const std = @import("std");

/// An object, which is either a pair or and atom. Objects are stored in a big
/// contiguous arraylist.
const Object = union(enum) {
    /// A pair or list. A list `(1 2 3)` is equivalent to `(1 . (2 . (3 . ())))`
    pair: Pair,
    /// An integer number. TODO: Other kinds of numbers.
    number: i64,
    /// A symbol, such as `+`, `foo`, or `the-word-recursion-has-many-meanings`.
    /// TODO: Interning.
    symbol: []const u8,
    /// Represented as `()`.
    nil,

    /// An index with which to refer to an Object in the storage array.
    const Handle = enum(u32) { _ };
};

/// Represented as `(car . cdr)`.
const Pair = struct {
    car: Object.Handle,
    cdr: Object.Handle,
};

/// A scope or environment in which bindings may be looked up.
const Environment = struct {
    bindings: std.StringHashMap(Object.Handle),
    parent: ?*Environment = null,
};

/// The overall interpreter that holds all objects.
const Machine = struct {
    objects: std.ArrayList(Object),
    root_env: Environment,

    fn init(allocator: std.mem.Allocator) Machine {
        const result: Machine = .{
            .objects = .{},
            .root_env = .{ .bindings = .init(allocator) },
        };

        // TODO: add basic math fns to root environment.

        return result;
    }
};