proof-engine 0.1.1

//! Stack-based bytecode virtual machine.
//!
//! Executes `Chunk` bytecode produced by `Compiler::compile_script`.

use std::collections::HashMap;
use std::fmt;
use std::sync::Arc;

use super::compiler::{Chunk, Constant, Instruction};

// ── Value ────────────────────────────────────────────────────────────────────

/// A runtime scripting value.
#[derive(Clone, Debug)]
pub enum Value {
    Nil,
    Bool(bool),
    Int(i64),
    Float(f64),
    Str(Arc<String>),
    Table(Table),
    Function(Arc<Closure>),
    NativeFunction(Arc<NativeFunc>),
}

impl PartialEq for Value {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (Value::Nil, Value::Nil)                 => true,
            (Value::Bool(a), Value::Bool(b))         => a == b,
            (Value::Int(a), Value::Int(b))           => a == b,
            (Value::Float(a), Value::Float(b))       => a == b,
            (Value::Str(a), Value::Str(b))           => a == b,
            (Value::Int(a), Value::Float(b))         => (*a as f64) == *b,
            (Value::Float(a), Value::Int(b))         => *a == (*b as f64),
            (Value::Table(a), Value::Table(b))       => Arc::ptr_eq(&a.inner, &b.inner),
            (Value::Function(a), Value::Function(b)) => Arc::ptr_eq(a, b),
            _ => false,
        }
    }
}

impl fmt::Display for Value {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Value::Nil              => write!(f, "nil"),
            Value::Bool(b)         => write!(f, "{}", b),
            Value::Int(n)          => write!(f, "{}", n),
            Value::Float(n)        => write!(f, "{}", n),
            Value::Str(s)          => write!(f, "{}", s),
            Value::Table(_)        => write!(f, "table"),
            Value::Function(_)     => write!(f, "function"),
            Value::NativeFunction(n) => write!(f, "function: {}", n.name),
        }
    }
}

impl Value {
    pub fn is_truthy(&self) -> bool {
        !matches!(self, Value::Nil | Value::Bool(false))
    }

    pub fn type_name(&self) -> &'static str {
        match self {
            Value::Nil              => "nil",
            Value::Bool(_)         => "boolean",
            Value::Int(_)          => "integer",
            Value::Float(_)        => "float",
            Value::Str(_)          => "string",
            Value::Table(_)        => "table",
            Value::Function(_)     => "function",
            Value::NativeFunction(_) => "function",
        }
    }

    pub fn to_float(&self) -> Option<f64> {
        match self {
            Value::Float(f) => Some(*f),
            Value::Int(i)   => Some(*i as f64),
            Value::Str(s)   => s.parse::<f64>().ok(),
            _ => None,
        }
    }

    pub fn to_int(&self) -> Option<i64> {
        match self {
            Value::Int(i)   => Some(*i),
            Value::Float(f) => if f.fract() == 0.0 { Some(*f as i64) } else { None },
            Value::Str(s)   => s.parse::<i64>().ok(),
            _ => None,
        }
    }

    pub fn to_str_repr(&self) -> Option<String> {
        match self {
            Value::Str(s)   => Some(s.as_ref().clone()),
            Value::Int(i)   => Some(i.to_string()),
            Value::Float(f) => Some(f.to_string()),
            _ => None,
        }
    }
}

impl From<&Constant> for Value {
    fn from(c: &Constant) -> Self {
        match c {
            Constant::Nil      => Value::Nil,
            Constant::Bool(b)  => Value::Bool(*b),
            Constant::Int(i)   => Value::Int(*i),
            Constant::Float(f) => Value::Float(*f),
            Constant::Str(s)   => Value::Str(Arc::new(s.clone())),
        }
    }
}

// ── Table ─────────────────────────────────────────────────────────────────────

#[derive(Clone, Debug)]
pub struct Table {
    pub(crate) inner: Arc<std::cell::RefCell<TableData>>,
}

#[derive(Debug)]
struct TableData {
    hash:      HashMap<TableKey, Value>,
    array:     Vec<Value>,
    metatable: Option<Table>,
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
enum TableKey {
    Str(String),
    Int(i64),
    Bool(bool),
}

impl TableKey {
    fn from_value(v: &Value) -> Option<Self> {
        match v {
            Value::Str(s)  => Some(TableKey::Str(s.as_ref().clone())),
            Value::Int(i)  => Some(TableKey::Int(*i)),
            Value::Bool(b) => Some(TableKey::Bool(*b)),
            Value::Float(f) => {
                let i = *f as i64;
                if i as f64 == *f { Some(TableKey::Int(i)) } else { None }
            }
            _ => None,
        }
    }
}

impl Table {
    pub fn new() -> Self {
        Table { inner: Arc::new(std::cell::RefCell::new(TableData {
            hash: HashMap::new(), array: Vec::new(), metatable: None,
        }))}
    }

    pub fn get(&self, key: &Value) -> Value {
        let d = self.inner.borrow();
        if let Some(i) = int_key(key) {
            if i >= 1 {
                return d.array.get((i - 1) as usize).cloned().unwrap_or(Value::Nil);
            }
        }
        TableKey::from_value(key)
            .and_then(|k| d.hash.get(&k).cloned())
            .unwrap_or(Value::Nil)
    }

    pub fn rawget_str(&self, key: &str) -> Value {
        self.inner.borrow().hash
            .get(&TableKey::Str(key.to_string()))
            .cloned()
            .unwrap_or(Value::Nil)
    }

    pub fn set(&self, key: Value, val: Value) {
        let mut d = self.inner.borrow_mut();
        if let Some(i) = int_key(&key) {
            if i >= 1 {
                let idx = (i - 1) as usize;
                if idx <= d.array.len() {
                    if matches!(val, Value::Nil) {
                        if idx < d.array.len() { d.array[idx] = Value::Nil; }
                    } else if idx == d.array.len() {
                        d.array.push(val);
                    } else {
                        d.array[idx] = val;
                    }
                    return;
                }
            }
        }
        if let Some(k) = TableKey::from_value(&key) {
            match val {
                Value::Nil => { d.hash.remove(&k); }
                v          => { d.hash.insert(k, v); }
            }
        }
    }

    pub fn rawset_str(&self, key: &str, val: Value) {
        let mut d = self.inner.borrow_mut();
        let k = TableKey::Str(key.to_string());
        match val {
            Value::Nil => { d.hash.remove(&k); }
            v          => { d.hash.insert(k, v); }
        }
    }

    pub fn length(&self) -> i64 {
        self.inner.borrow().array.len() as i64
    }

    pub fn push(&self, v: Value) {
        self.inner.borrow_mut().array.push(v);
    }

    pub fn array_values(&self) -> Vec<Value> {
        self.inner.borrow().array.clone()
    }

    pub fn set_metatable(&self, mt: Option<Table>) {
        self.inner.borrow_mut().metatable = mt;
    }

    pub fn get_metatable(&self) -> Option<Table> {
        self.inner.borrow().metatable.clone()
    }

    pub fn next(&self, after: &Value) -> Option<(Value, Value)> {
        let d = self.inner.borrow();
        match after {
            Value::Nil => {
                if let Some(v) = d.array.first() {
                    return Some((Value::Int(1), v.clone()));
                }
                return d.hash.iter().next().map(|(k, v)| (tk_to_val(k), v.clone()));
            }
            _ => {
                if let Some(i) = int_key(after) {
                    if i >= 1 {
                        let next_i = i as usize;
                        if next_i < d.array.len() {
                            return Some((Value::Int(i + 1), d.array[next_i].clone()));
                        }
                        return d.hash.iter().next().map(|(k, v)| (tk_to_val(k), v.clone()));
                    }
                }
                if let Some(k) = TableKey::from_value(after) {
                    let mut found = false;
                    for (hk, hv) in &d.hash {
                        if found { return Some((tk_to_val(hk), hv.clone())); }
                        if *hk == k { found = true; }
                    }
                }
                None
            }
        }
    }
}

fn int_key(v: &Value) -> Option<i64> {
    match v {
        Value::Int(i) => Some(*i),
        Value::Float(f) if f.fract() == 0.0 => Some(*f as i64),
        _ => None,
    }
}

fn tk_to_val(k: &TableKey) -> Value {
    match k {
        TableKey::Str(s)  => Value::Str(Arc::new(s.clone())),
        TableKey::Int(i)  => Value::Int(*i),
        TableKey::Bool(b) => Value::Bool(*b),
    }
}

// ── Closure / NativeFunc ──────────────────────────────────────────────────────

#[derive(Debug)]
pub struct Closure {
    pub chunk:    Arc<Chunk>,
    pub upvalues: Vec<UpvalueCell>,
}

#[derive(Debug, Clone)]
pub struct UpvalueCell(Arc<std::cell::RefCell<Value>>);

impl UpvalueCell {
    pub fn new(v: Value) -> Self { UpvalueCell(Arc::new(std::cell::RefCell::new(v))) }
    pub fn get(&self) -> Value { self.0.borrow().clone() }
    pub fn set(&self, v: Value) { *self.0.borrow_mut() = v; }
}

pub struct NativeFunc {
    pub name: String,
    pub func: Box<dyn Fn(&mut Vm, Vec<Value>) -> Result<Vec<Value>, ScriptError> + Send + Sync>,
}

impl fmt::Debug for NativeFunc {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "NativeFunc({})", self.name)
    }
}

// ── ScriptError ───────────────────────────────────────────────────────────────

#[derive(Debug, Clone)]
pub struct ScriptError {
    pub message: String,
    pub line:    Option<u32>,
}

impl ScriptError {
    pub fn new(msg: impl Into<String>) -> Self {
        ScriptError { message: msg.into(), line: None }
    }

    pub fn at(msg: impl Into<String>, line: u32) -> Self {
        ScriptError { message: msg.into(), line: Some(line) }
    }

    fn runtime(msg: impl Into<String>) -> Self { ScriptError::new(msg) }
}

impl fmt::Display for ScriptError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(l) = self.line { write!(f, "[line {}] {}", l, self.message) }
        else { write!(f, "{}", self.message) }
    }
}

impl std::error::Error for ScriptError {}

// ── CallFrame ─────────────────────────────────────────────────────────────────

struct CallFrame {
    chunk:    Arc<Chunk>,
    upvalues: Vec<UpvalueCell>,
    ip:       usize,
    base:     usize,
}

// ── Vm ────────────────────────────────────────────────────────────────────────

const MAX_DEPTH: usize = 200;

pub struct Vm {
    stack:   Vec<Value>,
    frames:  Vec<CallFrame>,
    globals: HashMap<String, Value>,
    depth:   usize,
    pub output: Vec<String>,
}

impl Vm {
    pub fn new() -> Self {
        Vm {
            stack:   Vec::with_capacity(64),
            frames:  Vec::with_capacity(32),
            globals: HashMap::new(),
            depth:   0,
            output:  Vec::new(),
        }
    }

    pub fn set_global(&mut self, name: impl Into<String>, v: Value) {
        self.globals.insert(name.into(), v);
    }

    pub fn get_global(&self, name: &str) -> Value {
        self.globals.get(name).cloned().unwrap_or(Value::Nil)
    }

    pub fn register_native(
        &mut self,
        name: impl Into<String>,
        f: impl Fn(&mut Vm, Vec<Value>) -> Result<Vec<Value>, ScriptError> + Send + Sync + 'static,
    ) {
        let name = name.into();
        let nf = Arc::new(NativeFunc { name: name.clone(), func: Box::new(f) });
        self.globals.insert(name, Value::NativeFunction(nf));
    }

    /// Execute a compiled `Chunk` as the top-level script.
    pub fn execute(&mut self, chunk: Arc<Chunk>) -> Result<Vec<Value>, ScriptError> {
        self.run_chunk(chunk, vec![], vec![])
    }

    /// Call any callable Value.
    pub fn call(&mut self, callee: Value, args: Vec<Value>) -> Result<Vec<Value>, ScriptError> {
        match callee {
            Value::Function(c) => {
                let chunk    = Arc::clone(&c.chunk);
                let upvalues = c.upvalues.clone();
                self.run_chunk(chunk, upvalues, args)
            }
            Value::NativeFunction(nf) => {
                let func = Arc::clone(&nf);
                (func.func)(self, args)
            }
            other => Err(ScriptError::runtime(format!(
                "attempt to call a {} value", other.type_name()
            ))),
        }
    }

    fn run_chunk(
        &mut self,
        chunk:    Arc<Chunk>,
        upvalues: Vec<UpvalueCell>,
        args:     Vec<Value>,
    ) -> Result<Vec<Value>, ScriptError> {
        self.depth += 1;
        if self.depth > MAX_DEPTH {
            self.depth -= 1;
            return Err(ScriptError::runtime("stack overflow"));
        }
        let base = self.stack.len();
        for a in args { self.stack.push(a); }
        let param_count = chunk.param_count as usize;
        while self.stack.len() < base + param_count {
            self.stack.push(Value::Nil);
        }
        self.frames.push(CallFrame { chunk, upvalues, ip: 0, base });
        let result = self.run();
        self.depth -= 1;
        result
    }

    fn run(&mut self) -> Result<Vec<Value>, ScriptError> {
        loop {
            let fi = self.frames.len() - 1;
            let ip = self.frames[fi].ip;
            let code = self.frames[fi].chunk.instructions.clone();

            if ip >= code.len() {
                let base = self.frames[fi].base;
                self.stack.truncate(base);
                self.frames.pop();
                return Ok(vec![]);
            }

            let instr = code[ip].clone();
            self.frames[fi].ip += 1;

            match instr {
                Instruction::LoadNil => self.push(Value::Nil),

                Instruction::LoadBool(b) => self.push(Value::Bool(b)),

                Instruction::LoadInt(n) => self.push(Value::Int(n)),

                Instruction::LoadFloat(n) => self.push(Value::Float(n)),

                Instruction::LoadStr(s) => self.push(Value::Str(Arc::new(s))),

                Instruction::LoadConst(idx) => {
                    let fi = self.frames.len() - 1;
                    let v = self.frames[fi].chunk.constants[idx].clone();
                    self.push(v);
                }

                Instruction::Pop  => { self.stack.pop(); }
                Instruction::Dup  => {
                    let v = self.stack.last().cloned().unwrap_or(Value::Nil);
                    self.push(v);
                }
                Instruction::Swap => {
                    let len = self.stack.len();
                    if len >= 2 { self.stack.swap(len - 1, len - 2); }
                }

                Instruction::GetLocal(slot) => {
                    let fi   = self.frames.len() - 1;
                    let base = self.frames[fi].base;
                    let v = self.stack.get(base + slot).cloned().unwrap_or(Value::Nil);
                    self.push(v);
                }
                Instruction::SetLocal(slot) => {
                    let fi   = self.frames.len() - 1;
                    let base = self.frames[fi].base;
                    let v = self.pop();
                    let idx = base + slot;
                    while self.stack.len() <= idx { self.stack.push(Value::Nil); }
                    self.stack[idx] = v;
                }

                Instruction::GetUpvalue(idx) => {
                    let fi = self.frames.len() - 1;
                    let v = self.frames[fi].upvalues.get(idx)
                        .map(|uv| uv.get())
                        .unwrap_or(Value::Nil);
                    self.push(v);
                }
                Instruction::SetUpvalue(idx) => {
                    let v = self.pop();
                    let fi = self.frames.len() - 1;
                    if let Some(uv) = self.frames[fi].upvalues.get(idx) {
                        uv.set(v);
                    }
                }

                Instruction::GetGlobal(name) => {
                    let v = self.globals.get(&name).cloned().unwrap_or(Value::Nil);
                    self.push(v);
                }
                Instruction::SetGlobal(name) => {
                    let v = self.pop();
                    self.globals.insert(name, v);
                }

                Instruction::NewTable => self.push(Value::Table(Table::new())),

                Instruction::SetField(name) => {
                    let val   = self.pop();
                    let table = self.peek();
                    match table {
                        Value::Table(t) => t.rawset_str(&name, val),
                        _ => return Err(ScriptError::runtime("SetField on non-table")),
                    }
                }
                Instruction::GetField(name) => {
                    let table = self.pop();
                    let v = match &table {
                        Value::Table(t) => t.rawget_str(&name),
                        Value::Str(_) => {
                            self.globals.get("string")
                                .and_then(|s| if let Value::Table(t) = s {
                                    Some(t.rawget_str(&name))
                                } else { None })
                                .unwrap_or(Value::Nil)
                        }
                        other => return Err(ScriptError::runtime(format!(
                            "attempt to index a {} value", other.type_name()
                        ))),
                    };
                    self.push(v);
                }
                Instruction::SetIndex => {
                    let val   = self.pop();
                    let key   = self.pop();
                    let table = self.pop();
                    match table {
                        Value::Table(t) => t.set(key, val),
                        other => return Err(ScriptError::runtime(format!(
                            "attempt to index a {} value", other.type_name()
                        ))),
                    }
                }
                Instruction::GetIndex => {
                    let key   = self.pop();
                    let table = self.pop();
                    let v = match &table {
                        Value::Table(t) => t.get(&key),
                        other => return Err(ScriptError::runtime(format!(
                            "attempt to index a {} value", other.type_name()
                        ))),
                    };
                    self.push(v);
                }
                Instruction::TableAppend => {
                    let val   = self.pop();
                    let table = self.peek();
                    if let Value::Table(t) = table { t.push(val); }
                }

                Instruction::Len => {
                    let a = self.pop();
                    let n = match a {
                        Value::Table(t) => t.length(),
                        Value::Str(s)   => s.len() as i64,
                        other => return Err(ScriptError::runtime(format!(
                            "attempt to get length of {} value", other.type_name()
                        ))),
                    };
                    self.push(Value::Int(n));
                }
                Instruction::Neg => {
                    let a = self.pop();
                    let r = match a {
                        Value::Int(i)   => Value::Int(-i),
                        Value::Float(f) => Value::Float(-f),
                        other => return Err(ScriptError::runtime(format!(
                            "unary - on {}", other.type_name()
                        ))),
                    };
                    self.push(r);
                }
                Instruction::Not    => { let a = self.pop(); self.push(Value::Bool(!a.is_truthy())); }
                Instruction::BitNot => {
                    let a = self.pop();
                    let i = a.to_int().ok_or_else(|| ScriptError::runtime(
                        format!("bitwise not on {}", a.type_name())))?;
                    self.push(Value::Int(!i));
                }

                Instruction::Add => self.arith2(|a, b| num_arith(a, b, i64::wrapping_add, |x, y| x + y))?,
                Instruction::Sub => self.arith2(|a, b| num_arith(a, b, i64::wrapping_sub, |x, y| x - y))?,
                Instruction::Mul => self.arith2(|a, b| num_arith(a, b, i64::wrapping_mul, |x, y| x * y))?,
                Instruction::Div => {
                    let b = self.pop(); let a = self.pop();
                    let af = a.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", a.type_name())))?;
                    let bf = b.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", b.type_name())))?;
                    self.push(Value::Float(af / bf));
                }
                Instruction::IDiv => {
                    let b = self.pop(); let a = self.pop();
                    let ai = a.to_int().ok_or_else(|| ScriptError::runtime("floor div requires integers"))?;
                    let bi = b.to_int().ok_or_else(|| ScriptError::runtime("floor div requires integers"))?;
                    if bi == 0 { return Err(ScriptError::runtime("integer divide by zero")); }
                    self.push(Value::Int(ai.div_euclid(bi)));
                }
                Instruction::Mod => {
                    let b = self.pop(); let a = self.pop();
                    let r = match (&a, &b) {
                        (Value::Int(ai), Value::Int(bi)) => {
                            if *bi == 0 { return Err(ScriptError::runtime("modulo by zero")); }
                            Value::Int(ai.rem_euclid(*bi))
                        }
                        _ => {
                            let af = a.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", a.type_name())))?;
                            let bf = b.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", b.type_name())))?;
                            Value::Float(af % bf)
                        }
                    };
                    self.push(r);
                }
                Instruction::Pow => {
                    let b = self.pop(); let a = self.pop();
                    let af = a.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", a.type_name())))?;
                    let bf = b.to_float().ok_or_else(|| ScriptError::runtime(format!("arith on {}", b.type_name())))?;
                    self.push(Value::Float(af.powf(bf)));
                }
                Instruction::Concat => {
                    let b = self.pop(); let a = self.pop();
                    let sa = a.to_str_repr().ok_or_else(|| ScriptError::runtime(format!("concat on {}", a.type_name())))?;
                    let sb = b.to_str_repr().ok_or_else(|| ScriptError::runtime(format!("concat on {}", b.type_name())))?;
                    self.push(Value::Str(Arc::new(sa + &sb)));
                }

                Instruction::BitAnd => self.bitwise(|a, b| a & b)?,
                Instruction::BitOr  => self.bitwise(|a, b| a | b)?,
                Instruction::BitXor => self.bitwise(|a, b| a ^ b)?,
                Instruction::Shl    => self.bitwise(|a, b| a.wrapping_shl(b as u32))?,
                Instruction::Shr    => self.bitwise(|a, b| a.wrapping_shr(b as u32))?,

                Instruction::Eq    => { let b = self.pop(); let a = self.pop(); self.push(Value::Bool(a == b)); }
                Instruction::NotEq => { let b = self.pop(); let a = self.pop(); self.push(Value::Bool(a != b)); }
                Instruction::Lt    => self.cmp(|a, b| a < b)?,
                Instruction::LtEq  => self.cmp(|a, b| a <= b)?,
                Instruction::Gt    => self.cmp(|a, b| a > b)?,
                Instruction::GtEq  => self.cmp(|a, b| a >= b)?,

                Instruction::Jump(off) => {
                    let fi = self.frames.len() - 1;
                    self.frames[fi].ip = (self.frames[fi].ip as isize + off) as usize;
                }
                Instruction::JumpIf(off) => {
                    if self.stack.last().map(|v| v.is_truthy()).unwrap_or(false) {
                        let fi = self.frames.len() - 1;
                        self.frames[fi].ip = (self.frames[fi].ip as isize + off) as usize;
                    }
                }
                Instruction::JumpIfNot(off) => {
                    if !self.stack.last().map(|v| v.is_truthy()).unwrap_or(false) {
                        let fi = self.frames.len() - 1;
                        self.frames[fi].ip = (self.frames[fi].ip as isize + off) as usize;
                    }
                }
                Instruction::JumpIfNotPop(off) => {
                    let top = self.pop();
                    if !top.is_truthy() {
                        let fi = self.frames.len() - 1;
                        self.frames[fi].ip = (self.frames[fi].ip as isize + off) as usize;
                    } else {
                        self.push(top);
                    }
                }
                Instruction::JumpIfPop(off) => {
                    let top = self.pop();
                    if top.is_truthy() {
                        let fi = self.frames.len() - 1;
                        self.frames[fi].ip = (self.frames[fi].ip as isize + off) as usize;
                    } else {
                        self.push(top);
                    }
                }
                Instruction::JumpAbs(abs_ip) => {
                    let fi = self.frames.len() - 1;
                    self.frames[fi].ip = abs_ip;
                }

                Instruction::Call(nargs) => {
                    let top  = self.stack.len();
                    let base = top.saturating_sub(nargs + 1);
                    let args: Vec<Value> = self.stack.drain(base + 1..).collect();
                    let callee = self.stack.pop().unwrap_or(Value::Nil);
                    let results = self.call(callee, args)?;
                    for r in results { self.stack.push(r); }
                }

                Instruction::CallMethod(method_name, nargs) => {
                    let top  = self.stack.len();
                    let base = top.saturating_sub(nargs + 1);
                    let extra: Vec<Value> = self.stack.drain(base + 1..).collect();
                    let obj = self.stack.pop().unwrap_or(Value::Nil);
                    let method = match &obj {
                        Value::Table(t) => t.rawget_str(&method_name),
                        other => return Err(ScriptError::runtime(format!(
                            "method call on {} value", other.type_name()
                        ))),
                    };
                    let mut args = vec![obj];
                    args.extend(extra);
                    let results = self.call(method, args)?;
                    for r in results { self.stack.push(r); }
                }

                Instruction::Return(nret) => {
                    let top = self.stack.len();
                    let ret_start = if nret == 0 {
                        self.frames[self.frames.len() - 1].base
                    } else {
                        top.saturating_sub(nret)
                    };
                    let returns: Vec<Value> = self.stack.drain(ret_start..).collect();
                    let fi   = self.frames.len() - 1;
                    let base = self.frames[fi].base;
                    self.stack.truncate(base);
                    self.frames.pop();
                    return Ok(returns);
                }

                Instruction::MakeFunction(idx) => {
                    let fi  = self.frames.len() - 1;
                    let sub = Arc::clone(&self.frames[fi].chunk.sub_chunks[idx]);
                    let closure = Arc::new(Closure { chunk: sub, upvalues: Vec::new() });
                    self.push(Value::Function(closure));
                }

                Instruction::MakeClosure(idx, captures) => {
                    let fi  = self.frames.len() - 1;
                    let sub = Arc::clone(&self.frames[fi].chunk.sub_chunks[idx]);
                    let mut upvalues = Vec::new();
                    for (is_local, slot) in captures {
                        if is_local {
                            let base = self.frames[fi].base;
                            let v = self.stack.get(base + slot).cloned().unwrap_or(Value::Nil);
                            upvalues.push(UpvalueCell::new(v));
                        } else {
                            let v = self.frames[fi].upvalues.get(slot)
                                .map(|uv| uv.clone())
                                .unwrap_or_else(|| UpvalueCell::new(Value::Nil));
                            upvalues.push(v);
                        }
                    }
                    let closure = Arc::new(Closure { chunk: sub, upvalues });
                    self.push(Value::Function(closure));
                }

                Instruction::CloseUpvalue(_slot) => {
                    // Upvalues are captured by value on closure creation; nothing to do here.
                }

                Instruction::ForPrep(_nvars) => {
                    // Generic for: iterator function, state, control are on stack.
                    // Body sets locals from results; handled by subsequent ForStep.
                }

                Instruction::ForStep(local_idx, jump_off) => {
                    // Numeric for: locals at [local_idx]=current, [local_idx+1]=limit, [local_idx+2]=step
                    let fi   = self.frames.len() - 1;
                    let base = self.frames[fi].base;
                    let cur   = self.stack.get(base + local_idx).cloned().unwrap_or(Value::Nil);
                    let limit = self.stack.get(base + local_idx + 1).cloned().unwrap_or(Value::Nil);
                    let step  = self.stack.get(base + local_idx + 2).cloned().unwrap_or(Value::Nil);
                    let cv = cur.to_float().unwrap_or(0.0);
                    let lv = limit.to_float().unwrap_or(0.0);
                    let sv = step.to_float().unwrap_or(1.0);
                    let should_continue = if sv > 0.0 { cv <= lv } else { cv >= lv };
                    if !should_continue {
                        let fi = self.frames.len() - 1;
                        self.frames[fi].ip = (self.frames[fi].ip as isize + jump_off) as usize;
                    } else {
                        let next = match (&cur, &step) {
                            (Value::Int(c), Value::Int(s)) => Value::Int(c.wrapping_add(*s)),
                            _ => Value::Float(cv + sv),
                        };
                        let fi   = self.frames.len() - 1;
                        let base = self.frames[fi].base;
                        let idx  = base + local_idx;
                        while self.stack.len() <= idx { self.stack.push(Value::Nil); }
                        self.stack[idx] = next;
                    }
                }

                Instruction::Nop => {}
            }
        }
    }

    // ── Stack helpers ─────────────────────────────────────────────────────

    #[inline] fn push(&mut self, v: Value) { self.stack.push(v); }
    #[inline] fn pop(&mut self) -> Value { self.stack.pop().unwrap_or(Value::Nil) }
    #[inline] fn peek(&self) -> Value { self.stack.last().cloned().unwrap_or(Value::Nil) }

    fn arith2<F>(&mut self, f: F) -> Result<(), ScriptError>
    where F: Fn(Value, Value) -> Result<Value, ScriptError>
    {
        let b = self.pop(); let a = self.pop();
        self.push(f(a, b)?);
        Ok(())
    }

    fn cmp<F>(&mut self, op: F) -> Result<(), ScriptError>
    where F: Fn(f64, f64) -> bool
    {
        let b = self.pop(); let a = self.pop();
        let av = a.to_float().ok_or_else(|| ScriptError::runtime(format!("compare on {}", a.type_name())))?;
        let bv = b.to_float().ok_or_else(|| ScriptError::runtime(format!("compare on {}", b.type_name())))?;
        self.push(Value::Bool(op(av, bv)));
        Ok(())
    }

    fn bitwise<F>(&mut self, op: F) -> Result<(), ScriptError>
    where F: Fn(i64, i64) -> i64
    {
        let b = self.pop(); let a = self.pop();
        let ai = a.to_int().ok_or_else(|| ScriptError::runtime(format!("bitwise on {}", a.type_name())))?;
        let bi = b.to_int().ok_or_else(|| ScriptError::runtime(format!("bitwise on {}", b.type_name())))?;
        self.push(Value::Int(op(ai, bi)));
        Ok(())
    }
}

fn num_arith<FI, FF>(a: Value, b: Value, fi: FI, ff: FF) -> Result<Value, ScriptError>
where
    FI: Fn(i64, i64) -> i64,
    FF: Fn(f64, f64) -> f64,
{
    match (&a, &b) {
        (Value::Int(ai), Value::Int(bi))     => Ok(Value::Int(fi(*ai, *bi))),
        (Value::Float(af), Value::Float(bf)) => Ok(Value::Float(ff(*af, *bf))),
        (Value::Int(ai), Value::Float(bf))   => Ok(Value::Float(ff(*ai as f64, *bf))),
        (Value::Float(af), Value::Int(bi))   => Ok(Value::Float(ff(*af, *bi as f64))),
        _ => Err(ScriptError::runtime(format!(
            "attempt to perform arithmetic on {} and {} values",
            a.type_name(), b.type_name()
        ))),
    }
}

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

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

    fn run(src: &str) -> Result<Vec<Value>, ScriptError> {
        let script = Parser::from_source("test", src)
            .map_err(|e| ScriptError::new(e.to_string()))?;
        let chunk = Compiler::compile_script(&script);
        let mut vm = Vm::new();
        vm.execute(chunk)
    }

    #[test]
    fn test_return_int() {
        let r = run("return 42").unwrap();
        assert_eq!(r[0], Value::Int(42));
    }

    #[test]
    fn test_arithmetic() {
        let r = run("return 2 + 3 * 4").unwrap();
        assert_eq!(r[0], Value::Int(14));
    }

    #[test]
    fn test_string_concat() {
        let r = run("return \"hello\" .. \" world\"").unwrap();
        assert!(matches!(&r[0], Value::Str(s) if s.as_ref() == "hello world"));
    }

    #[test]
    fn test_local_variable() {
        let r = run("local x = 10 return x").unwrap();
        assert_eq!(r[0], Value::Int(10));
    }

    #[test]
    fn test_if_else() {
        let r = run("if true then return 1 else return 2 end").unwrap();
        assert_eq!(r[0], Value::Int(1));
    }

    #[test]
    fn test_function_call() {
        let r = run("local function add(a, b) return a + b end return add(3, 4)").unwrap();
        assert_eq!(r[0], Value::Int(7));
    }
}