev().find(|l| l.name == *name) {
                        l.is_captured = true;
                    }
                    upvalue_entries.push(UpvalueEntry {
                        name: name.clone(),
                        def: UpvalueDef {
                            is_local: true,
                            index: reg,
                        },
                        is_const,
                    });
                } else if let Some(parent_uv_idx) = fc.find_upvalue(name) {
                    let is_const = fc.is_upvalue_const(parent_uv_idx);
                    upvalue_entries.push(UpvalueEntry {
                        name: name.clone(),
                        def: UpvalueDef {
                            is_local: false,
                            index: parent_uv_idx,
                        },
                        is_const,
                    });
                }
            }

            let param_count = params.len().min(255) as u8;
            let mut inner = FunctionCompiler::new("<arrow>".into(), param_count);

            // Copy upvalue entries.
            for entry in &upvalue_entries {
                inner.upvalues.push(UpvalueEntry {
                    name: entry.name.clone(),
                    def: entry.def.clone(),
                    is_const: entry.is_const,
                });
            }

            // Pre-scan for inner captures within the arrow body.
            match body {
                ArrowBody::Expr(_) => {}
                ArrowBody::Block(stmts) => {
                    inner.captured_names = collect_inner_captures(stmts);
                }
            }

            for p in params {
                if let PatternKind::Identifier(pname) = &p.kind {
                    let is_captured = inner.captured_names.contains(pname.as_str());
                    inner.define_local_ext(pname, is_captured, false);
                } else {
                    let _ = inner.alloc_reg();
                }
            }

            // Box captured parameters.
            for p in params {
                if let PatternKind::Identifier(pname) = &p.kind {
                    if let Some(local) = inner.find_local_info(pname) {
                        if local.is_captured {
                            let reg = local.reg;
                            let tmp = inner.alloc_reg();
                            inner.builder.emit_reg_reg(Op::Move, tmp, reg);
                            inner.builder.emit_reg(Op::NewCell, reg);
                            inner.builder.emit_reg_reg(Op::CellStore, reg, tmp);
                            inner.free_reg(tmp);
                        }
                    }
                }
            }

            let result = inner.alloc_reg();
            match body {
                ArrowBody::Expr(e) => {
                    compile_expr(&mut inner, e, result)?;
                }
                ArrowBody::Block(stmts) => {
                    inner.builder.emit_reg(Op::LoadUndefined, result);
                    compile_stmts(&mut inner, stmts, result)?;
                }
            }
            inner.builder.emit_reg(Op::Return, result);
            let mut inner_func = inner.builder.finish();
            inner_func.upvalue_defs = upvalue_entries.iter().map(|e| e.def.clone()).collect();
            inner_func.is_async = *is_async;
            let func_idx = fc.builder.add_function(inner_func);
            fc.builder.emit_reg_u16(Op::CreateClosure, dst, func_idx);
        }

        ExprKind::Class(class_def) => {
            // Class expression: compile like class decl but into dst.
            let name = class_def.id.clone().unwrap_or_default();
            // Find constructor.
            let ctor = class_def.body.iter().find(|m| {
                matches!(
                    &m.kind,
                    ClassMemberKind::Method {
                        kind: MethodKind::Constructor,
                        ..
                    }
                )
            });
            if let Some(member) = ctor {
                if let ClassMemberKind::Method { value, .. } = &member.kind {
                    let inner = compile_function_body_with_captures(fc, value)?;
                    let func_idx = fc.builder.add_function(inner);
                    fc.builder.emit_reg_u16(Op::CreateClosure, dst, func_idx);
                }
            } else {
                let mut empty = BytecodeBuilder::new(name, 0);
                let r = 0u8;
                empty.func.register_count = 1;
                empty.emit_reg(Op::LoadUndefined, r);
                empty.emit_reg(Op::Return, r);
                let func_idx = fc.builder.add_function(empty.finish());
                fc.builder.emit_reg_u16(Op::CreateClosure, dst, func_idx);
            }

            // Compile methods as properties on the constructor.
            for member in &class_def.body {
                match &member.kind {
                    ClassMemberKind::Method {
                        key,
                        value,
                        kind,
                        is_static: _,
                        computed: _,
                    } => {
                        if matches!(kind, MethodKind::Constructor) {
                            continue;
                        }
                        let method_name = match key {
                            PropertyKey::Identifier(s) | PropertyKey::String(s) => s.clone(),
                            _ => continue,
                        };
                        let inner = compile_function_body_with_captures(fc, value)?;
                        let func_idx = fc.builder.add_function(inner);
                        let method_reg = fc.alloc_reg();
                        fc.builder
                            .emit_reg_u16(Op::CreateClosure, method_reg, func_idx);
                        let name_idx = fc.builder.add_name(&method_name);
                        fc.builder.emit_set_prop_name(dst, name_idx, method_reg);
                        fc.free_reg(method_reg);
                    }
                    ClassMemberKind::Property { .. } => {}
                }
            }
        }

        ExprKind::Sequence(exprs) => {
            for e in exprs {
                compile_expr(fc, e, dst)?;
            }
        }

        ExprKind::Spread(inner) => {
            compile_expr(fc, inner, dst)?;
        }

        ExprKind::TemplateLiteral {
            quasis,
            expressions,
        } => {
            // Compile template literal as string concatenation.
            if quasis.len() == 1 && expressions.is_empty() {
                let ci = fc.builder.add_constant(Constant::String(quasis[0].clone()));
                fc.builder.emit_reg_u16(Op::LoadConst, dst, ci);
            } else {
                // Start with first quasi.
                let ci = fc.builder.add_constant(Constant::String(quasis[0].clone()));
                fc.builder.emit_reg_u16(Op::LoadConst, dst, ci);
                for (i, expr) in expressions.iter().enumerate() {
                    let tmp = fc.alloc_reg();
                    compile_expr(fc, expr, tmp)?;
                    fc.builder.emit_reg3(Op::Add, dst, dst, tmp);
                    fc.free_reg(tmp);
                    if i + 1 < quasis.len() {
                        let qi = fc
                            .builder
                            .add_constant(Constant::String(quasis[i + 1].clone()));
                        let tmp2 = fc.alloc_reg();
                        fc.builder.emit_reg_u16(Op::LoadConst, tmp2, qi);
                        fc.builder.emit_reg3(Op::Add, dst, dst, tmp2);
                        fc.free_reg(tmp2);
                    }
                }
            }
        }

        ExprKind::TaggedTemplate { tag, quasi } => {
            // Simplified: call tag with the template as argument.
            let func_reg = fc.alloc_reg();
            compile_expr(fc, tag, func_reg)?;
            let arg_reg = fc.alloc_reg();
            compile_expr(fc, quasi, arg_reg)?;
            fc.builder.emit_call(dst, func_reg, arg_reg, 1);
            fc.free_reg(arg_reg);
            fc.free_reg(func_reg);
        }

        ExprKind::Yield { argument, delegate } => {
            if *delegate {
                // yield* expr: iterate the sub-iterator and yield each value.
                let iter_r = fc.alloc_reg();
                if let Some(arg) = argument {
                    compile_expr(fc, arg, iter_r)?;
                } else {
                    fc.builder.emit_reg(Op::LoadUndefined, iter_r);
                }

                // Get iterator from the expression.
                let iter_method_r = fc.alloc_reg();
                let sym_iter_ni = fc.builder.add_name("@@iterator");
                fc.builder
                    .emit_get_prop_name(iter_method_r, iter_r, sym_iter_ni);
                let this_ni = fc.builder.add_name("this");
                fc.builder.emit_store_global(this_ni, iter_r);
                let iterator_r = fc.alloc_reg();
                let args_start = fc.next_reg;
                fc.builder
                    .emit_call(iterator_r, iter_method_r, args_start, 0);

                // Get next method.
                let next_r = fc.alloc_reg();
                let next_ni = fc.builder.add_name("next");
                fc.builder.emit_get_prop_name(next_r, iterator_r, next_ni);

                let result_r = fc.alloc_reg();
                let done_r = fc.alloc_reg();
                let val_r = fc.alloc_reg();

                let loop_start = fc.builder.offset();

                // Call next().
                fc.builder.emit_store_global(this_ni, iterator_r);
                fc.builder.emit_call(result_r, next_r, args_start, 0);

                let done_ni = fc.builder.add_name("done");
                let value_ni = fc.builder.add_name("value");
                fc.builder.emit_get_prop_name(done_r, result_r, done_ni);

                let exit_patch = fc.builder.emit_cond_jump(Op::JumpIfTrue, done_r);

                fc.builder.emit_get_prop_name(val_r, result_r, value_ni);

                // Yield the value.
                fc.builder.emit_yield(dst, val_r);

                // Jump back.
                fc.builder.emit_jump_to(loop_start);

                // Exit: the last result's value is the yield* expression value.
                fc.builder.patch_jump(exit_patch);
                fc.builder.emit_get_prop_name(dst, result_r, value_ni);

                fc.free_reg(val_r);
                fc.free_reg(done_r);
                fc.free_reg(result_r);
                fc.free_reg(next_r);
                fc.free_reg(iterator_r);
                fc.free_reg(iter_method_r);
                fc.free_reg(iter_r);
            } else {
                // yield expr: emit Yield opcode.
                let src = fc.alloc_reg();
                if let Some(arg) = argument {
                    compile_expr(fc, arg, src)?;
                } else {
                    fc.builder.emit_reg(Op::LoadUndefined, src);
                }
                fc.builder.emit_yield(dst, src);
                fc.free_reg(src);
            }
        }

        ExprKind::Await(inner) => {
            let src = fc.alloc_reg();
            compile_expr(fc, inner, src)?;
            fc.builder.emit_await(dst, src);
            fc.free_reg(src);
        }

        ExprKind::RegExp { pattern, flags } => {
            // Compile as: RegExp(pattern, flags) — a call to the global constructor.
            let func_reg = fc.alloc_reg();
            let name_idx = fc.builder.add_name("RegExp");
            fc.builder.emit_reg_u16(Op::LoadGlobal, func_reg, name_idx);

            let args_start = fc.next_reg;
            let pat_reg = fc.alloc_reg();
            let pat_idx = fc.builder.add_constant(Constant::String(pattern.clone()));
            fc.builder.emit_reg_u16(Op::LoadConst, pat_reg, pat_idx);

            let flags_reg = fc.alloc_reg();
            let flags_idx = fc.builder.add_constant(Constant::String(flags.clone()));
            fc.builder.emit_reg_u16(Op::LoadConst, flags_reg, flags_idx);

            fc.builder.emit_call(dst, func_reg, args_start, 2);

            fc.next_reg -= 1; // flags_reg
            fc.next_reg -= 1; // pat_reg
            fc.free_reg(func_reg);
        }

        ExprKind::OptionalChain { base } => {
            compile_expr(fc, base, dst)?;
        }
    }
    Ok(())
}

/// Compile a store operation (assignment target).
fn compile_store(fc: &mut FunctionCompiler, target: &Expr, src: Reg) -> Result<(), JsError> {
    match &target.kind {
        ExprKind::Identifier(name) => {
            if let Some(local) = fc.find_local_info(name) {
                if local.is_const {
                    return Err(JsError::SyntaxError(format!(
                        "Assignment to constant variable '{name}'"
                    )));
                }
                let reg = local.reg;
                let captured = local.is_captured;
                if captured {
                    fc.builder.emit_reg_reg(Op::CellStore, reg, src);
                } else if reg != src {
                    fc.builder.emit_reg_reg(Op::Move, reg, src);
                }
            } else if let Some(uv_idx) = fc.find_upvalue(name) {
                if fc.is_upvalue_const(uv_idx) {
                    return Err(JsError::SyntaxError(format!(
                        "Assignment to constant variable '{name}'"
                    )));
                }
                fc.builder.emit_store_upvalue(uv_idx, src);
            } else {
                let ni = fc.builder.add_name(name);
                fc.builder.emit_store_global(ni, src);
            }
        }
        ExprKind::Member {
            object,
            property,
            computed,
        } => {
            let obj_reg = fc.alloc_reg();
            compile_expr(fc, object, obj_reg)?;
            if !computed {
                if let ExprKind::Identifier(name) = &property.kind {
                    let ni = fc.builder.add_name(name);
                    fc.builder.emit_set_prop_name(obj_reg, ni, src);
                } else {
                    let key_reg = fc.alloc_reg();
                    compile_expr(fc, property, key_reg)?;
                    fc.builder.emit_reg3(Op::SetProperty, obj_reg, key_reg, src);
                    fc.free_reg(key_reg);
                }
            } else {
                let key_reg = fc.alloc_reg();
                compile_expr(fc, property, key_reg)?;
                fc.builder.emit_reg3(Op::SetProperty, obj_reg, key_reg, src);
                fc.free_reg(key_reg);
            }
            fc.free_reg(obj_reg);
        }
        _ => {
            // Other assignment targets (destructuring) not handled here.
        }
    }
    Ok(())
}

fn binary_op_to_opcode(op: BinaryOp) -> Op {
    match op {
        BinaryOp::Add => Op::Add,
        BinaryOp::Sub => Op::Sub,
        BinaryOp::Mul => Op::Mul,
        BinaryOp::Div => Op::Div,
        BinaryOp::Rem => Op::Rem,
        BinaryOp::Exp => Op::Exp,
        BinaryOp::Eq => Op::Eq,
        BinaryOp::Ne => Op::NotEq,
        BinaryOp::StrictEq => Op::StrictEq,
        BinaryOp::StrictNe => Op::StrictNotEq,
        BinaryOp::Lt => Op::LessThan,
        BinaryOp::Le => Op::LessEq,
        BinaryOp::Gt => Op::GreaterThan,
        BinaryOp::Ge => Op::GreaterEq,
        BinaryOp::Shl => Op::ShiftLeft,
        BinaryOp::Shr => Op::ShiftRight,
        BinaryOp::Ushr => Op::UShiftRight,
        BinaryOp::BitAnd => Op::BitAnd,
        BinaryOp::BitOr => Op::BitOr,
        BinaryOp::BitXor => Op::BitXor,
        BinaryOp::In => Op::In,
        BinaryOp::Instanceof => Op::InstanceOf,
    }
}

fn compound_assign_op(op: AssignOp) -> Op {
    match op {
        AssignOp::AddAssign => Op::Add,
        AssignOp::SubAssign => Op::Sub,
        AssignOp::MulAssign => Op::Mul,
        AssignOp::DivAssign => Op::Div,
        AssignOp::RemAssign => Op::Rem,
        AssignOp::ExpAssign => Op::Exp,
        AssignOp::ShlAssign => Op::ShiftLeft,
        AssignOp::ShrAssign => Op::ShiftRight,
        AssignOp::UshrAssign => Op::UShiftRight,
        AssignOp::BitAndAssign => Op::BitAnd,
        AssignOp::BitOrAssign => Op::BitOr,
        AssignOp::BitXorAssign => Op::BitXor,
        AssignOp::AndAssign => Op::BitAnd, // logical AND assignment uses short-circuit; simplified here
        AssignOp::OrAssign => Op::BitOr,   // likewise
        AssignOp::NullishAssign => Op::Move, // simplified
        AssignOp::Assign => unreachable!(),
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::parser::Parser;

    /// Helper: parse and compile source, return the top-level function.
    fn compile_src(src: &str) -> Function {
        let program = Parser::parse(src).expect("parse failed");
        compile(&program).expect("compile failed")
    }

    #[test]
    fn test_compile_number_literal() {
        let f = compile_src("42;");
        let dis = f.disassemble();
        assert!(dis.contains("LoadInt8 r0, 42"), "got:\n{dis}");
        assert!(dis.contains("Return r0"));
    }

    #[test]
    fn test_compile_large_number() {
        let f = compile_src("3.14;");
        let dis = f.disassemble();
        assert!(dis.contains("LoadConst r0, #0"), "got:\n{dis}");
        assert!(
            f.constants.contains(&Constant::Number(3.14)),
            "constants: {:?}",
            f.constants
        );
    }

    #[test]
    fn test_compile_string() {
        let f = compile_src("\"hello\";");
        let dis = f.disassemble();
        assert!(dis.contains("LoadConst r0, #0"));
        assert!(f.constants.contains(&Constant::String("hello".into())));
    }

    #[test]
    fn test_compile_bool_null() {
        let f = compile_src("true; false; null;");
        let dis = f.disassemble();
        assert!(dis.contains("LoadTrue r0"));
        assert!(dis.contains("LoadFalse r0"));
        assert!(dis.contains("LoadNull r0"));
    }

    #[test]
    fn test_compile_binary_arithmetic() {
        let f = compile_src("1 + 2;");
        let dis = f.disassemble();
        assert!(dis.contains("Add r0, r1, r2"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_nested_arithmetic() {
        let f = compile_src("(1 + 2) * 3;");
        let dis = f.disassemble();
        assert!(dis.contains("Add"), "got:\n{dis}");
        assert!(dis.contains("Mul"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_var_decl() {
        let f = compile_src("var x = 10; x;");
        let dis = f.disassemble();
        // x should get a register, then be loaded from that register.
        assert!(dis.contains("LoadInt8"), "got:\n{dis}");
        assert!(
            dis.contains("Move") || dis.contains("LoadInt8"),
            "got:\n{dis}"
        );
    }

    #[test]
    fn test_compile_let_const() {
        let f = compile_src("let a = 1; const b = 2; a + b;");
        let dis = f.disassemble();
        assert!(dis.contains("Add"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_if_else() {
        let f = compile_src("if (true) { 1; } else { 2; }");
        let dis = f.disassemble();
        assert!(dis.contains("JumpIfFalse"), "got:\n{dis}");
        assert!(dis.contains("Jump"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_while() {
        let f = compile_src("var i = 0; while (i < 10) { i = i + 1; }");
        let dis = f.disassemble();
        assert!(dis.contains("LessThan"), "got:\n{dis}");
        assert!(dis.contains("JumpIfFalse"), "got:\n{dis}");
        assert!(
            dis.contains("Jump"),
            "backward jump should be present: {dis}"
        );
    }

    #[test]
    fn test_compile_do_while() {
        let f = compile_src("var i = 0; do { i = i + 1; } while (i < 5);");
        let dis = f.disassemble();
        assert!(dis.contains("JumpIfTrue"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_for_loop() {
        let f = compile_src("for (var i = 0; i < 10; i = i + 1) { i; }");
        let dis = f.disassemble();
        assert!(dis.contains("LessThan"), "got:\n{dis}");
        assert!(dis.contains("JumpIfFalse"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_function_decl() {
        let f = compile_src("function add(a, b) { return a + b; }");
        let dis = f.disassemble();
        assert!(dis.contains("CreateClosure"), "got:\n{dis}");
        assert!(!f.functions.is_empty(), "should have nested function");
        let inner = &f.functions[0];
        assert_eq!(inner.name, "add");
        assert_eq!(inner.param_count, 2);
        let inner_dis = inner.disassemble();
        assert!(inner_dis.contains("Add"), "inner:\n{inner_dis}");
        assert!(inner_dis.contains("Return"), "inner:\n{inner_dis}");
    }

    #[test]
    fn test_compile_function_call() {
        let f = compile_src("function f() { return 42; } f();");
        let dis = f.disassemble();
        assert!(dis.contains("Call"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_arrow_function() {
        let f = compile_src("var add = (a, b) => a + b;");
        let dis = f.disassemble();
        assert!(dis.contains("CreateClosure"), "got:\n{dis}");
        let inner = &f.functions[0];
        assert_eq!(inner.param_count, 2);
    }

    #[test]
    fn test_compile_assignment() {
        let f = compile_src("var x = 1; x = x + 2;");
        let dis = f.disassemble();
        assert!(dis.contains("Add"), "got:\n{dis}");
        assert!(
            dis.contains("Move"),
            "assignment should produce Move:\n{dis}"
        );
    }

    #[test]
    fn test_compile_compound_assignment() {
        let f = compile_src("var x = 10; x += 5;");
        let dis = f.disassemble();
        assert!(dis.contains("Add"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_member_access() {
        let f = compile_src("var obj = {}; obj.x;");
        let dis = f.disassemble();
        assert!(dis.contains("CreateObject"), "got:\n{dis}");
        assert!(dis.contains("GetPropertyByName"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_computed_member() {
        let f = compile_src("var arr = []; arr[0];");
        let dis = f.disassemble();
        assert!(dis.contains("GetProperty"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_object_literal() {
        let f = compile_src("var obj = { a: 1, b: 2 };");
        let dis = f.disassemble();
        assert!(dis.contains("CreateObject"), "got:\n{dis}");
        assert!(dis.contains("SetPropertyByName"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_array_literal() {
        let f = compile_src("[1, 2, 3];");
        let dis = f.disassemble();
        assert!(dis.contains("CreateArray"), "got:\n{dis}");
        assert!(dis.contains("SetProperty"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_conditional() {
        let f = compile_src("true ? 1 : 2;");
        let dis = f.disassemble();
        assert!(dis.contains("JumpIfFalse"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_logical_and() {
        let f = compile_src("true && false;");
        let dis = f.disassemble();
        assert!(dis.contains("JumpIfFalse"), "short-circuit:\n{dis}");
    }

    #[test]
    fn test_compile_logical_or() {
        let f = compile_src("false || true;");
        let dis = f.disassemble();
        assert!(dis.contains("JumpIfTrue"), "short-circuit:\n{dis}");
    }

    #[test]
    fn test_compile_typeof() {
        let f = compile_src("typeof 42;");
        let dis = f.disassemble();
        assert!(dis.contains("TypeOf"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_unary_minus() {
        let f = compile_src("-42;");
        let dis = f.disassemble();
        assert!(dis.contains("Neg"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_not() {
        let f = compile_src("!true;");
        let dis = f.disassemble();
        assert!(dis.contains("LogicalNot"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_return() {
        let f = compile_src("function f() { return 42; }");
        let inner = &f.functions[0];
        let dis = inner.disassemble();
        assert!(dis.contains("Return"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_empty_return() {
        let f = compile_src("function f() { return; }");
        let inner = &f.functions[0];
        let dis = inner.disassemble();
        assert!(dis.contains("LoadUndefined"), "got:\n{dis}");
        assert!(dis.contains("Return"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_throw() {
        let f = compile_src("function f() { throw 42; }");
        let inner = &f.functions[0];
        let dis = inner.disassemble();
        assert!(dis.contains("Throw"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_this() {
        let f = compile_src("this;");
        let dis = f.disassemble();
        assert!(dis.contains("LoadGlobal"), "got:\n{dis}");
        assert!(f.names.contains(&"this".to_string()));
    }

    #[test]
    fn test_compile_global_var() {
        let f = compile_src("console;");
        let dis = f.disassemble();
        assert!(dis.contains("LoadGlobal"), "got:\n{dis}");
        assert!(f.names.contains(&"console".to_string()));
    }

    #[test]
    fn test_compile_template_literal() {
        let f = compile_src("`hello`;");
        assert!(
            f.constants.contains(&Constant::String("hello".into())),
            "constants: {:?}",
            f.constants
        );
    }

    #[test]
    fn test_compile_switch() {
        let f = compile_src("switch (1) { case 1: 42; break; case 2: 99; break; }");
        let dis = f.disassemble();
        assert!(dis.contains("StrictEq"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_class() {
        let f = compile_src("class Foo { constructor() {} greet() { return 1; } }");
        let dis = f.disassemble();
        assert!(dis.contains("CreateClosure"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_update_prefix() {
        let f = compile_src("var x = 0; ++x;");
        let dis = f.disassemble();
        assert!(dis.contains("Add"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_comparison() {
        let f = compile_src("1 === 2;");
        let dis = f.disassemble();
        assert!(dis.contains("StrictEq"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_bitwise() {
        let f = compile_src("1 & 2;");
        let dis = f.disassemble();
        assert!(dis.contains("BitAnd"), "got:\n{dis}");
    }

    #[test]
    fn test_compile_void() {
        let f = compile_src("void 0;");
        let dis = f.disassemble();
        assert!(dis.contains("Void"), "got:\n{dis}");
    }

    #[test]
    fn test_disassembler_output_format() {
        let f = compile_src("var x = 42; x + 1;");
        let dis = f.disassemble();
        // Should contain function header.
        assert!(dis.contains("function <main>"));
        // Should contain code section.
        assert!(dis.contains("code:"));
        // Should have hex offsets.
        assert!(dis.contains("0000"));
    }

    #[test]
    fn test_register_allocation_is_minimal() {
        // `var a = 1; var b = 2; a + b;` should use few registers.
        let f = compile_src("var a = 1; var b = 2; a + b;");
        // r0 = result, r1 = a, r2 = b, r3/r4 = temps for addition
        assert!(
            f.register_count <= 6,
            "too many registers: {}",
            f.register_count
        );
    }

    #[test]
    fn test_nested_function_closure() {
        let f = compile_src("function outer() { function inner() { return 1; } return inner; }");
        assert_eq!(f.functions.len(), 1);
        let outer = &f.functions[0];
        assert_eq!(outer.name, "outer");
        assert_eq!(outer.functions.len(), 1);
        let inner = &outer.functions[0];
        assert_eq!(inner.name, "inner");
    }

    #[test]
    fn test_for_with_no_parts() {
        // `for (;;) { break; }` — infinite loop with immediate break.
        let f = compile_src("for (;;) { break; }");
        let dis = f.disassemble();
        assert!(dis.contains("Jump"), "got:\n{dis}");
    }

    #[test]
    fn test_for_continue_targets_update() {
        // `continue` in a for-loop must jump to the update expression, not back
        // to the condition check. Verify the continue jump goes to the Add (i + 1)
        // rather than to the LessThan condition.
        let f = compile_src("for (var i = 0; i < 10; i = i + 1) { continue; }");
        let dis = f.disassemble();
        // The for-loop should contain: LessThan (test), JumpIfFalse (exit),
        // Jump (continue), Add (update), Jump (back to test).
        assert!(dis.contains("LessThan"), "missing test: {dis}");
        assert!(dis.contains("Add"), "missing update: {dis}");
        // There should be at least 2 Jump instructions (continue + back-edge).
        let jump_count = dis.matches("Jump ").count();
        assert!(
            jump_count >= 2,
            "expected >= 2 jumps for continue + back-edge, got {jump_count}: {dis}"
        );
    }

    #[test]
    fn test_do_while_continue_targets_condition() {
        // `continue` in do-while must jump to the condition, not the body start.
        let f = compile_src("var i = 0; do { i = i + 1; continue; } while (i < 5);");
        let dis = f.disassemble();
        assert!(dis.contains("LessThan"), "missing condition: {dis}");
        assert!(dis.contains("JumpIfTrue"), "missing back-edge: {dis}");
    }

    #[test]
    fn test_switch_default_case() {
        // Default case must not corrupt bytecode.
        let f = compile_src("switch (1) { case 1: 10; break; default: 20; break; }");
        let dis = f.disassemble();
        assert!(dis.contains("StrictEq"), "missing case test: {dis}");
        // The first instruction should NOT be corrupted.
        assert!(
            dis.contains("LoadUndefined r0"),
            "first instruction corrupted: {dis}"
        );
    }

    #[test]
    fn test_switch_only_default() {
        // Switch with only a default case.
        let f = compile_src("switch (42) { default: 99; }");
        let dis = f.disassemble();
        // Should compile without panicking and contain the default body.
        assert!(dis.contains("LoadInt8"), "got:\n{dis}");
    }

    #[test]
    fn test_class_empty_constructor_has_return() {
        // A class without an explicit constructor should produce a function with Return.
        let f = compile_src("class Foo {}");
        assert!(!f.functions.is_empty(), "should have constructor function");
        let ctor = &f.functions[0];
        let dis = ctor.disassemble();
        assert!(
            dis.contains("Return"),
            "empty constructor must have Return: {dis}"
        );
    }

    #[test]
    fn test_class_expression_compiles_methods() {
        // Class expression should compile methods, not just the constructor.
        let f = compile_src("var C = class { greet() { return 1; } };");
        let dis = f.disassemble();
        assert!(
            dis.contains("SetPropertyByName"),
            "method should be set as property: {dis}"
        );
    }
}