//! Pure-Rust regular expression engine for the JavaScript RegExp built-in.
//!
//! Implements a regex parser (pattern → IR) and a backtracking matcher.
//! Supports: character classes, quantifiers (greedy/lazy), anchors, groups
//! (capturing, non-capturing, named), alternation, backreferences, lookahead,
//! and standard escape sequences.

// ── Regex AST ───────────────────────────────────────────────

/// A parsed regex node.
#[derive(Debug, Clone)]
pub enum Node {
    /// Match a single literal character.
    Literal(char),
    /// `.` — match any character (respects dotAll flag).
    Dot,
    /// `^` — start anchor.
    Start,
    /// `$` — end anchor.
    End,
    /// `\b` — word boundary.
    WordBoundary,
    /// `\B` — non-word boundary.
    NonWordBoundary,
    /// Character class `[...]` or `[^...]`.
    CharClass {
        negated: bool,
        ranges: Vec<ClassRange>,
    },
    /// Alternation `a|b`.
    Alternation(Vec<Node>),
    /// Sequence of nodes (implicit concatenation).
    Sequence(Vec<Node>),
    /// Quantifier applied to a sub-node.
    Quantifier {
        node: Box<Node>,
        min: u32,
        max: Option<u32>, // None = unbounded
        greedy: bool,
    },
    /// Capturing group `(...)`.
    Group {
        index: u32,
        name: Option<String>,
        node: Box<Node>,
    },
    /// Non-capturing group `(?:...)`.
    NonCapturingGroup(Box<Node>),
    /// Backreference `\1` or `\k<name>`.
    Backref(u32),
    /// Positive lookahead `(?=...)`.
    Lookahead(Box<Node>),
    /// Negative lookahead `(?!...)`.
    NegativeLookahead(Box<Node>),
}

/// A range within a character class.
#[derive(Debug, Clone)]
pub enum ClassRange {
    /// Single character.
    Char(char),
    /// Character range `a-z`.
    Range(char, char),
    /// Predefined class like `\d`, `\w`, `\s`.
    Predefined(PredefinedClass),
}

/// Predefined character classes.
#[derive(Debug, Clone, Copy)]
pub enum PredefinedClass {
    Digit,    // \d
    NonDigit, // \D
    Word,     // \w
    NonWord,  // \W
    Space,    // \s
    NonSpace, // \S
}

// ── Regex flags ─────────────────────────────────────────────

/// Parsed regex flags.
#[derive(Debug, Clone, Copy, Default)]
pub struct RegexFlags {
    pub global: bool,
    pub ignore_case: bool,
    pub multiline: bool,
    pub dot_all: bool,
    pub unicode: bool,
    pub sticky: bool,
}

impl RegexFlags {
    pub fn parse(flags_str: &str) -> Result<Self, String> {
        let mut f = RegexFlags::default();
        for ch in flags_str.chars() {
            match ch {
                'g' => {
                    if f.global {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.global = true;
                }
                'i' => {
                    if f.ignore_case {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.ignore_case = true;
                }
                'm' => {
                    if f.multiline {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.multiline = true;
                }
                's' => {
                    if f.dot_all {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.dot_all = true;
                }
                'u' => {
                    if f.unicode {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.unicode = true;
                }
                'y' => {
                    if f.sticky {
                        return Err(format!("duplicate flag '{}'", ch));
                    }
                    f.sticky = true;
                }
                _ => return Err(format!("invalid flag '{}'", ch)),
            }
        }
        Ok(f)
    }

    pub fn as_flag_string(&self) -> String {
        let mut s = String::new();
        if self.global {
            s.push('g');
        }
        if self.ignore_case {
            s.push('i');
        }
        if self.multiline {
            s.push('m');
        }
        if self.dot_all {
            s.push('s');
        }
        if self.unicode {
            s.push('u');
        }
        if self.sticky {
            s.push('y');
        }
        s
    }
}

// ── Regex Parser ────────────────────────────────────────────

/// Parse a regex pattern string into a `Node` tree.
pub fn parse_pattern(pattern: &str) -> Result<(Node, u32), String> {
    let chars: Vec<char> = pattern.chars().collect();
    let mut parser = PatternParser {
        chars: &chars,
        pos: 0,
        group_count: 0,
        group_names: Vec::new(),
    };
    let node = parser.parse_alternation()?;
    if parser.pos < parser.chars.len() {
        return Err(format!(
            "unexpected '{}' at position {}",
            parser.chars[parser.pos], parser.pos
        ));
    }
    Ok((node, parser.group_count))
}

struct PatternParser<'a> {
    chars: &'a [char],
    pos: usize,
    group_count: u32,
    group_names: Vec<(String, u32)>,
}

impl<'a> PatternParser<'a> {
    fn peek(&self) -> Option<char> {
        self.chars.get(self.pos).copied()
    }

    fn advance(&mut self) -> Option<char> {
        let ch = self.chars.get(self.pos).copied();
        if ch.is_some() {
            self.pos += 1;
        }
        ch
    }

    fn expect(&mut self, expected: char) -> Result<(), String> {
        match self.advance() {
            Some(ch) if ch == expected => Ok(()),
            Some(ch) => Err(format!("expected '{}', got '{}'", expected, ch)),
            None => Err(format!("expected '{}', got end of pattern", expected)),
        }
    }

    /// Parse alternation: `a|b|c`
    fn parse_alternation(&mut self) -> Result<Node, String> {
        let mut branches = vec![self.parse_sequence()?];
        while self.peek() == Some('|') {
            self.advance(); // consume '|'
            branches.push(self.parse_sequence()?);
        }
        if branches.len() == 1 {
            Ok(branches.pop().unwrap())
        } else {
            Ok(Node::Alternation(branches))
        }
    }

    /// Parse a sequence of atoms (concatenation).
    fn parse_sequence(&mut self) -> Result<Node, String> {
        let mut nodes = Vec::new();
        while let Some(ch) = self.peek() {
            if ch == '|' || ch == ')' {
                break;
            }
            nodes.push(self.parse_quantified()?);
        }
        if nodes.len() == 1 {
            Ok(nodes.pop().unwrap())
        } else {
            Ok(Node::Sequence(nodes))
        }
    }

    /// Parse an atom with optional quantifier.
    fn parse_quantified(&mut self) -> Result<Node, String> {
        let node = self.parse_atom()?;
        if let Some(ch) = self.peek() {
            match ch {
                '*' | '+' | '?' => {
                    self.advance();
                    let (min, max) = match ch {
                        '*' => (0, None),
                        '+' => (1, None),
                        '?' => (0, Some(1)),
                        _ => unreachable!(),
                    };
                    let greedy = if self.peek() == Some('?') {
                        self.advance();
                        false
                    } else {
                        true
                    };
                    Ok(Node::Quantifier {
                        node: Box::new(node),
                        min,
                        max,
                        greedy,
                    })
                }
                '{' => self.parse_brace_quantifier(node),
                _ => Ok(node),
            }
        } else {
            Ok(node)
        }
    }

    /// Parse `{n}`, `{n,}`, `{n,m}` quantifier.
    fn parse_brace_quantifier(&mut self, node: Node) -> Result<Node, String> {
        let save = self.pos;
        self.advance(); // consume '{'
        let min = match self.parse_decimal() {
            Some(n) => n,
            None => {
                // Not a valid quantifier — treat '{' as literal.
                self.pos = save;
                return Ok(node);
            }
        };
        let max;
        match self.peek() {
            Some('}') => {
                self.advance();
                max = Some(min);
            }
            Some(',') => {
                self.advance();
                if self.peek() == Some('}') {
                    self.advance();
                    max = None;
                } else {
                    match self.parse_decimal() {
                        Some(n) => {
                            max = Some(n);
                            if self.peek() != Some('}') {
                                self.pos = save;
                                return Ok(node);
                            }
                            self.advance();
                        }
                        None => {
                            self.pos = save;
                            return Ok(node);
                        }
                    }
                }
            }
            _ => {
                self.pos = save;
                return Ok(node);
            }
        }
        let greedy = if self.peek() == Some('?') {
            self.advance();
            false
        } else {
            true
        };
        Ok(Node::Quantifier {
            node: Box::new(node),
            min,
            max,
            greedy,
        })
    }

    fn parse_decimal(&mut self) -> Option<u32> {
        let start = self.pos;
        while let Some(ch) = self.peek() {
            if ch.is_ascii_digit() {
                self.advance();
            } else {
                break;
            }
        }
        if self.pos == start {
            return None;
        }
        let s: String = self.chars[start..self.pos].iter().collect();
        s.parse().ok()
    }

    /// Parse a single atom.
    fn parse_atom(&mut self) -> Result<Node, String> {
        match self.peek() {
            None => Err("unexpected end of pattern".to_string()),
            Some('.') => {
                self.advance();
                Ok(Node::Dot)
            }
            Some('^') => {
                self.advance();
                Ok(Node::Start)
            }
            Some('$') => {
                self.advance();
                Ok(Node::End)
            }
            Some('\\') => self.parse_escape(),
            Some('[') => self.parse_char_class(),
            Some('(') => self.parse_group(),
            Some(ch) => {
                self.advance();
                Ok(Node::Literal(ch))
            }
        }
    }

    /// Parse an escape sequence.
    fn parse_escape(&mut self) -> Result<Node, String> {
        self.advance(); // consume '\'
        match self.advance() {
            None => Err("unexpected end of pattern after '\\'".to_string()),
            Some('d') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::Digit)],
            }),
            Some('D') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::NonDigit)],
            }),
            Some('w') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::Word)],
            }),
            Some('W') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::NonWord)],
            }),
            Some('s') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::Space)],
            }),
            Some('S') => Ok(Node::CharClass {
                negated: false,
                ranges: vec![ClassRange::Predefined(PredefinedClass::NonSpace)],
            }),
            Some('b') => Ok(Node::WordBoundary),
            Some('B') => Ok(Node::NonWordBoundary),
            Some('n') => Ok(Node::Literal('\n')),
            Some('r') => Ok(Node::Literal('\r')),
            Some('t') => Ok(Node::Literal('\t')),
            Some('f') => Ok(Node::Literal('\x0C')),
            Some('v') => Ok(Node::Literal('\x0B')),
            Some('0') => Ok(Node::Literal('\0')),
            Some('x') => {
                let hi = self.advance().ok_or("expected hex digit")?;
                let lo = self.advance().ok_or("expected hex digit")?;
                let code = hex2(hi, lo)?;
                Ok(Node::Literal(char::from(code)))
            }
            Some('u') => {
                if self.peek() == Some('{') {
                    self.advance();
                    let mut code_str = String::new();
                    while self.peek() != Some('}') {
                        code_str.push(self.advance().ok_or("expected '}'")?);
                    }
                    self.advance(); // consume '}'
                    let code =
                        u32::from_str_radix(&code_str, 16).map_err(|_| "invalid unicode escape")?;
                    let ch = char::from_u32(code).ok_or("invalid unicode code point")?;
                    Ok(Node::Literal(ch))
                } else {
                    let a = self.advance().ok_or("expected hex digit")?;
                    let b = self.advance().ok_or("expected hex digit")?;
                    let c = self.advance().ok_or("expected hex digit")?;
                    let d = self.advance().ok_or("expected hex digit")?;
                    let code = hex4(a, b, c, d)?;
                    let ch = char::from_u32(code).ok_or("invalid unicode escape")?;
                    Ok(Node::Literal(ch))
                }
            }
            Some('k') => {
                // Named backreference \k<name>
                self.expect('<')?;
                let mut name = String::new();
                while self.peek() != Some('>') {
                    name.push(self.advance().ok_or("expected '>'")?);
                }
                self.advance(); // consume '>'
                                // Resolve name to group index.
                for &(ref n, idx) in &self.group_names {
                    if *n == name {
                        return Ok(Node::Backref(idx));
                    }
                }
                Err(format!("unknown group name '{}'", name))
            }
            Some(ch) if ch.is_ascii_digit() && ch != '0' => {
                // Numeric backreference \1, \12, etc.
                let mut num_str = String::new();
                num_str.push(ch);
                while let Some(next) = self.peek() {
                    if next.is_ascii_digit() {
                        num_str.push(next);
                        self.advance();
                    } else {
                        break;
                    }
                }
                let idx: u32 = num_str.parse().map_err(|_| "invalid backreference")?;
                Ok(Node::Backref(idx))
            }
            // Escaped metacharacters — treat as literal.
            Some(ch) => Ok(Node::Literal(ch)),
        }
    }

    /// Parse a character class `[...]` or `[^...]`.
    fn parse_char_class(&mut self) -> Result<Node, String> {
        self.advance(); // consume '['
        let negated = if self.peek() == Some('^') {
            self.advance();
            true
        } else {
            false
        };
        let mut ranges = Vec::new();
        // Allow ']' as first character in class.
        if self.peek() == Some(']') {
            self.advance();
            ranges.push(ClassRange::Char(']'));
        }
        while self.peek() != Some(']') {
            if self.peek().is_none() {
                return Err("unterminated character class".to_string());
            }
            let item = self.parse_class_atom()?;
            // Check for range `a-b`.
            if self.peek() == Some('-')
                && self.chars.get(self.pos + 1) != Some(&']')
                && self.chars.get(self.pos + 1).is_some()
            {
                if let ClassRange::Char(start) = item {
                    self.advance(); // consume '-'
                    let end_item = self.parse_class_atom()?;
                    if let ClassRange::Char(end) = end_item {
                        if start > end {
                            return Err("character class range out of order".to_string());
                        }
                        ranges.push(ClassRange::Range(start, end));
                        continue;
                    } else {
                        // Not a valid range, push start, '-', and end_item.
                        ranges.push(ClassRange::Char(start));
                        ranges.push(ClassRange::Char('-'));
                        ranges.push(end_item);
                        continue;
                    }
                }
            }
            ranges.push(item);
        }
        self.advance(); // consume ']'
        Ok(Node::CharClass { negated, ranges })
    }

    fn parse_class_atom(&mut self) -> Result<ClassRange, String> {
        match self.peek() {
            Some('\\') => {
                self.advance(); // consume '\'
                match self.advance() {
                    None => Err("unexpected end of class".to_string()),
                    Some('d') => Ok(ClassRange::Predefined(PredefinedClass::Digit)),
                    Some('D') => Ok(ClassRange::Predefined(PredefinedClass::NonDigit)),
                    Some('w') => Ok(ClassRange::Predefined(PredefinedClass::Word)),
                    Some('W') => Ok(ClassRange::Predefined(PredefinedClass::NonWord)),
                    Some('s') => Ok(ClassRange::Predefined(PredefinedClass::Space)),
                    Some('S') => Ok(ClassRange::Predefined(PredefinedClass::NonSpace)),
                    Some('n') => Ok(ClassRange::Char('\n')),
                    Some('r') => Ok(ClassRange::Char('\r')),
                    Some('t') => Ok(ClassRange::Char('\t')),
                    Some('f') => Ok(ClassRange::Char('\x0C')),
                    Some('v') => Ok(ClassRange::Char('\x0B')),
                    Some('0') => Ok(ClassRange::Char('\0')),
                    Some('x') => {
                        let hi = self.advance().ok_or("expected hex digit")?;
                        let lo = self.advance().ok_or("expected hex digit")?;
                        let code = hex2(hi, lo)?;
                        Ok(ClassRange::Char(char::from(code)))
                    }
                    Some('u') => {
                        if self.peek() == Some('{') {
                            self.advance();
                            let mut code_str = String::new();
                            while self.peek() != Some('}') {
                                code_str.push(self.advance().ok_or("expected '}'")?);
                            }
                            self.advance();
                            let code = u32::from_str_radix(&code_str, 16)
                                .map_err(|_| "invalid unicode escape")?;
                            let ch = char::from_u32(code)
                                .ok_or_else(|| "invalid unicode code point".to_string())?;
                            Ok(ClassRange::Char(ch))
                        } else {
                            let a = self.advance().ok_or("expected hex digit")?;
                            let b = self.advance().ok_or("expected hex digit")?;
                            let c = self.advance().ok_or("expected hex digit")?;
                            let d = self.advance().ok_or("expected hex digit")?;
                            let code = hex4(a, b, c, d)?;
                            let ch = char::from_u32(code)
                                .ok_or_else(|| "invalid unicode escape".to_string())?;
                            Ok(ClassRange::Char(ch))
                        }
                    }
                    Some(ch) => Ok(ClassRange::Char(ch)),
                }
            }
            Some(ch) => {
                self.advance();
                Ok(ClassRange::Char(ch))
            }
            None => Err("unexpected end of character class".to_string()),
        }
    }

    /// Parse a group `(...)`.
    fn parse_group(&mut self) -> Result<Node, String> {
        self.advance(); // consume '('
        if self.peek() == Some('?') {
            self.advance(); // consume '?'
            match self.peek() {
                Some(':') => {
                    self.advance();
                    let inner = self.parse_alternation()?;
                    self.expect(')')?;
                    Ok(Node::NonCapturingGroup(Box::new(inner)))
                }
                Some('=') => {
                    self.advance();
                    let inner = self.parse_alternation()?;
                    self.expect(')')?;
                    Ok(Node::Lookahead(Box::new(inner)))
                }
                Some('!') => {
                    self.advance();
                    let inner = self.parse_alternation()?;
                    self.expect(')')?;
                    Ok(Node::NegativeLookahead(Box::new(inner)))
                }
                Some('<') => {
                    self.advance(); // consume '<'
                    let mut name = String::new();
                    while self.peek() != Some('>') {
                        name.push(self.advance().ok_or("expected '>'")?);
                    }
                    self.advance(); // consume '>'
                    self.group_count += 1;
                    let idx = self.group_count;
                    self.group_names.push((name.clone(), idx));
                    let inner = self.parse_alternation()?;
                    self.expect(')')?;
                    Ok(Node::Group {
                        index: idx,
                        name: Some(name),
                        node: Box::new(inner),
                    })
                }
                _ => Err("invalid group specifier".to_string()),
            }
        } else {
            self.group_count += 1;
            let idx = self.group_count;
            let inner = self.parse_alternation()?;
            self.expect(')')?;
            Ok(Node::Group {
                index: idx,
                name: None,
                node: Box::new(inner),
            })
        }
    }
}

// ── Hex helpers ─────────────────────────────────────────────

fn hex_digit(ch: char) -> Result<u32, String> {
    match ch {
        '0'..='9' => Ok(ch as u32 - '0' as u32),
        'a'..='f' => Ok(ch as u32 - 'a' as u32 + 10),
        'A'..='F' => Ok(ch as u32 - 'A' as u32 + 10),
        _ => Err(format!("invalid hex digit '{}'", ch)),
    }
}

fn hex2(hi: char, lo: char) -> Result<u8, String> {
    Ok((hex_digit(hi)? * 16 + hex_digit(lo)?) as u8)
}

fn hex4(a: char, b: char, c: char, d: char) -> Result<u32, String> {
    Ok(hex_digit(a)? * 4096 + hex_digit(b)? * 256 + hex_digit(c)? * 16 + hex_digit(d)?)
}

// ── Compiled regex ──────────────────────────────────────────

/// A compiled regular expression ready for matching.
#[derive(Debug, Clone)]
pub struct CompiledRegex {
    pub pattern: String,
    pub flags: RegexFlags,
    pub node: Node,
    pub group_count: u32,
}

impl CompiledRegex {
    pub fn new(pattern: &str, flags_str: &str) -> Result<Self, String> {
        let flags = RegexFlags::parse(flags_str)?;
        let (node, group_count) = parse_pattern(pattern)?;
        Ok(CompiledRegex {
            pattern: pattern.to_string(),
            flags,
            node,
            group_count,
        })
    }
}

// ── Backtracking matcher ────────────────────────────────────

/// Result of a successful regex match.
#[derive(Debug, Clone)]
pub struct MatchResult {
    /// Overall match start index (in chars).
    pub start: usize,
    /// Overall match end index (in chars).
    pub end: usize,
    /// Capture group contents: index 0 = full match, 1..n = groups.
    pub captures: Vec<Option<(usize, usize)>>,
}

/// Execute the regex on the given string, starting the search at `start_pos` (char index).
pub fn exec(regex: &CompiledRegex, input: &str, start_pos: usize) -> Option<MatchResult> {
    let chars: Vec<char> = input.chars().collect();
    let group_count = regex.group_count as usize;

    if regex.flags.sticky {
        // Sticky: only try at start_pos.
        let mut captures = vec![None; group_count + 1];
        let mut ctx = MatchContext {
            chars: &chars,
            flags: &regex.flags,
            captures: &mut captures,
            backtrack_limit: 1_000_000,
            backtrack_count: 0,
        };
        if let Some(end) = match_node(&mut ctx, &regex.node, start_pos) {
            captures = ctx.captures.to_vec();
            captures[0] = Some((start_pos, end));
            return Some(MatchResult {
                start: start_pos,
                end,
                captures,
            });
        }
        return None;
    }

    // Non-sticky: try each position starting from start_pos.
    for i in start_pos..=chars.len() {
        let mut captures = vec![None; group_count + 1];
        let mut ctx = MatchContext {
            chars: &chars,
            flags: &regex.flags,
            captures: &mut captures,
            backtrack_limit: 1_000_000,
            backtrack_count: 0,
        };
        if let Some(end) = match_node(&mut ctx, &regex.node, i) {
            captures = ctx.captures.to_vec();
            captures[0] = Some((i, end));
            return Some(MatchResult {
                start: i,
                end,
                captures,
            });
        }
    }
    None
}

struct MatchContext<'a> {
    chars: &'a [char],
    flags: &'a RegexFlags,
    captures: &'a mut Vec<Option<(usize, usize)>>,
    backtrack_limit: u32,
    backtrack_count: u32,
}

/// Try to match `node` at position `pos`, returning the end position on success.
fn match_node(ctx: &mut MatchContext, node: &Node, pos: usize) -> Option<usize> {
    ctx.backtrack_count += 1;
    if ctx.backtrack_count > ctx.backtrack_limit {
        return None;
    }

    match node {
        Node::Literal(ch) => {
            if pos < ctx.chars.len() {
                let input_ch = ctx.chars[pos];
                if ctx.flags.ignore_case {
                    if char_eq_ignore_case(input_ch, *ch) {
                        Some(pos + 1)
                    } else {
                        None
                    }
                } else if input_ch == *ch {
                    Some(pos + 1)
                } else {
                    None
                }
            } else {
                None
            }
        }

        Node::Dot => {
            if pos < ctx.chars.len() {
                let ch = ctx.chars[pos];
                if ctx.flags.dot_all || !is_line_terminator(ch) {
                    Some(pos + 1)
                } else {
                    None
                }
            } else {
                None
            }
        }

        Node::Start => {
            if pos == 0
                || (ctx.flags.multiline && pos > 0 && is_line_terminator(ctx.chars[pos - 1]))
            {
                Some(pos)
            } else {
                None
            }
        }

        Node::End => {
            if pos == ctx.chars.len()
                || (ctx.flags.multiline
                    && pos < ctx.chars.len()
                    && is_line_terminator(ctx.chars[pos]))
            {
                Some(pos)
            } else {
                None
            }
        }

        Node::WordBoundary => {
            let before = if pos > 0 {
                is_word_char(ctx.chars[pos - 1])
            } else {
                false
            };
            let after = if pos < ctx.chars.len() {
                is_word_char(ctx.chars[pos])
            } else {
                false
            };
            if before != after {
                Some(pos)
            } else {
                None
            }
        }

        Node::NonWordBoundary => {
            let before = if pos > 0 {
                is_word_char(ctx.chars[pos - 1])
            } else {
                false
            };
            let after = if pos < ctx.chars.len() {
                is_word_char(ctx.chars[pos])
            } else {
                false
            };
            if before == after {
                Some(pos)
            } else {
                None
            }
        }

        Node::CharClass { negated, ranges } => {
            if pos >= ctx.chars.len() {
                return None;
            }
            let ch = ctx.chars[pos];
            let matched = class_matches(ranges, ch, ctx.flags.ignore_case);
            if matched != *negated {
                Some(pos + 1)
            } else {
                None
            }
        }

        Node::Sequence(nodes) => match_sequence(ctx, nodes, 0, pos),

        Node::Alternation(branches) => {
            for branch in branches {
                let saved = ctx.captures.clone();
                if let Some(end) = match_node(ctx, branch, pos) {
                    return Some(end);
                }
                *ctx.captures = saved;
            }
            None
        }

        Node::Quantifier {
            node: inner,
            min,
            max,
            greedy,
        } => match_quantifier_standalone(ctx, inner, pos, *min, *max, *greedy),

        Node::Group {
            index, node: inner, ..
        } => {
            let idx = *index as usize;
            let saved = if idx < ctx.captures.len() {
                ctx.captures[idx]
            } else {
                None
            };
            let result = match_node(ctx, inner, pos);
            if let Some(end) = result {
                if idx < ctx.captures.len() {
                    ctx.captures[idx] = Some((pos, end));
                }
                Some(end)
            } else {
                if idx < ctx.captures.len() {
                    ctx.captures[idx] = saved;
                }
                None
            }
        }

        Node::NonCapturingGroup(inner) => match_node(ctx, inner, pos),

        Node::Backref(idx) => {
            let idx = *idx as usize;
            if idx >= ctx.captures.len() {
                return Some(pos); // Unmatched backref matches empty.
            }
            match ctx.captures[idx] {
                Some((start, end)) => {
                    let cap_len = end - start;
                    if pos + cap_len > ctx.chars.len() {
                        return None;
                    }
                    for i in 0..cap_len {
                        let a = ctx.chars[start + i];
                        let b = ctx.chars[pos + i];
                        if ctx.flags.ignore_case {
                            if !char_eq_ignore_case(a, b) {
                                return None;
                            }
                        } else if a != b {
                            return None;
                        }
                    }
                    Some(pos + cap_len)
                }
                None => Some(pos), // Unmatched group — backref matches empty.
            }
        }

        Node::Lookahead(inner) => {
            let saved = ctx.captures.clone();
            if match_node(ctx, inner, pos).is_some() {
                Some(pos) // Lookahead doesn't consume.
            } else {
                *ctx.captures = saved;
                None
            }
        }

        Node::NegativeLookahead(inner) => {
            let saved = ctx.captures.clone();
            if match_node(ctx, inner, pos).is_some() {
                *ctx.captures = saved;
                None
            } else {
                *ctx.captures = saved;
                Some(pos)
            }
        }
    }
}

/// Match a sequence of nodes with backtracking support for quantifiers.
fn match_sequence(ctx: &mut MatchContext, nodes: &[Node], idx: usize, pos: usize) -> Option<usize> {
    if idx >= nodes.len() {
        return Some(pos);
    }

    let node = &nodes[idx];

    // For quantifiers, try each count with the remaining sequence as continuation.
    if let Node::Quantifier {
        node: inner,
        min,
        max,
        greedy,
    } = node
    {
        return match_quantifier_in_seq(ctx, inner, pos, *min, *max, *greedy, &nodes[idx + 1..]);
    }

    // For non-quantifier nodes, match and continue.
    let saved = ctx.captures.clone();
    match match_node(ctx, node, pos) {
        Some(next) => match_sequence(ctx, nodes, idx + 1, next).or_else(|| {
            *ctx.captures = saved;
            None
        }),
        None => None,
    }
}

/// Match a quantifier within a sequence, trying each count with the continuation.
fn match_quantifier_in_seq(
    ctx: &mut MatchContext,
    inner: &Node,
    pos: usize,
    min: u32,
    max: Option<u32>,
    greedy: bool,
    continuation: &[Node],
) -> Option<usize> {
    // Collect all reachable positions (match inner 0 to max times).
    let mut positions = vec![pos]; // positions[0] = 0 matches
    let mut cur = pos;

    loop {
        let count = positions.len() - 1;
        if let Some(m) = max {
            if count as u32 >= m {
                break;
            }
        }
        let saved = ctx.captures.clone();
        match match_node(ctx, inner, cur) {
            Some(next) if next > cur => {
                cur = next;
                positions.push(cur);
            }
            Some(next) => {
                // Zero-width match — record once and stop to prevent infinite loop.
                *ctx.captures = saved;
                if next == cur && (count as u32) < min {
                    positions.push(cur);
                }
                break;
            }
            None => {
                *ctx.captures = saved;
                break;
            }
        }
    }

    let max_count = positions.len() - 1;
    let min_count = min as usize;
    if max_count < min_count {
        return None;
    }

    if greedy {
        // Try from most matches to fewest.
        for &p in positions[min_count..=max_count].iter().rev() {
            let saved = ctx.captures.clone();
            if let Some(end) = match_sequence(ctx, continuation, 0, p) {
                return Some(end);
            }
            *ctx.captures = saved;
        }
        None
    } else {
        // Lazy: try from fewest to most.
        for &p in &positions[min_count..=max_count] {
            let saved = ctx.captures.clone();
            if let Some(end) = match_sequence(ctx, continuation, 0, p) {
                return Some(end);
            }
            *ctx.captures = saved;
        }
        None
    }
}

/// Match a standalone quantifier (not in a sequence context).
fn match_quantifier_standalone(
    ctx: &mut MatchContext,
    inner: &Node,
    pos: usize,
    min: u32,
    max: Option<u32>,
    greedy: bool,
) -> Option<usize> {
    let mut positions = vec![pos];
    let mut cur = pos;

    loop {
        let count = positions.len() - 1;
        if let Some(m) = max {
            if count as u32 >= m {
                break;
            }
        }
        let saved = ctx.captures.clone();
        match match_node(ctx, inner, cur) {
            Some(next) if next > cur => {
                cur = next;
                positions.push(cur);
            }
            Some(next) => {
                *ctx.captures = saved;
                if next == cur && (count as u32) < min {
                    positions.push(cur);
                }
                break;
            }
            None => {
                *ctx.captures = saved;
                break;
            }
        }
    }

    let max_count = positions.len() - 1;
    let min_count = min as usize;
    if max_count < min_count {
        return None;
    }

    if greedy {
        Some(positions[max_count])
    } else {
        Some(positions[min_count])
    }
}

// ── Character helpers ───────────────────────────────────────

fn is_line_terminator(ch: char) -> bool {
    matches!(ch, '\n' | '\r' | '\u{2028}' | '\u{2029}')
}

fn is_word_char(ch: char) -> bool {
    ch.is_ascii_alphanumeric() || ch == '_'
}

fn char_eq_ignore_case(a: char, b: char) -> bool {
    a.eq_ignore_ascii_case(&b)
}

fn class_matches(ranges: &[ClassRange], ch: char, ignore_case: bool) -> bool {
    for range in ranges {
        match range {
            ClassRange::Char(c) => {
                if ignore_case {
                    if char_eq_ignore_case(ch, *c) {
                        return true;
                    }
                } else if ch == *c {
                    return true;
                }
            }
            ClassRange::Range(start, end) => {
                if ignore_case {
                    let ch_lower = ch.to_ascii_lowercase();
                    let start_lower = start.to_ascii_lowercase();
                    let end_lower = end.to_ascii_lowercase();
                    if ch_lower >= start_lower && ch_lower <= end_lower {
                        return true;
                    }
                } else if ch >= *start && ch <= *end {
                    return true;
                }
            }
            ClassRange::Predefined(class) => {
                if predefined_matches(*class, ch) {
                    return true;
                }
            }
        }
    }
    false
}

fn predefined_matches(class: PredefinedClass, ch: char) -> bool {
    match class {
        PredefinedClass::Digit => ch.is_ascii_digit(),
        PredefinedClass::NonDigit => !ch.is_ascii_digit(),
        PredefinedClass::Word => is_word_char(ch),
        PredefinedClass::NonWord => !is_word_char(ch),
        PredefinedClass::Space => matches!(
            ch,
            ' ' | '\t' | '\n' | '\r' | '\x0B' | '\x0C' | '\u{00A0}' | '\u{FEFF}'
        ),
        PredefinedClass::NonSpace => !matches!(
            ch,
            ' ' | '\t' | '\n' | '\r' | '\x0B' | '\x0C' | '\u{00A0}' | '\u{FEFF}'
        ),
    }
}

// ── Tests ───────────────────────────────────────────────────

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

    fn match_at(pattern: &str, flags: &str, input: &str, start: usize) -> Option<MatchResult> {
        let regex = CompiledRegex::new(pattern, flags).unwrap();
        exec(&regex, input, start)
    }

    fn assert_match(pattern: &str, input: &str, expected_start: usize, expected_end: usize) {
        let m = match_at(pattern, "", input, 0).expect("expected match");
        assert_eq!(
            m.start, expected_start,
            "pattern={} input={}",
            pattern, input
        );
        assert_eq!(m.end, expected_end, "pattern={} input={}", pattern, input);
    }

    fn assert_no_match(pattern: &str, input: &str) {
        assert!(
            match_at(pattern, "", input, 0).is_none(),
            "expected no match: pattern={} input={}",
            pattern,
            input
        );
    }

    #[test]
    fn test_literal() {
        assert_match("abc", "xabcx", 1, 4);
        assert_no_match("abc", "xyz");
    }

    #[test]
    fn test_dot() {
        assert_match("a.c", "abc", 0, 3);
        assert_match("a.c", "axc", 0, 3);
        assert_no_match("a.c", "a\nc");
        // dotAll flag
        let m = match_at("a.c", "s", "a\nc", 0).unwrap();
        assert_eq!(m.start, 0);
        assert_eq!(m.end, 3);
    }

    #[test]
    fn test_anchors() {
        assert_match("^abc", "abc", 0, 3);
        assert_no_match("^abc", "xabc");
        assert_match("abc$", "abc", 0, 3);
        assert_no_match("abc$", "abcx");
    }

    #[test]
    fn test_quantifiers() {
        assert_match("a*", "aaa", 0, 3);
        assert_match("a+", "aaa", 0, 3);
        assert_no_match("a+", "bbb");
        assert_match("a?b", "ab", 0, 2);
        assert_match("a?b", "b", 0, 1);
        assert_match("a{2}", "aaa", 0, 2);
        assert_match("a{2,3}", "aaaa", 0, 3);
        assert_match("a{2,}", "aaaa", 0, 4);
    }

    #[test]
    fn test_char_class() {
        assert_match("[abc]", "b", 0, 1);
        assert_no_match("[abc]", "d");
        assert_match("[a-z]", "m", 0, 1);
        assert_no_match("[a-z]", "M");
        assert_match("[^abc]", "d", 0, 1);
        assert_no_match("[^abc]", "a");
    }

    #[test]
    fn test_predefined_classes() {
        assert_match("\\d+", "abc123", 3, 6);
        assert_match("\\w+", "hello world", 0, 5);
        assert_match("\\s+", "a b", 1, 2);
    }

    #[test]
    fn test_alternation() {
        assert_match("cat|dog", "dog", 0, 3);
        assert_match("cat|dog", "catdog", 0, 3);
    }

    #[test]
    fn test_groups() {
        let m = match_at("(a)(b)(c)", "", "abc", 0).unwrap();
        assert_eq!(m.captures[1], Some((0, 1)));
        assert_eq!(m.captures[2], Some((1, 2)));
        assert_eq!(m.captures[3], Some((2, 3)));
    }

    #[test]
    fn test_non_capturing_group() {
        let m = match_at("(?:ab)(c)", "", "abc", 0).unwrap();
        assert_eq!(m.captures[1], Some((2, 3)));
    }

    #[test]
    fn test_named_group() {
        let m = match_at("(?<word>\\w+)", "", "hello", 0).unwrap();
        assert_eq!(m.captures[1], Some((0, 5)));
    }

    #[test]
    fn test_backreference() {
        assert_match("(a)\\1", "aa", 0, 2);
        assert_no_match("(a)\\1", "ab");
    }

    #[test]
    fn test_word_boundary() {
        assert_match("\\bfoo\\b", "a foo b", 2, 5);
        assert_no_match("\\bfoo\\b", "afoo");
    }

    #[test]
    fn test_lookahead() {
        assert_match("a(?=b)", "ab", 0, 1);
        assert_no_match("a(?=b)", "ac");
        assert_match("a(?!b)", "ac", 0, 1);
        assert_no_match("a(?!b)", "ab");
    }

    #[test]
    fn test_ignore_case() {
        let m = match_at("abc", "i", "ABC", 0).unwrap();
        assert_eq!(m.start, 0);
        assert_eq!(m.end, 3);
    }

    #[test]
    fn test_multiline() {
        let m = match_at("^b", "m", "a\nb", 0).unwrap();
        assert_eq!(m.start, 2);
    }

    #[test]
    fn test_global_multiple() {
        let regex = CompiledRegex::new("a", "g").unwrap();
        let m1 = exec(&regex, "aba", 0).unwrap();
        assert_eq!(m1.start, 0);
        let m2 = exec(&regex, "aba", m1.end).unwrap();
        assert_eq!(m2.start, 2);
        assert!(exec(&regex, "aba", m2.end).is_none());
    }

    #[test]
    fn test_escape_sequences() {
        assert_match("\\n", "\n", 0, 1);
        assert_match("\\t", "\t", 0, 1);
        assert_match("\\x41", "A", 0, 1);
        assert_match("\\u0041", "A", 0, 1);
    }

    #[test]
    fn test_lazy_quantifiers() {
        let m = match_at("a+?", "", "aaa", 0).unwrap();
        assert_eq!(m.end, 1); // Lazy: match as few as possible.
    }

    #[test]
    fn test_empty_pattern() {
        let m = match_at("", "", "abc", 0).unwrap();
        assert_eq!(m.start, 0);
        assert_eq!(m.end, 0);
    }

    #[test]
    fn test_flags_parse() {
        let f = RegexFlags::parse("gims").unwrap();
        assert!(f.global);
        assert!(f.ignore_case);
        assert!(f.multiline);
        assert!(f.dot_all);
        assert!(!f.unicode);
        assert!(!f.sticky);

        assert!(RegexFlags::parse("gg").is_err());
        assert!(RegexFlags::parse("x").is_err());
    }
}