// Lua support (for different versions)

#[cfg(not(any(feature = "Lua5_3", feature = "Lua5_4")))]
compile_error!("no feature for supported Lua version");

use super::{cmach::cmach_lua_t, *};
use sandkiste::prelude::*;

use std::borrow::{Cow, Cow::Borrowed, Cow::Owned};
use std::error::Error;
use std::ffi::{c_int, c_void, CString};
use std::hash::{Hash, Hasher};
use std::marker::PhantomData;
use std::num::TryFromIntError;
use std::rc::Rc;
use std::slice;
use std::str;

/// Virtual machine that executes Lua code
///
/// # Basic usage
///
/// 1. A new virtual machine can be created with [`LuaMachine::new`].
/// 2. Use [`LuaMachine::load_stdlib_sealed`] to load a sealed version of the
///    Lua standard library; that is a modified version which does not allow
///    breaking out of the sandbox.
///    Alternatively, a full version of the standard library can be loaded with
///    [`LuaMachine::load_stdlib`], which, for example, can be used to load
///    further dependencies, and then be sealed afterwards with
///    [`LuaMachine::seal`]. Note, however, that this may introduce security
///    vulnerabilites depending on which libraries have been loaded or how the
///    state has been modified prior to calling `seal`.
/// 3. Set resource limits with [`LuaMachine::set_memory_limit`] and
///    [`LuaMachine::set_execution_limit`], if desired.
/// 4. Use the API provided by the [`sandkiste`] crate to compile and execute
///    Lua programs. For a concrete example using Lua, see the
///    [top-level module documentation of `sandkiste_lua`](crate).
///
/// # Lifetimes
///
/// Lifetime argument `'a` is a lower bound for closures passed to the Lua
/// machine. It can be inferred automatically.
#[derive(Debug)]
pub struct LuaMachine<'a> {
    c_machine: *mut cmach_lua_t,
    phantom: PhantomData<fn(&'a ()) -> &'a ()>,
}

/// A `LuaMachine` is only equal to itself
impl<'a> PartialEq for LuaMachine<'a> {
    fn eq(&self, other: &Self) -> bool {
        self.c_machine == other.c_machine
    }
}

/// A `LuaMachine` is only equal to itself
impl<'a> Eq for LuaMachine<'a> {}

impl<'a> Hash for LuaMachine<'a> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.c_machine.hash(state);
    }
}

impl<'a> Machine<'a> for LuaMachine<'a> {
    type Datum<'b, 'c> = LuaDatum<'a, 'b, 'c>
    where
        Self: 'b;
    type Function<'b> = LuaFunction<'a, 'b>
    where
        Self: 'b;
}

/// Key used to reference values that are stored in the Lua registry
///
/// Lua objects that are referenced from Rust are stored with a numeric key in
/// the Lua registry. This type reflects the numeric key used in the Lua
/// registry and is an alias to [`c_int`]. It can be used to manually interact
/// with the Lua machine and is normally not needed for user code.
/// See [`LuaMachine::get_reference_key`].
pub type LuaReferenceKey = c_int;

#[derive(Debug)]
struct LuaInnerReference<'a, 'b> {
    machine: &'b LuaMachine<'a>,
    key: LuaReferenceKey,
    identity: *const c_void,
}

impl<'a, 'b> PartialEq for LuaInnerReference<'a, 'b> {
    fn eq(&self, other: &Self) -> bool {
        self.identity == other.identity
    }
}

impl<'a, 'b> Eq for LuaInnerReference<'a, 'b> {}

impl<'a, 'b> Hash for LuaInnerReference<'a, 'b> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.identity.hash(state);
    }
}

/// Reference to value inside Lua machine
///
/// This type reflects an opaque reference to a value inside the Lua machine.
/// A `LuaReference` may be obtained by calling [`MaybeOpaque::try_as_opaque`]
/// on a [`LuaDatum`]. A `LuaReference` may be cloned, compared, or hashed, but
/// provides no other means of accessing the underlying value.
///
/// # Lifetimes
///
/// Lifetime argument `'a` corresponds to the lifetime argument `'a` of
/// [`LuaMachine`]. It is a lower bound for closures passed to the Lua machine.
///
/// Lifetime argument `'b` corresponds to the lifetime of the shared reference
/// to the `LuaMachine` being worked with.
#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct LuaReference<'a, 'b>(Rc<LuaInnerReference<'a, 'b>>);

impl<'a, 'b> LuaReference<'a, 'b> {
    fn new(inner_ref: LuaInnerReference<'a, 'b>) -> Self {
        LuaReference(Rc::new(inner_ref))
    }
}

impl<'a, 'b> AsRef<LuaInnerReference<'a, 'b>> for LuaReference<'a, 'b> {
    fn as_ref(&self) -> &LuaInnerReference<'a, 'b> {
        self.0.as_ref()
    }
}

/// Lua function
///
/// A function inside the Lua machine can be referenced from Rust through a
/// `LuaFunction`. See [`Function`] trait for information on how to use this
/// type.
///
/// # Lifetimes
///
/// Lifetime argument `'a` corresponds to the lifetime argument `'a` of
/// [`LuaMachine`]. It is a lower bound for closures passed to the Lua machine.
///
/// Lifetime argument `'b` corresponds to the lifetime of the shared reference
/// to the `LuaMachine` being worked with.
#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct LuaFunction<'a, 'b> {
    lua_ref: LuaReference<'a, 'b>,
    chunk_name: Option<String>,
}

#[derive(Clone, Debug)]
enum LuaType<'a, 'b, 'c> {
    Nil,
    Boolean(bool),
    Function(LuaFunction<'a, 'b>),
    Float(cmach::lua_Number),
    Integer(cmach::lua_Integer),
    String(Cow<'c, str>),
    Binary(Cow<'c, [u8]>),
    Table(LuaReference<'a, 'b>),
    Userdata(LuaReference<'a, 'b>),
    Thread(LuaReference<'a, 'b>),
}

/// Lua value (of any supported type)
///
/// A `LuaDatum` is used to pass values from Rust to Lua and vice versa. It can
/// contain values of several types (dynamic typing). See module
/// [`sandkiste::types`] for more information.
///
/// # Lifetimes
///
/// Lifetime argument `'a` corresponds to the lifetime argument `'a` of
/// [`LuaMachine`]. It is a lower bound for closures passed to the Lua machine.
///
/// Lifetime argument `'b` corresponds to the lifetime of the shared reference
/// to the `LuaMachine` being worked with.
///
/// Lifetime argument `'c` is often `'static` but can be a shorter lifetime for
/// temporary `LuaDatum`s created from a `&'c str` reference, for example.
#[derive(Clone, Debug)]
pub struct LuaDatum<'a, 'b, 'c>(LuaType<'a, 'b, 'c>);

impl<'a, 'b, 'c, T> From<Option<T>> for LuaDatum<'a, 'b, 'c>
where
    T: Into<LuaDatum<'a, 'b, 'c>>,
{
    fn from(value: Option<T>) -> Self {
        match value {
            None => LuaDatum::null(),
            Some(v) => v.into(),
        }
    }
}

impl<'a, 'b, 'c> Nullable for LuaDatum<'a, 'b, 'c> {
    fn null() -> Self {
        LuaDatum(LuaType::Nil)
    }
    fn is_null(&self) -> bool {
        match self.0 {
            LuaType::Nil => true,
            _ => false,
        }
    }
}

impl<'a, 'b, 'c> From<bool> for LuaDatum<'a, 'b, 'c> {
    fn from(value: bool) -> Self {
        LuaDatum(LuaType::Boolean(value))
    }
}

impl<'a, 'b, 'c> MaybeBoolean for LuaDatum<'a, 'b, 'c> {
    fn try_as_bool(&self) -> Result<bool, DatumViewError> {
        match self.0 {
            LuaType::Boolean(false) => Ok(false),
            LuaType::Boolean(true) => Ok(true),
            _ => Err(DatumViewError::new("boolean expected")),
        }
    }
}

impl<'a, 'b, 'c> From<LuaFunction<'a, 'b>> for LuaDatum<'a, 'b, 'c> {
    fn from(value: LuaFunction<'a, 'b>) -> Self {
        LuaDatum(LuaType::Function(value))
    }
}

impl<'a, 'b, 'c> MaybeFunction for LuaDatum<'a, 'b, 'c> {
    type Function = LuaFunction<'a, 'b>;
    type FunctionRef<'d> = &'d LuaFunction<'a, 'b>
    where
        Self: 'd;
    fn try_as_function(&self) -> Result<&LuaFunction<'a, 'b>, DatumViewError> {
        match self.0 {
            LuaType::Function(ref f) => Ok(f),
            _ => Err(DatumViewError::new("function expected")),
        }
    }
}

impl<'a, 'b, 'c> TryFrom<LuaDatum<'a, 'b, 'c>> for LuaReference<'a, 'b> {
    type Error = DatumConversionError<LuaDatum<'a, 'b, 'c>>;
    fn try_from(value: LuaDatum<'a, 'b, 'c>) -> Result<Self, Self::Error> {
        match value.0 {
            LuaType::Function(f) => Ok(f.lua_ref),
            LuaType::Table(r) => Ok(r),
            LuaType::Userdata(r) => Ok(r),
            LuaType::Thread(r) => Ok(r),
            _ => Err(DatumConversionError::new(value, "reference type expected")),
        }
    }
}

impl<'a, 'b, 'c> MaybeOpaque for LuaDatum<'a, 'b, 'c> {
    type Opaque = LuaReference<'a, 'b>;
    type OpaqueRef<'d> = &'d LuaReference<'a, 'b>
    where
        Self: 'd;
    fn is_opaque(&self) -> bool {
        match self.0 {
            LuaType::Function(_) => true,
            LuaType::Table(_) => true,
            LuaType::Userdata(_) => true,
            LuaType::Thread(_) => true,
            _ => false,
        }
    }
    fn try_as_opaque(&self) -> Result<&LuaReference<'a, 'b>, DatumViewError> {
        match self.0 {
            LuaType::Function(ref f) => Ok(&f.lua_ref),
            LuaType::Table(ref r) => Ok(r),
            LuaType::Userdata(ref r) => Ok(r),
            LuaType::Thread(ref r) => Ok(r),
            _ => Err(DatumViewError::new("reference type expected")),
        }
    }
}

impl<'a, 'b, 'c> From<String> for LuaDatum<'a, 'b, 'c> {
    fn from(value: String) -> LuaDatum<'a, 'b, 'c> {
        LuaDatum(LuaType::String(Owned(value)))
    }
}

impl<'a, 'b, 'c> From<&'c str> for LuaDatum<'a, 'b, 'c> {
    fn from(value: &'c str) -> LuaDatum<'a, 'b, 'c> {
        LuaDatum(LuaType::String(Borrowed(value)))
    }
}

impl<'a, 'b, 'c> TryFrom<LuaDatum<'a, 'b, 'c>> for String {
    type Error = DatumConversionError<LuaDatum<'a, 'b, 'c>>;
    fn try_from(value: LuaDatum<'a, 'b, 'c>) -> Result<Self, Self::Error> {
        match value.0 {
            LuaType::String(s) => Ok(s.into_owned()),
            LuaType::Binary(_) => Err(DatumConversionError::new(
                value,
                "Lua string contains invalid UTF-8 sequence",
            )),
            _ => Err(DatumConversionError::new(value, "string expected")),
        }
    }
}

impl<'a, 'b, 'c> MaybeString<'c> for LuaDatum<'a, 'b, 'c> {
    fn try_as_str(&self) -> Result<&str, DatumViewError> {
        match self.0 {
            LuaType::String(ref s) => Ok(s),
            LuaType::Binary(_) => Err(DatumViewError::new(
                "Lua string contains invalid UTF-8 sequence",
            )),
            _ => Err(DatumViewError::new("string expected")),
        }
    }
}

impl<'a, 'b, 'c> From<Vec<u8>> for LuaDatum<'a, 'b, 'c> {
    fn from(value: Vec<u8>) -> LuaDatum<'a, 'b, 'c> {
        match String::from_utf8(value) {
            Ok(s) => LuaDatum(LuaType::String(Owned(s))),
            Err(err) => LuaDatum(LuaType::Binary(Owned(err.into_bytes()))),
        }
    }
}

impl<'a, 'b, 'c> From<&'c [u8]> for LuaDatum<'a, 'b, 'c> {
    fn from(value: &'c [u8]) -> LuaDatum<'a, 'b, 'c> {
        match str::from_utf8(value) {
            Ok(s) => LuaDatum(LuaType::String(Borrowed(s))),
            Err(_) => LuaDatum(LuaType::Binary(Borrowed(value))),
        }
    }
}

impl<'a, 'b, 'c> TryFrom<LuaDatum<'a, 'b, 'c>> for Vec<u8> {
    type Error = DatumConversionError<LuaDatum<'a, 'b, 'c>>;
    fn try_from(value: LuaDatum<'a, 'b, 'c>) -> Result<Self, Self::Error> {
        match value.0 {
            LuaType::String(s) => Ok(s.into_owned().into()),
            LuaType::Binary(b) => Ok(b.into_owned()),
            _ => Err(DatumConversionError::new(value, "binary string expected")),
        }
    }
}

impl<'a, 'b, 'c> MaybeBinary<'c> for LuaDatum<'a, 'b, 'c> {
    fn try_as_bin(&self) -> Result<&[u8], DatumViewError> {
        match self.0 {
            LuaType::String(ref s) => Ok(s.as_bytes()),
            LuaType::Binary(ref b) => Ok(b),
            _ => Err(DatumViewError::new("binary string expected")),
        }
    }
}

impl<'a, 'b, 'c> From<f64> for LuaDatum<'a, 'b, 'c> {
    fn from(value: f64) -> Self {
        LuaDatum(LuaType::Float(value))
    }
}

impl<'a, 'b, 'c> From<f32> for LuaDatum<'a, 'b, 'c> {
    fn from(value: f32) -> Self {
        LuaDatum(LuaType::Float(value as f64))
    }
}

impl<'a, 'b, 'c> MaybeFloat for LuaDatum<'a, 'b, 'c> {
    fn try_as_f64(&self) -> Result<f64, DatumViewError> {
        match self.0 {
            LuaType::Float(value) => Ok(value as f64),
            LuaType::Integer(value) => Ok(value as f64),
            _ => Err(DatumViewError::new("float (or integer) expected")),
        }
    }
}

impl<'a, 'b, 'c> TryFrom<usize> for LuaDatum<'a, 'b, 'c> {
    type Error = TryFromIntError;
    fn try_from(value: usize) -> Result<Self, Self::Error> {
        Ok(LuaDatum(LuaType::Integer(value.try_into()?)))
    }
}

impl<'a, 'b, 'c> TryFrom<isize> for LuaDatum<'a, 'b, 'c> {
    type Error = TryFromIntError;
    fn try_from(value: isize) -> Result<Self, Self::Error> {
        Ok(LuaDatum(LuaType::Integer(value.try_into()?)))
    }
}

impl<'a, 'b, 'c> TryFrom<u64> for LuaDatum<'a, 'b, 'c> {
    type Error = TryFromIntError;
    fn try_from(value: u64) -> Result<Self, Self::Error> {
        Ok(LuaDatum(LuaType::Integer(value.try_into()?)))
    }
}

impl<'a, 'b, 'c> TryFrom<i64> for LuaDatum<'a, 'b, 'c> {
    type Error = TryFromIntError;
    fn try_from(value: i64) -> Result<Self, Self::Error> {
        Ok(LuaDatum(LuaType::Integer(value.try_into()?)))
    }
}

impl<'a, 'b, 'c> TryFrom<u32> for LuaDatum<'a, 'b, 'c> {
    type Error = TryFromIntError;
    fn try_from(value: u32) -> Result<Self, Self::Error> {
        Ok(LuaDatum(LuaType::Integer(value.try_into()?)))
    }
}

impl<'a, 'b, 'c> From<i32> for LuaDatum<'a, 'b, 'c> {
    fn from(value: i32) -> Self {
        LuaDatum(LuaType::Integer(value.into()))
    }
}

impl<'a, 'b, 'c> From<u16> for LuaDatum<'a, 'b, 'c> {
    fn from(value: u16) -> Self {
        LuaDatum(LuaType::Integer(value.into()))
    }
}

impl<'a, 'b, 'c> From<i16> for LuaDatum<'a, 'b, 'c> {
    fn from(value: i16) -> Self {
        LuaDatum(LuaType::Integer(value.into()))
    }
}

impl<'a, 'b, 'c> From<u8> for LuaDatum<'a, 'b, 'c> {
    fn from(value: u8) -> Self {
        LuaDatum(LuaType::Integer(value.into()))
    }
}

impl<'a, 'b, 'c> From<i8> for LuaDatum<'a, 'b, 'c> {
    fn from(value: i8) -> Self {
        LuaDatum(LuaType::Integer(value.into()))
    }
}

impl<'a, 'b, 'c> MaybeInteger for LuaDatum<'a, 'b, 'c> {
    fn try_as_i64(&self) -> Result<i64, DatumViewError> {
        match self.0 {
            LuaType::Integer(value) => value
                .try_into()
                .map_err(|_| DatumViewError::new("Lua integer does not fit into i64")),
            LuaType::Float(_) => Err(DatumViewError::new("integer expected, but got float")),
            _ => Err(DatumViewError::new("integer expected")),
        }
    }
}

impl<'a, 'b, 'c> MaybeArray for LuaDatum<'a, 'b, 'c> {
    type Element<'d> = LuaDatum<'a, 'b, 'd>;
    fn try_array(&self) -> Result<(), DatumViewError> {
        match self.0 {
            LuaType::Table(_) => Ok(()),
            _ => Err(DatumViewError::new("array (table) expected")),
        }
    }
    fn array_len(&self) -> Result<usize, MachineError> {
        match self.0 {
            LuaType::Table(ref lua_ref) => {
                let machine = lua_ref.as_ref().machine;
                let l = machine.lua_state();
                unsafe {
                    machine.push_reference(lua_ref);
                    machine.pop_error_with_backtrace(cmach::cmach_lua_len(l))?;
                    let result = machine.pop_datum()?;
                    Ok(result.try_as_usize()?)
                }
            }
            _ => Err(self.try_array().unwrap_err())?,
        }
    }
    fn array_get(&self, index: usize) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        match self.0 {
            LuaType::Table(ref lua_ref) => {
                let machine = lua_ref.as_ref().machine;
                let l = machine.lua_state();
                unsafe {
                    let index = cmach::lua_Integer::try_from(index)
                        .ok()
                        .and_then(|x| x.checked_add(1))
                        .ok_or_else(|| MachineError {
                            message: Some("lua_Integer too small to store array index".to_string()),
                            ..Default::default()
                        })?;
                    machine.push_reference(lua_ref);
                    cmach::lua_pushinteger(l, index);
                    machine.pop_error_with_backtrace(cmach::cmach_lua_gettable(l))?;
                    Ok(machine.pop_datum()?)
                }
            }
            _ => Err(self.try_array().unwrap_err())?,
        }
    }
    fn array_set<'d>(
        &self,
        index: usize,
        element: LuaDatum<'a, 'b, 'd>,
    ) -> Result<(), MachineError> {
        match self.0 {
            LuaType::Table(ref lua_ref) => {
                let machine = lua_ref.as_ref().machine;
                let l = machine.lua_state();
                unsafe {
                    let index = cmach::lua_Integer::try_from(index)
                        .ok()
                        .and_then(|x| x.checked_add(1))
                        .ok_or_else(|| MachineError {
                            message: Some("lua_Integer too small to store array index".to_string()),
                            ..Default::default()
                        })?;
                    machine.push_reference(lua_ref);
                    cmach::lua_pushinteger(l, index);
                    match machine.push_datum(&element) {
                        Ok(_) => (),
                        Err(err) => {
                            cmach::lua_pop(l, 2);
                            Err(err)?
                        }
                    }
                    machine.pop_error_with_backtrace(cmach::cmach_lua_settable(l))?;
                    Ok(())
                }
            }
            _ => Err(self.try_array().unwrap_err())?,
        }
    }
    fn array_push<'d>(&self, element: LuaDatum<'a, 'b, 'd>) -> Result<(), MachineError> {
        self.array_set(self.array_len()?, element)
    }
    fn array_truncate(&self, len: usize) -> Result<(), MachineError> {
        for index in (len..self.array_len()?).rev() {
            self.array_set(index, LuaDatum::null())?;
        }
        Ok(())
    }
}

impl<'a, 'b, 'c> MaybeStringMap for LuaDatum<'a, 'b, 'c> {
    type Value<'d> = LuaDatum<'a, 'b, 'd>;
    fn try_string_map(&self) -> Result<(), DatumViewError> {
        match self.0 {
            LuaType::Table(_) => Ok(()),
            _ => Err(DatumViewError::new("string map (table) expected")),
        }
    }
    fn string_map_get(&self, key: &str) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        match self.0 {
            LuaType::Table(ref lua_ref) => {
                let machine = lua_ref.as_ref().machine;
                let l = machine.lua_state();
                unsafe {
                    machine.push_reference(lua_ref);
                    match machine.push_str(key) {
                        Ok(_) => (),
                        Err(err) => {
                            cmach::lua_pop(l, 1);
                            Err(err)?
                        }
                    }
                    machine.pop_error_with_backtrace(cmach::cmach_lua_gettable(l))?;
                    Ok(machine.pop_datum()?)
                }
            }
            _ => Err(self.try_string_map().unwrap_err())?,
        }
    }
    fn string_map_set<'d, 'e>(
        &self,
        key: &'d str,
        value: LuaDatum<'a, 'b, 'e>,
    ) -> Result<(), MachineError> {
        match self.0 {
            LuaType::Table(ref lua_ref) => {
                let machine = lua_ref.as_ref().machine;
                let l = machine.lua_state();
                unsafe {
                    machine.push_reference(lua_ref);
                    match machine.push_str(key) {
                        Ok(_) => (),
                        Err(err) => {
                            cmach::lua_pop(l, 1);
                            Err(err)?
                        }
                    }
                    match machine.push_datum(&value) {
                        Ok(_) => (),
                        Err(err) => {
                            cmach::lua_pop(l, 2);
                            Err(err)?
                        }
                    }
                    machine.pop_error_with_backtrace(cmach::cmach_lua_settable(l))?;
                    Ok(())
                }
            }
            _ => Err(self.try_string_map().unwrap_err())?,
        }
    }
}

impl<'a> HasArray<'a> for LuaMachine<'a> {
    fn new_empty_array<'b>(&'b self) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        self.new_table()
    }
}

impl<'a> HasStringMap<'a> for LuaMachine<'a> {
    fn new_empty_string_map<'b>(&'b self) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        self.new_table()
    }
}

/// (Sub-)module in a [`LuaMachine`] (which is a Lua table)
///
/// The type `LuaModule` is used to implement the abstract [`Module`] trait
/// which allows getting and setting variables (through [`Module::get`] and
/// [`Module::set`], respectively) a module (in this case: in a Lua table).
/// A `LuaModule` may be obtained by calling [`HasModules::module`] on the
/// [`LuaMachine`] (to obtain a top-level module/table) or [`Module::module`]
/// on a `LuaModule` (to obtain a sub module/table, respectively).
///
/// To set global variables, use [`Globals::get`] and [`Globals::set`] directly
/// on the [`LuaMachine`] instead.
///
/// # Lifetimes
///
/// Lifetime argument `'a` corresponds to the lifetime argument `'a` of
/// [`LuaMachine`]. It is a lower bound for closures passed to the Lua machine.
///
/// Lifetime argument `'b` corresponds to the lifetime of the shared reference
/// to the `LuaMachine` being worked with.
pub struct LuaModule<'a, 'b> {
    machine: &'b LuaMachine<'a>,
    table: LuaDatum<'a, 'b, 'static>,
}

impl<'a> HasModules<'a> for LuaMachine<'a> {
    type Module<'b> = LuaModule<'a, 'b>
    where
        'a: 'b;
    fn module<'b>(&'b self, name: &str) -> Result<LuaModule<'a, 'b>, MachineError> {
        let mut table = self.get(name)?;
        if table.is_null() {
            table = self.new_empty_string_map()?;
            self.set(name, table.clone())?;
        }
        Ok(LuaModule {
            machine: self,
            table,
        })
    }
}

impl<'a, 'b> Module<'a, 'b> for LuaModule<'a, 'b> {
    type Machine = LuaMachine<'a>;
    /// Open a sub-module (create if non-existent)
    fn module(&self, name: &str) -> Result<Self, MachineError> {
        let mut table = self.table.string_map_get(name)?;
        if table.is_null() {
            table = self.machine.new_empty_string_map()?;
            self.table.string_map_set(name, table.clone())?;
        }
        Ok(LuaModule {
            machine: self.machine,
            table,
        })
    }
    /// Get value from module
    fn get(
        &self,
        name: &str,
    ) -> Result<<Self::Machine as Machine<'a>>::Datum<'b, 'static>, MachineError> {
        Ok(self.table.string_map_get(name)?)
    }
    /// Set value in module
    fn set<'c>(
        &self,
        name: &str,
        datum: <Self::Machine as Machine<'a>>::Datum<'b, 'c>,
    ) -> Result<(), MachineError> {
        self.table.string_map_set(name, datum)?;
        Ok(())
    }
}

impl<'a, 'b> Drop for LuaInnerReference<'a, 'b> {
    fn drop(&mut self) {
        unsafe {
            cmach::luaL_unref(
                self.machine.lua_state(),
                cmach::LUA_REGISTRYINDEX.try_into().unwrap(),
                self.key,
            )
        };
    }
}

impl<'a> Drop for LuaMachine<'a> {
    fn drop(&mut self) {
        unsafe {
            cmach::cmach_lua_free(self.c_machine);
        };
    }
}

impl<'a> Globals<'a> for LuaMachine<'a> {
    fn get<'b>(&'b self, name: &str) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.push_str(name)?;
            self.pop_error_with_backtrace(cmach::cmach_lua_getglobal(l))?;
            self.pop_datum()
        }
    }
    fn set<'b, 'c>(&'b self, name: &str, value: LuaDatum<'a, 'b, 'c>) -> Result<(), MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.push_str(name)?;
            match self.push_datum(&value) {
                Ok(()) => (),
                Err(err) => {
                    cmach::lua_pop(l, 1);
                    Err(err)?;
                }
            }
            self.pop_error_with_backtrace(cmach::cmach_lua_setglobal(l))?;
            Ok(())
        }
    }
}

impl<'a> LuaMachine<'a> {
    /// Create a bare Lua machine
    pub fn new() -> Self {
        unsafe {
            let c_machine: *mut cmach_lua_t = cmach::cmach_lua_new();
            if c_machine.is_null() {
                panic!("memory allocation error in cmach_lua_new");
            }
            // `LuaMachine::drop` ensures cleanup
            LuaMachine {
                c_machine,
                phantom: PhantomData,
            }
        }
    }
    /// Retrieve pointer to `lua_State`
    pub fn lua_state(&self) -> *mut cmach::lua_State {
        unsafe { (*self.c_machine).L }
    }
    /// Assert that Lua stack is empty
    pub fn assert_empty_stack(&self) {
        unsafe {
            let l = self.lua_state();
            assert_eq!(cmach::lua_gettop(l), 0, "unexpected values on Lua stack");
        }
    }
    /// Set limit for executed instructions
    pub fn set_execution_limit(&self, instr: u64) {
        unsafe {
            let instr = c_int::try_from(instr).unwrap_or(c_int::MAX);
            cmach::cmach_lua_execlimit(self.c_machine, instr);
        }
    }
    /// Set memory limit
    pub fn set_memory_limit(&self, bytes: usize) {
        unsafe {
            (*self.c_machine).mem_limit = bytes;
        }
    }
    /// Load standard library
    pub fn load_stdlib(&self) -> Result<(), MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.pop_error_with_backtrace(cmach::cmach_lua_openlibs(l))?;
            Ok(())
        }
    }
    /// Load sealed version of standard library
    pub fn load_stdlib_sealed(&self) -> Result<(), MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.pop_error_with_backtrace(cmach::cmach_lua_openlibs_sealed(l))?;
            Ok(())
        }
    }
    /// Remove certain parts of standard library to seal machine as sandbox
    pub fn seal(&self) -> Result<(), MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.pop_error_with_backtrace(cmach::cmach_lua_seal(l))?;
            Ok(())
        }
    }
    /// Create empty table
    pub fn new_table<'b>(&'b self) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        self.assert_empty_stack();
        unsafe {
            let l = self.lua_state();
            self.pop_error_with_backtrace(cmach::cmach_lua_newtable(l))?;
            let result = self.extract_datum(-1);
            cmach::lua_pop(l, 1);
            result
        }
    }
    /// Get [`LuaReferenceKey`] for [`LuaReference`]
    pub fn get_reference_key<'b>(&'b self, lua_ref: &LuaReference<'a, 'b>) -> LuaReferenceKey {
        let inner_ref = lua_ref.as_ref();
        assert_eq!(
            self, inner_ref.machine,
            "reference used on wrong Lua machine"
        );
        inner_ref.key
    }
    /// Pop error value from Lua stack and convert into [`String`]
    pub unsafe fn pop_error_message(&self) -> String {
        let l = self.lua_state();
        let message = if cmach::cmach_lua_touserstr(l) == cmach::LUA_OK.try_into().unwrap() {
            let udata = cmach::lua_touserdata(l, -1);
            let len = cmach::lua_rawlen(l, -1);
            String::from_utf8_lossy(slice::from_raw_parts(udata as *const u8, len as usize))
                .into_owned()
        } else {
            "(unknown)".to_string()
        };
        cmach::lua_pop(l, 1);
        message
    }
    /// If `status != LUA_OK`, pop error value from Lua stack and convert into [`MachineError`]
    pub unsafe fn pop_error(&self, status: c_int) -> Result<(), MachineError> {
        if status == cmach::LUA_OK.try_into().unwrap() {
            Ok(())
        } else {
            let is_execlimit = cmach::cmach_lua_is_execlimit(self.c_machine) != 0;
            let message = Some(self.pop_error_message());
            Err(MachineError {
                kind: if status == cmach::LUA_ERRMEM.try_into().unwrap() {
                    MachineErrorKind::Memory
                } else if is_execlimit {
                    MachineErrorKind::ExecLimit
                } else if status == cmach::LUA_ERRSYNTAX.try_into().unwrap() {
                    MachineErrorKind::Syntax
                } else if status == cmach::LUA_ERRRUN.try_into().unwrap() {
                    MachineErrorKind::Runtime
                } else {
                    MachineErrorKind::Unknown
                },
                message,
                message_incl_chunk_name: true,
                message_incl_pos: true,
                ..Default::default()
            })
        }
    }
    /// Same as [`LuaMachine::pop_error`] but also extract traceback
    /// (requires `cmach_lua_errmsgh` to be used during `lua_pcall`)
    pub unsafe fn pop_error_with_backtrace(&self, status: c_int) -> Result<(), MachineError> {
        if status == cmach::LUA_ERRRUN.try_into().unwrap() {
            let l = self.lua_state();
            cmach::lua_rawgeti(l, -1, 1);
            let is_execlimit = cmach::cmach_lua_is_execlimit(self.c_machine) != 0;
            let message = Some(self.pop_error_message());
            cmach::lua_rawgeti(l, -1, 2);
            let machine_backtrace = Some(self.pop_error_message());
            cmach::lua_rawgeti(l, -1, 3);
            let chunk_name = Some(self.pop_error_message());
            cmach::lua_rawgeti(l, -1, 4);
            let line = usize::try_from(cmach::lua_tointeger(l, -1)).ok();
            cmach::lua_pop(l, 2); // pop line number and error table
            Err(MachineError {
                kind: if is_execlimit {
                    MachineErrorKind::ExecLimit
                } else {
                    MachineErrorKind::Runtime
                },
                message,
                machine_backtrace,
                message_incl_chunk_name: true,
                chunk_name,
                message_incl_pos: true,
                line,
                ..Default::default()
            })
        } else {
            self.pop_error(status)
        }
    }
    /// Convert value on Lua stack into `LuaDatum`
    ///
    /// NOTE: requires some free space on Lua stack for operation
    pub unsafe fn extract_datum<'b>(
        &'b self,
        index: c_int,
    ) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        let l = self.lua_state();
        let type_id = cmach::lua_type(l, index);
        let lua_type = if type_id == cmach::LUA_TNIL.try_into().unwrap() {
            LuaType::Nil
        } else if type_id == cmach::LUA_TBOOLEAN.try_into().unwrap() {
            LuaType::Boolean(cmach::lua_toboolean(l, index) != 0)
        } else if type_id == cmach::LUA_TNUMBER.try_into().unwrap() {
            if cmach::lua_isinteger(l, index) != 0 {
                LuaType::Integer(cmach::lua_tointeger(l, index))
            } else {
                LuaType::Float(cmach::lua_tonumber(l, index))
            }
        } else if type_id == cmach::LUA_TSTRING.try_into().unwrap() {
            cmach::lua_pushvalue(l, index);
            self.pop_error_with_backtrace(cmach::cmach_lua_touserstr(l))?;
            let udata = cmach::lua_touserdata(l, -1);
            let len = cmach::lua_rawlen(l, -1);
            let vec = slice::from_raw_parts(udata as *const u8, len as usize).to_vec();
            cmach::lua_pop(l, 1);
            match String::from_utf8(vec) {
                Ok(s) => LuaType::String(Owned(s)),
                Err(err) => LuaType::Binary(Owned(err.into_bytes())),
            }
        } else {
            let identity = cmach::lua_topointer(l, index);
            cmach::lua_pushvalue(l, index);
            self.pop_error_with_backtrace(cmach::cmach_lua_ref(l))?;
            let key = cmach::lua_tointeger(l, -1) as LuaReferenceKey;
            cmach::lua_pop(l, 1);
            // `LuaInnerReference::drop` ensures cleanup
            let inner_ref = LuaInnerReference {
                machine: &self,
                key,
                identity,
            };
            let lua_ref = LuaReference::new(inner_ref);
            if type_id == cmach::LUA_TFUNCTION.try_into().unwrap() {
                LuaType::Function(LuaFunction {
                    lua_ref,
                    chunk_name: None,
                })
            } else if type_id == cmach::LUA_TTABLE.try_into().unwrap() {
                LuaType::Table(lua_ref)
            } else if type_id == cmach::LUA_TUSERDATA.try_into().unwrap() {
                LuaType::Userdata(lua_ref)
            } else if type_id == cmach::LUA_TTHREAD.try_into().unwrap() {
                LuaType::Thread(lua_ref)
            } else {
                panic!("unexpected type {} on Lua stack", type_id)
            }
        };
        Ok(LuaDatum(lua_type))
    }
    /// Pop value from Lua stack and convert into `LuaDatum`
    ///
    /// NOTE: requires some free space on Lua stack for operation
    pub unsafe fn pop_datum<'b>(&'b self) -> Result<LuaDatum<'a, 'b, 'static>, MachineError> {
        let result = self.extract_datum(-1);
        cmach::lua_pop(self.lua_state(), 1);
        result
    }
    /// Push `LuaReference` on top of Lua stack
    pub unsafe fn push_reference<'b>(&'b self, lua_ref: &LuaReference<'a, 'b>) {
        cmach::lua_rawgeti(
            self.lua_state(),
            cmach::LUA_REGISTRYINDEX.try_into().unwrap(),
            self.get_reference_key(lua_ref) as cmach::lua_Integer,
        );
    }
    /// Push binary data on top of Lua stack
    ///
    /// NOTE: requires some free space on Lua stack for operation
    pub unsafe fn push_bin(&self, binary: &[u8]) -> Result<(), MachineError> {
        self.pop_error_with_backtrace(cmach::cmach_lua_pushlstring(
            self.lua_state(),
            binary.as_ptr() as *const _,
            cmach::lua_Integer::try_from(binary.len()).map_err(|_| MachineError {
                message: Some("lua_Integer too small to store length".to_string()),
                ..Default::default()
            })?,
        ))
    }
    /// Push string on top of Lua stack
    ///
    /// NOTE: requires some free space on Lua stack for operation
    pub unsafe fn push_str(&self, string: &str) -> Result<(), MachineError> {
        self.push_bin(string.as_ref())
    }
    /// Push `LuaDatum` on top of Lua stack
    ///
    /// NOTE: requires some free space on Lua stack for operation
    pub unsafe fn push_datum<'b, 'c>(
        &'b self,
        datum: &LuaDatum<'a, 'b, 'c>,
    ) -> Result<(), MachineError> {
        let l = self.lua_state();
        match datum.0 {
            LuaType::Nil => cmach::lua_pushnil(l),
            LuaType::Boolean(b) => cmach::lua_pushboolean(l, if b { 1 } else { 0 }),
            LuaType::Function(ref f) => self.push_reference(&f.lua_ref),
            LuaType::Float(x) => cmach::lua_pushnumber(l, x),
            LuaType::Integer(i) => cmach::lua_pushinteger(l, i),
            LuaType::String(ref s) => self.push_str(s)?,
            LuaType::Binary(ref b) => self.push_bin(b)?,
            LuaType::Table(ref lua_ref) => self.push_reference(lua_ref),
            LuaType::Userdata(ref lua_ref) => self.push_reference(lua_ref),
            LuaType::Thread(ref lua_ref) => self.push_reference(lua_ref),
        };
        Ok(())
    }
}

impl<'a, 'b> Function for LuaFunction<'a, 'b> {
    type Datum<'c> = LuaDatum<'a, 'b, 'c>;
    fn chunk_name(&self) -> Option<&str> {
        self.chunk_name.as_deref()
    }
    fn call<'c, A>(&self, args: A) -> Result<Vec<LuaDatum<'a, 'b, 'static>>, MachineError>
    where
        A: IntoIterator<Item = LuaDatum<'a, 'b, 'c>>,
        <A as IntoIterator>::IntoIter: ExactSizeIterator,
    {
        let args = args.into_iter();
        unsafe {
            let machine = self.lua_ref.as_ref().machine;
            machine.assert_empty_stack();
            let l = machine.lua_state();
            let old_top = cmach::lua_gettop(l);
            // TODO: use `try` expression here if/when `try_blocks` feature
            // becomes stable:
            let res = (|| {
                cmach::lua_pushcfunction(l, Some(cmach::cmach_lua_errmsgh));
                let base_top = old_top + 1;
                machine.push_reference(&self.lua_ref);
                let argc = c_int::try_from(args.len()).map_err(|_| MachineError {
                    message: Some("C integer too small to store argument count".to_string()),
                    ..Default::default()
                })?;
                let mut remaining_argc = argc;
                if cmach::lua_checkstack(l, argc) == 0 {
                    Err(MachineError {
                        message: Some(
                            "Lua stack exhausted while preparing function arguments".to_string(),
                        ),
                        chunk_name: self.chunk_name.clone(),
                        ..Default::default()
                    })?;
                }
                for arg in args {
                    if remaining_argc == 0 {
                        panic!("ExactSizeIterator for arguments provided more items than reported");
                    }
                    remaining_argc -= 1;
                    machine.push_datum(&arg)?;
                }
                machine.pop_error_with_backtrace(cmach::lua_pcall(
                    l,
                    argc,
                    cmach::LUA_MULTRET as c_int,
                    old_top + 1,
                ))?;
                // ensure there is still some space (10 elements) on Lua stack
                // (some extra space is required by `extract_datum`)
                if cmach::lua_checkstack(l, 10) == 0 {
                    Err(MachineError {
                        message: Some("Lua stack exhausted after calling function".to_string()),
                        chunk_name: self.chunk_name.clone(),
                        ..Default::default()
                    })?;
                }
                let new_top = cmach::lua_gettop(l);
                let mut retvals = Vec::with_capacity((new_top - base_top) as usize);
                for index in base_top..new_top {
                    retvals.push(machine.extract_datum(index + 1)?);
                }
                Ok(retvals)
            })();
            cmach::lua_settop(l, old_top);
            res
        }
    }
}

impl<'a> Compile<'a, String> for LuaMachine<'a> {
    fn compile(
        &self,
        chunk_name: Option<String>,
        code: String,
    ) -> Result<LuaFunction<'a, '_>, MachineError> {
        self.assert_empty_stack();
        let codelen = code.len();
        let code = CString::new(code).map_err(|err| MachineError {
            kind: MachineErrorKind::Syntax,
            message: Some(err.to_string()),
            ..Default::default()
        })?;
        let lua_name: Option<CString> = chunk_name.as_ref().map(|name| {
            let mut lua_name = String::with_capacity(name.len() + 2);
            lua_name.push('=');
            lua_name.push_str(name);
            CString::new(lua_name)
                .unwrap_or_else(|_| CString::new("(invalid name)".to_string()).unwrap())
        });
        let inner_ref = unsafe {
            let l = self.lua_state();
            self.pop_error(cmach::luaL_loadbufferx(
                self.lua_state(),
                code.as_ptr(),
                codelen
                    .try_into()
                    .expect("could not convert integer of code length"),
                // NOTE: taking reference (`&`) in following match is important for `.as_ptr()`!
                match &lua_name {
                    None => code.as_ptr(),
                    Some(n) => n.as_ptr(),
                },
                b"t\0".as_ptr() as *const _,
            ))?;
            let identity = cmach::lua_topointer(l, -1);
            self.pop_error(cmach::cmach_lua_ref(l))?;
            let key = cmach::lua_tointeger(l, -1) as LuaReferenceKey;
            cmach::lua_pop(l, 1);
            // `LuaInnerReference::drop` ensures cleanup
            LuaInnerReference {
                machine: &self,
                key,
                identity,
            }
        };
        Ok(LuaFunction {
            lua_ref: LuaReference::new(inner_ref),
            chunk_name,
        })
    }
}

impl<'a, 'b> Compile<'a, &'b str> for LuaMachine<'a> {
    fn compile(
        &self,
        chunk_name: Option<String>,
        code: &'b str,
    ) -> Result<LuaFunction<'a, '_>, MachineError> {
        self.compile(chunk_name, code.to_string())
    }
}

unsafe extern "C" fn callback_trampoline<F>(context: *mut c_void, nargs: c_int) -> c_int
where
    F: FnMut(c_int) -> c_int,
{
    let closure = &mut *(context as *mut F);
    closure(nargs)
}

fn get_callback_trampoline<F>(_closure: &F) -> cmach::cmach_callback_fptr
where
    F: FnMut(c_int) -> c_int,
{
    Some(callback_trampoline::<F>)
}

unsafe extern "C" fn release_trampoline<F>(context: *mut c_void)
where
    F: FnMut(c_int) -> c_int,
{
    drop(Box::from_raw(context as *mut F));
}

fn get_release_trampoline<F>(_closure: &F) -> cmach::cmach_callback_release_fptr
where
    F: FnMut(c_int) -> c_int,
{
    Some(release_trampoline::<F>)
}

impl<'a> Callback<'a> for LuaMachine<'a> {
    fn callback<'b, 'c, F, R>(&'b self, func: F) -> Result<LuaDatum<'a, 'b, 'static>, MachineError>
    where
        R: IntoIterator<Item = Self::Datum<'b, 'c>>,
        <R as IntoIterator>::IntoIter: ExactSizeIterator,
        F: 'a + Fn(Vec<LuaDatum<'a, 'b, 'static>>) -> Result<R, Box<dyn Error>>,
    {
        self.assert_empty_stack();
        let l = self.lua_state();
        let inner_closure = move |nargs| -> Result<c_int, MachineError> {
            unsafe {
                // ensure there is still some space (10 elements) on Lua stack
                // (some extra space is required by `extract_datum`)
                if cmach::lua_checkstack(l, 10) == 0 {
                    Err(MachineError {
                        message: Some("Lua stack exhausted".to_string()),
                        ..Default::default()
                    })?;
                }
                let mut args = Vec::with_capacity(nargs as usize);
                for i in 0..nargs {
                    args.push(self.extract_datum(i - nargs)?);
                }
                cmach::lua_pop(l, nargs);
                match func(args) {
                    Ok(retvals) => {
                        let retvals = retvals.into_iter();
                        // ensure there is still some space (number of
                        // return values + 10 elements) on Lua stack
                        // (some extra space is required by `push_datum`)
                        let retvalc = c_int::try_from(retvals.len()).map_err(|_| MachineError {
                            message: Some(
                                "C integer too small to store return value count".to_string(),
                            ),
                            ..Default::default()
                        })?;
                        let mut remaining_retvalc = retvalc;
                        let space = retvalc.checked_add(10).ok_or_else(|| MachineError {
                            message: Some("return value count too high".to_string()),
                            ..Default::default()
                        })?;
                        if cmach::lua_checkstack(l, space) == 0 {
                            Err(MachineError {
                                message: Some("Lua stack exhausted".to_string()),
                                ..Default::default()
                            })?;
                        }
                        for retval in retvals {
                            if remaining_retvalc == 0 {
                                panic!("ExactSizeIterator for arguments provided more items than reported");
                            }
                            remaining_retvalc -= 1;
                            self.push_datum(&retval)?;
                        }
                        Ok(retvalc)
                    }
                    Err(err) => {
                        self.push_str(&format!("callback returned error: {}", err))?;
                        Ok(-1)
                    }
                }
            }
        };
        let closure = move |nargs| match inner_closure(nargs) {
            Ok(retvals) => retvals,
            Err(err) => {
                let errstr = err.to_string();
                unsafe {
                    let (errstrptr, errlen) = match cmach::lua_Integer::try_from(errstr.len()) {
                        Ok(errlen) => (errstr.as_ptr(), errlen),
                        Err(_) => {
                            const TOOLONG: &'static str = "error message too long";
                            (TOOLONG.as_ptr(), TOOLONG.len() as cmach::lua_Integer)
                        }
                    };
                    cmach::cmach_lua_pushlstring(l, errstrptr as *const _, errlen);
                    // leave error on stack if `cmach_lua_pushlstring` threw error
                }
                -1
            }
        };
        let callback_trampoline = get_callback_trampoline(&closure);
        let release_trampoline = get_release_trampoline(&closure);
        let closure_ptr = Box::into_raw(Box::new(closure));
        unsafe {
            self.pop_error_with_backtrace(cmach::cmach_lua_pushclosure(
                l,
                closure_ptr as *mut c_void,
                callback_trampoline,
                release_trampoline,
            ))?;
            Ok(self.pop_datum()?)
        }
    }
}