lua_stdlib/

io_lib.rs

//! Standard I/O library — `io.*` functions and `file:*` methods.
//!
//! C source: `src/liolib.c` (841 lines, ~35 functions).
//!
//! PORT NOTE: This module necessarily requires file-system access. The PORTING.md
//! rule banning `std::fs` outside `lua-cli` conflicts with the crate assignment
//! (`lua-stdlib`). Every file-system call site carries a `TODO(port): std::fs`
//! marker. The architecture team must either relax the rule for this file, move
//! the module to `lua-cli`, or provide a thin IO-abstraction crate that wraps
//! `std::fs` under a permitted API.
//!
//! `popen` additionally requires `std::process::Command` and is stubbed.
//!
//! PORT NOTE: Rust's borrow checker prevents holding `&mut dyn LuaFileOps`
//! (extracted from userdata) and `&mut LuaState` simultaneously. The affected
//! functions (`io_read`, `f_read`, `io_write`, `f_write`, `io_flush`, `f_flush`,
//! `f_seek`, `f_setvbuf`, `get_io_file`) are marked with `TODO(port): borrow
//! split`. Phase B must restructure `g_read`/`g_write` to take a `StackIdx`
//! rather than a raw `&mut dyn LuaFileOps`, and use `RefCell` inside `LStream`
//! for interior mutability, or extract the file handle via a separate borrow
//! scope.

use std::cell::RefCell;
use std::collections::HashMap;
use std::io::{self, SeekFrom};
use std::rc::Rc;

use lua_types::{LuaError, LuaFileHandle, LuaType, LuaValue};
use crate::state_stub::{LuaState, LuaStateStubExt as _, lua_CFunction, upvalue_index, CompareOp, LuaDebug};

thread_local! {
    /// Side-table mapping userdata identity (the `Rc` pointer address from
    /// `GcRef::identity()`) to its associated `LStream`. The C port stores
    /// `LStream` directly inside the userdata payload; Rust cannot do that
    /// safely because `LStream` carries heap pointers (a `Box<dyn LuaFileOps>`
    /// and a fn pointer). Entries are inserted by `new_pre_file` and never
    /// removed in Phase A-C — leak is intentional per `PORTING.md` §2 #4.
    static LSTREAM_REGISTRY: RefCell<HashMap<usize, Rc<RefCell<LStream>>>>
        = RefCell::new(HashMap::new());
}

fn register_lstream(ud_id: usize, lstream: LStream) -> Rc<RefCell<LStream>> {
    let cell = Rc::new(RefCell::new(lstream));
    LSTREAM_REGISTRY.with(|reg| {
        reg.borrow_mut().insert(ud_id, cell.clone());
    });
    cell
}

fn lookup_lstream(ud_id: usize) -> Option<Rc<RefCell<LStream>>> {
    LSTREAM_REGISTRY.with(|reg| reg.borrow().get(&ud_id).cloned())
}

// ── Constants ────────────────────────────────────────────────────────────────

/// Name of the file-handle metatable in the Lua registry. C: `LUA_FILEHANDLE`.
pub const LUA_FILE_HANDLE: &[u8] = b"FILE*";

/// Registry key for the default input file. C: `IO_INPUT` = `"_IO_input"`.
const IO_INPUT_KEY: &[u8] = b"_IO_input";

/// Registry key for the default output file. C: `IO_OUTPUT` = `"_IO_output"`.
const IO_OUTPUT_KEY: &[u8] = b"_IO_output";

/// Number of bytes in the `"_IO_"` prefix, used to strip it in error messages.
const IO_PREFIX_LEN: usize = 4;

/// Maximum number of format-arguments passed to `file:lines`. C: `MAXARGLINE`.
const MAX_ARG_LINE: usize = 250;

/// Maximum byte-length of a numeric literal read from a file. C: `L_MAXLENNUM`.
const L_MAX_LEN_NUM: usize = 200;

/// End-of-file sentinel returned by `LuaFileOps::read_byte`. C: `EOF` == -1.
const EOF_SENTINEL: i32 = -1;

/// Bulk-read chunk size, mirroring C's `LUAL_BUFFERSIZE`.
const LUAL_BUFFER_SIZE: usize = 8192;

// ── Traits ───────────────────────────────────────────────────────────────────

/// Capabilities required by the io library from an OS file handle.
///
/// This trait extends [`LuaFileHandle`] (defined in `lua-types`) with the
/// additional `set_buf_mode` operation. Concrete implementations backed by
/// `std::fs::File` live in `lua-cli`; standard-stream implementations live in
/// this module. The split keeps `std::fs` out of `lua-stdlib` per PORTING.md §1.
pub trait LuaFileOps: LuaFileHandle {
    /// Control stream buffering. C: `setvbuf`.
    fn set_buf_mode(&mut self, mode: BufMode, size: usize) -> io::Result<()>;
}

// ── Enums ────────────────────────────────────────────────────────────────────

/// Seek anchor for `file:seek`. C: `{SEEK_SET, SEEK_CUR, SEEK_END}`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SeekWhence {
    Set,
    Cur,
    End,
}

/// Buffering mode for `file:setvbuf`. C: `{_IONBF, _IOFBF, _IOLBF}`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum BufMode {
    No,
    Full,
    Line,
}

/// Which standard stream to wrap in `create_std_file`.
pub enum StdFileKind {
    Stdin,
    Stdout,
    Stderr,
}

// ── Structs ──────────────────────────────────────────────────────────────────

/// Lua file handle stored as the typed payload of a `LuaUserData`.
///
/// C equivalent: `typedef luaL_Stream LStream` in `liolib.c`.
///
/// TODO(port): Phase B must arrange for `LStream` to live inside
/// `LuaUserData`'s opaque payload. The userdata system needs a typed-access
/// API, e.g. `state.check_arg_typed_userdata::<LStream>(1, LUA_FILE_HANDLE)?`.
///
/// TODO(port): `file` must be `Option<RefCell<Box<dyn LuaFileOps>>>` to allow
/// interior-mutability borrow splitting between the file handle and `LuaState`.
pub struct LStream {
    /// OS file handle. `None` = incompletely opened (pre-file pattern).
    /// Concrete implementations are installed via `GlobalState::file_open_hook`
    /// (registered by `lua-cli`) to keep `std::fs` out of `lua-stdlib`.
    pub file: Option<Box<dyn LuaFileHandle>>,
    /// Close callback. `None` means the stream is closed. C: `p->closef == NULL`.
    pub close_fn: Option<fn(&mut LuaState) -> Result<usize, LuaError>>,
}

impl LStream {
    /// `isclosed(p)` in C: true when `closef` is NULL.
    pub fn is_closed(&self) -> bool {
        self.close_fn.is_none()
    }
}

/// Minimal `LuaFileOps` placeholder for stdin/stdout/stderr while real
/// std::io wiring is deferred. All read/write/seek operations return
/// `Unsupported`, which is sufficient for the validation paths exercised
/// by `io.input(io.stdin)`, `io.output(io.stdout)`, and `io.type`.
struct StdStreamHandle {
    kind: StdFileKind,
}

impl LuaFileHandle for StdStreamHandle {
    fn read_byte(&mut self) -> i32 {
        use std::io::Read;
        match self.kind {
            StdFileKind::Stdin => {
                let mut buf = [0u8; 1];
                match std::io::stdin().read(&mut buf) {
                    Ok(1) => buf[0] as i32,
                    _ => EOF_SENTINEL,
                }
            }
            _ => EOF_SENTINEL,
        }
    }
    fn unread_byte(&mut self, _byte: i32) {}
    fn write_bytes(&mut self, data: &[u8]) -> io::Result<usize> {
        use std::io::Write;
        match self.kind {
            StdFileKind::Stderr => {
                std::io::stderr().write_all(data)?;
                Ok(data.len())
            }
            _ => {
                std::io::stdout().write_all(data)?;
                Ok(data.len())
            }
        }
    }
    fn flush(&mut self) -> io::Result<()> {
        use std::io::Write;
        match self.kind {
            StdFileKind::Stderr => std::io::stderr().flush(),
            _ => std::io::stdout().flush(),
        }
    }
    fn seek(&mut self, _pos: SeekFrom) -> io::Result<u64> {
        Err(io::Error::new(io::ErrorKind::Unsupported, "stdio seek"))
    }
    fn tell(&mut self) -> io::Result<u64> {
        Err(io::Error::new(io::ErrorKind::Unsupported, "stdio tell"))
    }
    fn clear_error(&mut self) {}
    fn has_error(&self) -> bool { false }
}

impl LuaFileOps for StdStreamHandle {
    fn set_buf_mode(&mut self, _mode: BufMode, _size: usize) -> io::Result<()> { Ok(()) }
}

impl StdStreamHandle {
    fn new(kind: StdFileKind) -> Self { StdStreamHandle { kind } }
}

/// State machine for reading a numeric literal byte-by-byte from a file.
struct ReadNumState {
    /// Current look-ahead byte, or `EOF_SENTINEL`.
    current: i32,
    /// Number of bytes accumulated in `buf`.
    count: usize,
    /// Accumulated characters of the numeral (NUL-terminated on finalise).
    buf: [u8; L_MAX_LEN_NUM + 1],
}

impl ReadNumState {
    fn new(first_byte: i32) -> Self {
        ReadNumState {
            current: first_byte,
            count: 0,
            buf: [0u8; L_MAX_LEN_NUM + 1],
        }
    }

    /// Save current char to `buf` and read the next byte from `file`.
    /// Returns `false` if the buffer is full (numeral too long). C: `nextc`.
    fn advance(&mut self, file: &mut dyn LuaFileHandle) -> bool {
        if self.count >= L_MAX_LEN_NUM {
            self.buf[0] = 0;
            return false;
        }
        self.buf[self.count] = self.current as u8;
        self.count += 1;
        self.current = file.read_byte();
        true
    }

    /// Accept current char if it equals either byte in `set`. C: `test2`.
    fn try2(&mut self, file: &mut dyn LuaFileHandle, set: [u8; 2]) -> bool {
        if self.current == set[0] as i32 || self.current == set[1] as i32 {
            self.advance(file)
        } else {
            false
        }
    }

    /// Consume a run of (hex)digits; return the count. C: `readdigits`.
    fn read_digits(&mut self, file: &mut dyn LuaFileHandle, hex: bool) -> usize {
        let mut count = 0usize;
        loop {
            let is_digit = if hex {
                (self.current as u8).is_ascii_hexdigit()
            } else {
                (self.current as u8).is_ascii_digit()
            };
            if !is_digit || self.current == EOF_SENTINEL {
                break;
            }
            if !self.advance(file) {
                break;
            }
            count += 1;
        }
        count
    }

    /// Return the accumulated bytes (without the NUL terminator).
    fn as_bytes(&self) -> &[u8] {
        &self.buf[..self.count]
    }
}

// ── Function registration tables ─────────────────────────────────────────────

/// `io.*` module functions. C: `static const luaL_Reg iolib[]`.
pub const IO_LIB: &[(&[u8], fn(&mut LuaState) -> Result<usize, LuaError>)] = &[
    (b"close",   io_close),
    (b"flush",   io_flush),
    (b"input",   io_input),
    (b"lines",   io_lines),
    (b"open",    io_open),
    (b"output",  io_output),
    (b"popen",   io_popen),
    (b"read",    io_read),
    (b"tmpfile", io_tmpfile),
    (b"type",    io_type),
    (b"write",   io_write),
];

/// `file:*` instance methods. C: `static const luaL_Reg meth[]`.
pub const FILE_METHODS: &[(&[u8], fn(&mut LuaState) -> Result<usize, LuaError>)] = &[
    (b"read",    f_read),
    (b"write",   f_write),
    (b"lines",   f_lines),
    (b"flush",   f_flush),
    (b"seek",    f_seek),
    (b"close",   f_close),
    (b"setvbuf", f_setvbuf),
];

/// File-handle metamethods. C: `static const luaL_Reg metameth[]`.
pub const FILE_METAMETHODS: &[(&[u8], fn(&mut LuaState) -> Result<usize, LuaError>)] = &[
    (b"__gc",       f_gc),
    (b"__close",    f_gc),
    (b"__tostring", f_tostring),
];

// ── Helpers ──────────────────────────────────────────────────────────────────

/// Validate an `fopen` mode string: must match `[rwa]\+?b*`. C: `l_checkmode`.
///
///           (*mode != '+' || ...) && strspn(mode, "b") == strlen(mode));`
fn check_mode(mode: &[u8]) -> bool {
    if mode.is_empty() {
        return false;
    }
    let mut idx = 0usize;
    if !matches!(mode[idx], b'r' | b'w' | b'a') {
        return false;
    }
    idx += 1;
    if idx < mode.len() && mode[idx] == b'+' {
        idx += 1;
    }
    mode[idx..].iter().all(|&b| b == b'b')
}

/// Validate a `popen` mode string: only `"r"` or `"w"`. C: `l_checkmodep`.
fn check_mode_popen(mode: &[u8]) -> bool {
    matches!(mode, b"r" | b"w")
}

/// Push success (`true`) or failure (`false`, msg, errno) per `luaL_fileresult`.
///
///     else { luaL_pushfail; pushstring(msg); pushinteger(errno); return 3; }`
fn file_result(
    state: &mut LuaState,
    success: bool,
    fname: Option<&[u8]>,
    os_err: io::Error,
) -> Result<usize, LuaError> {
    if success {
        state.push(LuaValue::Bool(true));
        return Ok(1);
    }
    state.push(LuaValue::Bool(false));
    let msg = os_err.to_string();
    match fname {
        Some(name) => {
            let mut s = Vec::with_capacity(name.len() + 2 + msg.len());
            s.extend_from_slice(name);
            s.extend_from_slice(b": ");
            s.extend_from_slice(msg.as_bytes());
            state.push_string(&s);
        }
        None => {
            state.push_string(msg.as_bytes());
        }
    }
    let errno_code = os_err.raw_os_error().unwrap_or(0) as i64;
    state.push(LuaValue::Int(errno_code));
    Ok(3)
}

/// Push popen/system exit-status results per `luaL_execresult`.
///
///     else { luaL_pushfail; pushlstring("exit"|"signal"); pushinteger(stat); return 3; }`
///
/// TODO(port): POSIX `WIFEXITED`/`WTERMSIG` macros not available on all platforms;
/// this stub always treats non-zero stat as an exit code.
fn exec_result(state: &mut LuaState, stat: i32) -> Result<usize, LuaError> {
    if stat == 0 {
        state.push(LuaValue::Bool(true));
        Ok(1)
    } else {
        state.push(LuaValue::Bool(false));
        // TODO(port): distinguish exit vs signal via POSIX macros
        state.push_string(b"exit");
        state.push(LuaValue::Int(stat as i64));
        Ok(3)
    }
}

/// Retrieve `LStream` from argument 1 via a userdata type-check.
///
/// Returns an `Rc<RefCell<LStream>>` from the side-table registry. The C port
/// returns a raw `LStream *` pointing into the userdata payload; Rust uses a
/// side table because `LStream` contains heap pointers that cannot be safely
/// reinterpreted from a raw byte buffer in safe Rust.
fn get_lstream(state: &mut LuaState) -> Result<Rc<RefCell<LStream>>, LuaError> {
    let ud = state.check_arg_userdata(1, LUA_FILE_HANDLE)?;
    lookup_lstream(ud.identity()).ok_or_else(|| {
        LuaError::runtime(format_args!("invalid file handle"))
    })
}

/// Look up the `LStream` registered for the userdata sitting at upvalue `idx`.
///
/// `aux_lines` stores the file-handle userdata as upvalue 1 of `io_readline`;
/// this helper performs the same registry round-trip that `get_lstream` does
/// for argument 1, but reads the value from the closure's upvalue slot instead
/// of the call stack.
fn lstream_from_upvalue(
    state: &mut LuaState,
    idx: i32,
) -> Result<Rc<RefCell<LStream>>, LuaError> {
    let v = state.value_at(crate::state_stub::upvalue_index(idx));
    let ud_id = match v {
        LuaValue::UserData(ud) => ud.identity(),
        _ => {
            return Err(LuaError::runtime(format_args!(
                "invalid file handle in upvalue {}",
                idx
            )));
        }
    };
    lookup_lstream(ud_id).ok_or_else(|| {
        LuaError::runtime(format_args!("invalid file handle in upvalue {}", idx))
    })
}

/// Validate that argument 1 is an open file handle; error if closed.
fn tofile(state: &mut LuaState) -> Result<Rc<RefCell<LStream>>, LuaError> {
    let p_rc = get_lstream(state)?;
    {
        let p = p_rc.borrow();
        if p.is_closed() {
            return Err(LuaError::runtime(format_args!(
                "attempt to use a closed file"
            )));
        }
        debug_assert!(p.file.is_some());
    }
    Ok(p_rc)
}

// ── File creation helpers ────────────────────────────────────────────────────

/// Allocate a "closed" file-handle userdata and push it; set its metatable.
/// Also registers an empty `LStream` in the side table keyed by the userdata
/// identity, and returns the `Rc<RefCell<LStream>>` so the caller may finish
/// initialising it (set `file`, set `close_fn`). C: `newprefile(L)`.
fn new_pre_file(state: &mut LuaState) -> Result<Rc<RefCell<LStream>>, LuaError> {
    let ud = state.new_userdata_typed(LUA_FILE_HANDLE, std::mem::size_of::<LStream>(), 0)?;
    state.set_metatable_by_name(LUA_FILE_HANDLE)?;
    let cell = register_lstream(ud.identity(), LStream { file: None, close_fn: None });
    Ok(cell)
}

/// Allocate a new regular-file handle with `io_fclose` as the close function.
fn new_file(state: &mut LuaState) -> Result<Rc<RefCell<LStream>>, LuaError> {
    let cell = new_pre_file(state)?;
    cell.borrow_mut().close_fn = Some(io_fclose);
    Ok(cell)
}

/// Open `fname` and push its handle; raise a runtime error on failure.
///
/// The file system is reached via `GlobalState::file_open_hook` (registered by
/// `lua-cli`) since `std::fs` is banned in `lua-stdlib` per PORTING.md §1.
fn opencheck(state: &mut LuaState, fname: &[u8], mode: &[u8]) -> Result<(), LuaError> {
    let hook = state.global().file_open_hook;
    let fh = match hook {
        Some(open_fn) => open_fn(fname, mode).map_err(|e| {
            LuaError::runtime(format_args!(
                "cannot open file '{}' ({})",
                fname.escape_ascii(),
                match &e {
                    LuaError::Runtime(LuaValue::Str(s)) => {
                        String::from_utf8_lossy(s.as_bytes()).into_owned()
                    }
                    other => format!("{:?}", other),
                }
            ))
        })?,
        None => {
            return Err(LuaError::runtime(format_args!(
                "cannot open file '{}' (no filesystem hook registered)",
                fname.escape_ascii()
            )));
        }
    };
    let cell = new_file(state)?;
    cell.borrow_mut().file = Some(fh);
    Ok(())
}

// ── Close functions ──────────────────────────────────────────────────────────

/// Close a regular file via `fclose`. C: `io_fclose`.
///
/// TODO(port): flush + drop `Box<dyn LuaFileOps>`, map io::Error to file_result.
fn io_fclose(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    // TODO(port): actually flush then drop p.file, capture any error
    let _closed = p_rc.borrow_mut().file.take();
    state.push(LuaValue::Bool(true));
    Ok(1)
}

/// Close a popen process pipe. C: `io_pclose`.
///
/// TODO(port): std::process::Child — popen not yet implemented.
fn io_pclose(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    let _closed = p_rc.borrow_mut().file.take();
    // TODO(port): wait on the child process and forward its exit code
    exec_result(state, 0)
}

/// Refuse to close a standard-stream handle. C: `io_noclose`.
fn io_noclose(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    p_rc.borrow_mut().close_fn = Some(io_noclose); // reinstall to keep the handle alive
    state.push(LuaValue::Bool(false));
    state.push_string(b"cannot close standard file");
    Ok(2)
}

/// Invoke the stream's close function and mark it closed. C: `aux_close`.
fn aux_close(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    let cf = p_rc.borrow_mut().close_fn.take().ok_or_else(|| {
        LuaError::runtime(format_args!("attempt to close an already-closed file"))
    })?;
    cf(state)
}

// ── io.type ──────────────────────────────────────────────────────────────────

/// `io.type(x)` — return `"file"`, `"closed file"`, or `false`. C: `io_type`.
pub fn io_type(state: &mut LuaState) -> Result<usize, LuaError> {
    state.check_arg_any(1)?;
    let maybe_userdata = state.test_arg_userdata(1, LUA_FILE_HANDLE);
    match maybe_userdata {
        None => {
            state.push(LuaValue::Bool(false));
        }
        Some(ud) => {
            let is_closed = match lookup_lstream(ud.identity()) {
                Some(rc) => rc.borrow().is_closed(),
                None => true, // unknown userdata with FILE* metatable: treat as closed
            };
            if is_closed {
                state.push_string(b"closed file");
            } else {
                state.push_string(b"file");
            }
        }
    }
    Ok(1)
}

// ── __tostring metamethod ────────────────────────────────────────────────────

/// `tostring(file)` metamethod. C: `f_tostring`.
fn f_tostring(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    let closed = p_rc.borrow().is_closed();
    if closed {
        state.push_string(b"file (closed)");
    } else {
        // TODO(port): pointer-address representation for the file handle
        state.push_string(b"file (0x?)");
    }
    Ok(1)
}

// ── close / gc ───────────────────────────────────────────────────────────────

/// `file:close()`. C: `f_close`.
fn f_close(state: &mut LuaState) -> Result<usize, LuaError> {
    let _ = tofile(state)?; // validates stream is open before closing
    aux_close(state)
}

/// `io.close([file])`. C: `io_close`.
pub fn io_close(state: &mut LuaState) -> Result<usize, LuaError> {
    // The pushed value naturally lands at position 1 (top advances by one from
    // func+1 to func+2). The C source does NOT call lua_replace here; adding one
    // would pop the value back out, since position 1 equals top-1 in this case.
    if state.type_at(1) == LuaType::None {
        state.registry_get(IO_OUTPUT_KEY)?;
    }
    f_close(state)
}

/// `__gc` / `__close` metamethod — silently close if still open. C: `f_gc`.
fn f_gc(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_lstream(state)?;
    let needs_close = {
        let p = p_rc.borrow();
        !p.is_closed() && p.file.is_some()
    };
    if needs_close {
        // ignore any error from aux_close during GC finalisation
        let _ = aux_close(state);
    }
    Ok(0)
}

// ── io.open / io.popen / io.tmpfile ─────────────────────────────────────────

/// `io.open(filename [, mode])`. C: `io_open`.
///
/// The file system is reached via `GlobalState::file_open_hook` (registered by
/// `lua-cli`) since `std::fs` is banned in `lua-stdlib` per PORTING.md §1.
pub fn io_open(state: &mut LuaState) -> Result<usize, LuaError> {
    let filename: Vec<u8> = state.check_arg_string(1)?;
    let mode: Vec<u8> = state.opt_arg_string(2, b"r")?;
    if !check_mode(&mode) {
        return Err(LuaError::arg_error(2, "invalid mode"));
    }
    let hook = state.global().file_open_hook;
    match hook {
        Some(open_fn) => match open_fn(&filename, &mode) {
            Ok(fh) => {
                let cell = new_file(state)?;
                cell.borrow_mut().file = Some(fh);
                Ok(1)
            }
            Err(e) => {
                let os_err = io::Error::new(
                    io::ErrorKind::Other,
                    match &e {
                        LuaError::Runtime(LuaValue::Str(s)) => {
                            String::from_utf8_lossy(s.as_bytes()).into_owned()
                        }
                        other => format!("{:?}", other),
                    },
                );
                file_result(state, false, Some(&filename), os_err)
            }
        },
        None => {
            let os_err = io::Error::new(
                io::ErrorKind::Unsupported,
                "no filesystem hook registered",
            );
            file_result(state, false, Some(&filename), os_err)
        }
    }
}

/// `io.popen(filename [, mode])`. C: `io_popen`.
///
/// `std::process::Command` is banned in `lua-stdlib`; the child process is
/// spawned via `GlobalState::popen_hook`, which `lua-cli` installs. When the
/// hook is absent (sandboxed embeddings), this returns a clean Lua failure
/// shape (`nil, errmsg, errno`) rather than panicking, so clients such as
/// LuaRocks that probe `io.popen` fall back gracefully instead of crashing
/// the host.
pub fn io_popen(state: &mut LuaState) -> Result<usize, LuaError> {
    let filename: Vec<u8> = state.check_arg_string(1)?;
    let mode: Vec<u8> = state.opt_arg_string(2, b"r")?;
    if !check_mode_popen(&mode) {
        return Err(LuaError::arg_error(2, "invalid mode"));
    }
    let hook = state.global().popen_hook;
    match hook {
        Some(spawn_fn) => match spawn_fn(&filename, &mode) {
            Ok(fh) => {
                let cell = new_pre_file(state)?;
                let mut p = cell.borrow_mut();
                p.file = Some(fh);
                p.close_fn = Some(io_pclose);
                drop(p);
                Ok(1)
            }
            Err(e) => {
                let os_err = io::Error::new(
                    io::ErrorKind::Other,
                    match &e {
                        LuaError::Runtime(LuaValue::Str(s)) => {
                            String::from_utf8_lossy(s.as_bytes()).into_owned()
                        }
                        other => format!("{:?}", other),
                    },
                );
                file_result(state, false, Some(&filename), os_err)
            }
        },
        None => {
            let os_err = io::Error::new(
                io::ErrorKind::Unsupported,
                "popen not enabled in this build",
            );
            file_result(state, false, Some(&filename), os_err)
        }
    }
}

/// `io.tmpfile()`. C: `io_tmpfile`.
pub fn io_tmpfile(state: &mut LuaState) -> Result<usize, LuaError> {
    let hook = state.global().file_open_hook;
    let Some(open_fn) = hook else {
        let os_err = io::Error::new(
            io::ErrorKind::Unsupported,
            "no filesystem hook registered",
        );
        return file_result(state, false, None, os_err);
    };

    let mut path = std::env::temp_dir().to_string_lossy().as_bytes().to_vec();
    if path.last().copied() != Some(b'/') && path.last().copied() != Some(b'\\') {
        path.push(b'/');
    }
    let unique = format!(
        "lua_tmpfile_{}_{}",
        std::process::id(),
        std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .map(|d| d.as_nanos())
            .unwrap_or(0)
    );
    path.extend_from_slice(unique.as_bytes());

    match open_fn(&path, b"w+b") {
        Ok(fh) => {
            let cell = new_file(state)?;
            cell.borrow_mut().file = Some(fh);
            Ok(1)
        }
        Err(e) => {
            let os_err = io::Error::new(
                io::ErrorKind::Other,
                match &e {
                    LuaError::Runtime(LuaValue::Str(s)) => {
                        String::from_utf8_lossy(s.as_bytes()).into_owned()
                    }
                    other => format!("{:?}", other),
                },
            );
            file_result(state, false, None, os_err)
        }
    }
}

// ── io.input / io.output ─────────────────────────────────────────────────────

/// Retrieve the current default IO file from the registry; error if closed.
///
/// TODO(port): borrow split — returns `&mut dyn LuaFileHandle` while caller also
/// needs `&mut LuaState`. Phase B: use `RefCell` inside `LStream`.
fn get_io_file<'a>(
    state: &'a mut LuaState,
    key: &[u8],
) -> Result<&'a mut dyn LuaFileHandle, LuaError> {
    state.registry_get(key)?;
    // TODO(port): extract &mut LStream from the registry value's userdata payload
    let label = &key[IO_PREFIX_LEN..]; // strip "_IO_" for the error message
    let p: &mut LStream = todo!("TODO(port): extract LStream from registry userdata");
    if p.is_closed() {
        return Err(LuaError::runtime(format_args!(
            "default {} file is closed",
            label.escape_ascii()
        )));
    }
    Ok(p.file.as_mut().expect("open stream has no file handle").as_mut())
}

/// Generic setter/getter for `io.input` and `io.output`. C: `g_iofile`.
fn g_iofile(state: &mut LuaState, key: &[u8], mode: &[u8]) -> Result<usize, LuaError> {
    if !matches!(state.type_at(1), LuaType::None | LuaType::Nil) {
        if state.type_at(1) == LuaType::String {
            let filename = state.check_arg_string(1)?;
            opencheck(state, &filename, mode)?;
        } else {
            let _ = tofile(state)?;
            state.push_value_at(1);
        }
        state.registry_set(key)?;
    }
    state.registry_get(key)?;
    Ok(1)
}

/// `io.input([file])`. C: `io_input`.
pub fn io_input(state: &mut LuaState) -> Result<usize, LuaError> {
    g_iofile(state, IO_INPUT_KEY, b"r")
}

/// `io.output([file])`. C: `io_output`.
pub fn io_output(state: &mut LuaState) -> Result<usize, LuaError> {
    g_iofile(state, IO_OUTPUT_KEY, b"w")
}

// ── Read helpers ─────────────────────────────────────────────────────────────

/// Read a numeric literal from `file` into an owned byte buffer.
fn read_number_bytes(file: &mut dyn LuaFileHandle) -> Vec<u8> {
    let first = loop {
        let b = file.read_byte();
        if b == EOF_SENTINEL || !(b as u8).is_ascii_whitespace() {
            break b;
        }
    };

    let mut rn = ReadNumState::new(first);

    rn.try2(file, [b'-', b'+']);

    let mut count: usize = 0;
    let hex = if rn.try2(file, [b'0', b'0']) {
        if rn.try2(file, [b'x', b'X']) {
            true
        } else {
            count = 1;
            false
        }
    } else {
        false
    };

    count += rn.read_digits(file, hex);

    // TODO(port): locale decimal-point character; defaulting to '.'
    let dec_point = b'.';
    if rn.try2(file, [dec_point, b'.']) {
        count += rn.read_digits(file, hex);
    }

    if count > 0 {
        let exp_chars = if hex { [b'p', b'P'] } else { [b'e', b'E'] };
        if rn.try2(file, exp_chars) {
            rn.try2(file, [b'-', b'+']);
            rn.read_digits(file, false);
        }
    }

    file.unread_byte(rn.current);
    rn.as_bytes().to_vec()
}

/// Peek for EOF: returns `true` if more input is available. C: `test_eof`
/// (the file-only half — caller still pushes `""` regardless).
fn test_eof(file: &mut dyn LuaFileHandle) -> bool {
    let c = file.read_byte();
    if c != EOF_SENTINEL {
        file.unread_byte(c);
    }
    c != EOF_SENTINEL
}

/// Read one line from `file` into an owned buffer. Returns `(bytes, had_content)`.
/// If `chop` is true the trailing `\n` is stripped. C: `read_line(L, f, chop)`.
///
/// PERF(port): C uses luaL_prepbuffer (large fixed stack buffer) to avoid
/// per-byte allocation; Rust's Vec grows here, which is slightly slower.
fn read_line(file: &mut dyn LuaFileHandle, chop: bool) -> (Vec<u8>, bool) {
    let mut buf: Vec<u8> = Vec::new();
    let mut c: i32 = EOF_SENTINEL;

    //          while (i < LUAL_BUFFERSIZE && (c = l_getc(f)) != EOF && c != '\n')
    //            buff[i++] = c;
    //          luaL_addsize(&b, i);
    //    } while (c != EOF && c != '\n');
    'outer: loop {
        for _ in 0..LUAL_BUFFER_SIZE {
            c = file.read_byte();
            if c == EOF_SENTINEL || c == b'\n' as i32 {
                break 'outer;
            }
            buf.push(c as u8);
        }
        // chunk full but no newline/EOF yet — continue reading
    }

    if !chop && c == b'\n' as i32 {
        buf.push(b'\n');
    }

    let had_content = c == b'\n' as i32 || !buf.is_empty();
    (buf, had_content)
}

/// Read the entire file into an owned buffer. C: `read_all(L, f)` (file-only half).
///
/// PERF(port): C uses `fread` with a large buffer; Rust reads byte-by-byte via
/// `LuaFileOps::read_byte`. Phase B should add `read_chunk(&mut buf)` to the
/// trait for bulk reads.
fn read_all(file: &mut dyn LuaFileHandle) -> Vec<u8> {
    let mut buf: Vec<u8> = Vec::new();
    loop {
        let mut chunk_read = 0usize;
        for _ in 0..LUAL_BUFFER_SIZE {
            let b = file.read_byte();
            if b == EOF_SENTINEL {
                break;
            }
            buf.push(b as u8);
            chunk_read += 1;
        }
        if chunk_read < LUAL_BUFFER_SIZE {
            break;
        }
    }
    buf
}

/// Read at most `n` bytes from `file`. Returns `(bytes, had_content)`.
fn read_chars(file: &mut dyn LuaFileHandle, n: usize) -> (Vec<u8>, bool) {
    let mut buf = Vec::with_capacity(n);
    for _ in 0..n {
        let b = file.read_byte();
        if b == EOF_SENTINEL {
            break;
        }
        buf.push(b as u8);
    }
    let nr = buf.len();
    (buf, nr > 0)
}

/// Dispatch one or more read formats; push results. C: `g_read(L, f, first)`.
///
/// Takes an `Rc<RefCell<LStream>>` so each I/O step can borrow the file briefly,
/// release the borrow, then push the result to `state`. This is the "collect
/// then borrow" pattern that resolves the `&mut state` vs `&mut file` conflict.
fn g_read(
    state: &mut LuaState,
    p_rc: &Rc<RefCell<LStream>>,
    first: i32,
) -> Result<usize, LuaError> {
    //
    // In C, `getiofile` leaves the default stream on the stack, so subtracting
    // one skips that extra value. This Rust port resolves registry streams into
    // an Rc and pops the registry value before reaching `g_read`, so count the
    // read formats directly from `first`.
    let nargs = (state.top() - first + 1).max(0);
    let mut n = first;
    let mut success = true;

    {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        fh.clear_error();
    }

    if nargs == 0 {
        let (bytes, had) = {
            let mut p = p_rc.borrow_mut();
            let fh = p.file.as_deref_mut().expect("open stream has no file handle");
            read_line(fh, true)
        };
        state.push_string(&bytes)?;
        success = had;
        n = first + 1;
    } else {
        state.ensure_stack((nargs as i32) + 20, "too many arguments")?;
        let mut remaining = nargs;
        while remaining > 0 && success {
            if state.type_at(n) == LuaType::Number {
                let l = state.check_arg_integer(n)? as usize;
                if l == 0 {
                    let not_eof = {
                        let mut p = p_rc.borrow_mut();
                        let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                        test_eof(fh)
                    };
                    state.push_string(b"")?;
                    success = not_eof;
                } else {
                    let (bytes, had) = {
                        let mut p = p_rc.borrow_mut();
                        let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                        read_chars(fh, l)
                    };
                    state.push_string(&bytes)?;
                    success = had;
                }
            } else {
                let s: Vec<u8> = state.check_arg_string(n)?;
                let pp: &[u8] = if s.first() == Some(&b'*') { &s[1..] } else { &s[..] };
                match pp.first() {
                    Some(&b'n') => {
                        let bytes = {
                            let mut p = p_rc.borrow_mut();
                            let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                            read_number_bytes(fh)
                        };
                        let pushed = state.string_to_number_push(&bytes)?;
                        if pushed != 0 {
                            success = true;
                        } else {
                            state.push(LuaValue::Nil);
                            success = false;
                        }
                    }
                    Some(&b'l') => {
                        let (bytes, had) = {
                            let mut p = p_rc.borrow_mut();
                            let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                            read_line(fh, true)
                        };
                        state.push_string(&bytes)?;
                        success = had;
                    }
                    Some(&b'L') => {
                        let (bytes, had) = {
                            let mut p = p_rc.borrow_mut();
                            let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                            read_line(fh, false)
                        };
                        state.push_string(&bytes)?;
                        success = had;
                    }
                    Some(&b'a') => {
                        let bytes = {
                            let mut p = p_rc.borrow_mut();
                            let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                            read_all(fh)
                        };
                        state.push_string(&bytes)?;
                        success = true;
                    }
                    _ => {
                        return Err(LuaError::arg_error(n, "invalid format"));
                    }
                }
            }
            n += 1;
            remaining -= 1;
        }
    }

    let has_err = {
        let p = p_rc.borrow();
        match p.file.as_deref() {
            Some(fh) => fh.has_error(),
            None => false,
        }
    };
    if has_err {
        let err = {
            let p = p_rc.borrow();
            match p.file.as_deref().and_then(|fh| fh.last_error_info()) {
                Some((code, _msg)) if code != 0 => io::Error::from_raw_os_error(code),
                Some((_code, msg)) => io::Error::new(io::ErrorKind::Other, msg),
                None => io::Error::new(io::ErrorKind::Other, "file read error"),
            }
        };
        return file_result(
            state,
            false,
            None,
            err,
        );
    }

    if !success {
        state.pop_n(1);
        state.push(LuaValue::Nil);
    }

    Ok((n - first) as usize)
}

/// Resolve the registry-default I/O file (IO_INPUT / IO_OUTPUT) into its
/// backing `Rc<RefCell<LStream>>`. Errors if the slot holds a closed handle
/// or a value that is not a registered file userdata.
///
fn get_io_file_rc(state: &mut LuaState, key: &[u8]) -> Result<Rc<RefCell<LStream>>, LuaError> {
    state.registry_get(key)?;
    let ud_id = state
        .test_arg_userdata(-1, LUA_FILE_HANDLE)
        .map(|ud| ud.identity());
    state.pop_n(1);
    let label = &key[IO_PREFIX_LEN..];
    let id = ud_id.ok_or_else(|| {
        LuaError::runtime(format_args!(
            "default {} file is invalid",
            label.escape_ascii()
        ))
    })?;
    let rc = lookup_lstream(id).ok_or_else(|| {
        LuaError::runtime(format_args!(
            "default {} file is invalid",
            label.escape_ascii()
        ))
    })?;
    if rc.borrow().is_closed() {
        return Err(LuaError::runtime(format_args!(
            "default {} file is closed",
            label.escape_ascii()
        )));
    }
    Ok(rc)
}

/// `io.read(...)`. C: `io_read`.
pub fn io_read(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = get_io_file_rc(state, IO_INPUT_KEY)?;
    g_read(state, &p_rc, 1)
}

/// `file:read(...)`. C: `f_read`.
pub fn f_read(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = tofile(state)?;
    g_read(state, &p_rc, 2)
}

// ── Write helpers ────────────────────────────────────────────────────────────

/// Dispatch one or more write values. C: `g_write(L, f, arg)`.
///
/// TODO(port): borrow split — same issue as g_read.
fn g_write(
    state: &mut LuaState,
    file: &mut dyn LuaFileHandle,
    arg: i32,
) -> Result<usize, LuaError> {
    let nargs = state.top() - arg;
    let mut overall_ok = true;

    for i in 0..nargs {
        let idx = arg + i;
        if state.type_at(idx) == LuaType::Number {
            // PERF(port): byte-by-byte write; Phase B add bulk write_fmt to LuaFileOps.
            // TODO(port): C's %.14g (significant digits) has no direct Rust equivalent.
            let s = if state.is_integer(idx) {
                let ival = state.to_integer(idx).unwrap_or(0);
                format!("{}", ival)
            } else {
                let fval = state.to_number(idx).unwrap_or(0.0);
                // TODO(port): implement proper %.14g (choose between %e and %f based on magnitude)
                format!("{:.14e}", fval)
            };
            match file.write_bytes(s.as_bytes()) {
                Ok(n) => overall_ok = overall_ok && n == s.len(),
                Err(_) => overall_ok = false,
            }
        } else {
            let s: Vec<u8> = state.check_arg_string(idx)?;
            match file.write_bytes(&s) {
                Ok(n) => overall_ok = overall_ok && n == s.len(),
                Err(_) => overall_ok = false,
            }
        }
    }

    if overall_ok {
        Ok(1) // file handle already at stack top; C returns it on success
    } else {
        file_result(
            state,
            false,
            None,
            io::Error::new(io::ErrorKind::Other, "write error"),
        )
    }
}

/// `io.write(...)`. C: `io_write`.
///
/// Writes all arguments to the current default output file (`IO_OUTPUT`). When
/// a file was set via `io.output(filename)`, writes go to that file; otherwise
/// they go to stdout via `state.write_output()`.
///
/// The borrow split (needing both `&mut LuaState` and `&mut dyn LuaFileHandle`)
/// is resolved by collecting all formatted strings first and then writing them
/// to the file handle obtained from the `LSTREAM_REGISTRY`.
pub fn io_write(state: &mut LuaState) -> Result<usize, LuaError> {
    // Step 1: collect all formatted byte strings before touching the file handle.
    let n = state.top();
    let mut chunks: Vec<Vec<u8>> = Vec::with_capacity(n as usize);
    for i in 1..=(n as i32) {
        if state.type_at(i) == LuaType::Number {
            let s = if state.is_integer(i) {
                let ival = state.to_integer(i).unwrap_or(0);
                format!("{}", ival).into_bytes()
            } else {
                let fval = state.to_number(i).unwrap_or(0.0);
                // TODO(port): proper %.14g (significant-digit) formatting.
                format!("{:.14e}", fval).into_bytes()
            };
            chunks.push(s);
        } else {
            let bytes: Vec<u8> = state.check_arg_string(i)?;
            chunks.push(bytes);
        }
    }

    // Step 2: resolve the current output file. C's `getiofile` errors when
    // the default output is closed; do not silently fall back to stdout.
    let p_rc = get_io_file_rc(state, IO_OUTPUT_KEY)?;
    {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        for chunk in &chunks {
            fh.write_bytes(chunk).map_err(|e| {
                LuaError::runtime(format_args!("io.write: {}", e))
            })?;
        }
    }
    state.registry_get(IO_OUTPUT_KEY)?;
    Ok(1)
}

/// `file:write(...)`. C: `f_write`.
pub fn f_write(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = tofile(state)?;

    // Step 1: collect args 2..=n as owned byte chunks before borrowing the file.
    let n = state.top();
    let mut chunks: Vec<Vec<u8>> = Vec::with_capacity(n.saturating_sub(1) as usize);
    for i in 2..=(n as i32) {
        if state.type_at(i) == LuaType::Number {
            let s = if state.is_integer(i) {
                let ival = state.to_integer(i).unwrap_or(0);
                format!("{}", ival).into_bytes()
            } else {
                let fval = state.to_number(i).unwrap_or(0.0);
                // TODO(port): proper %.14g formatting (significant digits).
                format!("{:.14e}", fval).into_bytes()
            };
            chunks.push(s);
        } else {
            let bytes: Vec<u8> = state.check_arg_string(i)?;
            chunks.push(bytes);
        }
    }

    // Step 2: write through the file with the LStream borrow scoped tightly.
    let result: io::Result<()> = {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        let mut r: io::Result<()> = Ok(());
        for chunk in &chunks {
            match fh.write_bytes(chunk) {
                Ok(written) if written == chunk.len() => {}
                Ok(_) => {
                    r = Err(io::Error::new(io::ErrorKind::Other, "short write"));
                    break;
                }
                Err(e) => {
                    r = Err(e);
                    break;
                }
            }
        }
        r
    };

    // Step 3: on success return the file handle (arg 1); on failure use file_result.
    match result {
        Ok(()) => {
            state.push_value_at(1);
            Ok(1)
        }
        Err(e) => file_result(state, false, None, e),
    }
}

// ── Seek / setvbuf / flush ───────────────────────────────────────────────────

/// `file:seek([whence [, offset]])`. C: `f_seek`.
pub fn f_seek(state: &mut LuaState) -> Result<usize, LuaError> {
    static MODE_NAMES: &[&[u8]] = &[b"set", b"cur", b"end"];

    let p_rc = tofile(state)?;
    let op = state.check_arg_option(2, Some(b"cur"), MODE_NAMES)?;
    let p3: i64 = state.opt_arg_integer(3, 0)?;

    let seek_pos = match op {
        0 => SeekFrom::Start(p3 as u64),
        1 => SeekFrom::Current(p3),
        2 => SeekFrom::End(p3),
        _ => unreachable!(),
    };

    let result = {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        fh.seek(seek_pos)
    };
    match result {
        Ok(pos) => {
            state.push(LuaValue::Int(pos as i64));
            Ok(1)
        }
        Err(e) => file_result(state, false, None, e),
    }
}

/// `file:setvbuf(mode [, size])`. C: `f_setvbuf`.
pub fn f_setvbuf(state: &mut LuaState) -> Result<usize, LuaError> {
    static MODE_NAMES: &[&[u8]] = &[b"no", b"full", b"line"];

    let p_rc = tofile(state)?;
    let op = state.check_arg_option(2, None, MODE_NAMES)?;
    let sz: i64 = state.opt_arg_integer(3, LUAL_BUFFER_SIZE as i64)?;
    let mode = match op {
        0 => BufMode::No,
        1 => BufMode::Full,
        2 => BufMode::Line,
        _ => unreachable!(),
    };
    let result = {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        let mode_index = match mode {
            BufMode::No => 0,
            BufMode::Full => 1,
            BufMode::Line => 2,
        };
        fh.set_buf_mode(mode_index, sz.max(0) as usize)
    };
    match result {
        Ok(()) => file_result(state, true, None, io::Error::last_os_error()),
        Err(e) => file_result(state, false, None, e),
    }
}

/// `io.flush()`. C: `io_flush`.
pub fn io_flush(state: &mut LuaState) -> Result<usize, LuaError> {
    let ud_id: Option<usize> = {
        state.registry_get(IO_OUTPUT_KEY)?;
        let id = state
            .test_arg_userdata(-1, LUA_FILE_HANDLE)
            .map(|ud| ud.identity());
        state.pop_n(1);
        id
    };
    if let Some(id) = ud_id {
        if let Some(rc) = lookup_lstream(id) {
            let result = {
                let mut p = rc.borrow_mut();
                if p.is_closed() {
                    return Err(LuaError::runtime(format_args!(
                        "default output file is closed"
                    )));
                }
                let fh = p.file.as_deref_mut().expect("open stream has no file handle");
                fh.flush()
            };
            return match result {
                Ok(()) => {
                    state.push(LuaValue::Bool(true));
                    Ok(1)
                }
                Err(e) => file_result(state, false, None, e),
            };
        }
    }
    // No live default output file: behave like a successful no-op flush of stdout.
    state.push(LuaValue::Bool(true));
    Ok(1)
}

/// `file:flush()`. C: `f_flush`.
pub fn f_flush(state: &mut LuaState) -> Result<usize, LuaError> {
    let p_rc = tofile(state)?;
    let result = {
        let mut p = p_rc.borrow_mut();
        let fh = p.file.as_mut().expect("open stream has no file handle");
        fh.flush()
    };
    match result {
        Ok(()) => {
            state.push(LuaValue::Bool(true));
            Ok(1)
        }
        Err(e) => file_result(state, false, None, e),
    }
}

// ── Lines iterator ───────────────────────────────────────────────────────────

/// Build the `io_readline` closure with its upvalues and push it.
///
/// Upvalue layout (C comment):
///   1) file handle (first stack value)
///   2) number of read-format arguments
///   3) toclose flag (bool)
///   4..n+3) format arguments
fn aux_lines(state: &mut LuaState, toclose: bool) -> Result<(), LuaError> {
    // `lua_gettop` is the stack count RELATIVE to the current frame, not the
    // absolute `top_idx`; using `state.top()` mirrors that.
    let n = state.top() - 1;
    if n > MAX_ARG_LINE as i32 {
        return Err(LuaError::arg_error(
            MAX_ARG_LINE as i32 + 2,
            "too many arguments",
        ));
    }
    state.push_value_at(1)?;
    state.push(LuaValue::Int(n as i64));
    state.push(LuaValue::Bool(toclose));
    state.rotate(2, 3)?;
    state.push_c_closure(io_readline, (3 + n) as i32)?;
    Ok(())
}

/// `file:lines(...)`. C: `f_lines`.
pub fn f_lines(state: &mut LuaState) -> Result<usize, LuaError> {
    let _ = tofile(state)?; // validates file is open
    aux_lines(state, false)?;
    Ok(1)
}

/// `io.lines([filename, ...])`. C: `io_lines`.
pub fn io_lines(state: &mut LuaState) -> Result<usize, LuaError> {
    if state.type_at(1) == LuaType::None {
        state.push(LuaValue::Nil);
    }
    let toclose = if state.type_at(1) == LuaType::Nil {
        state.registry_get(IO_INPUT_KEY)?;
        state.replace(1);
        let _ = tofile(state)?;
        false
    } else {
        let filename = state.check_arg_string(1)?;
        opencheck(state, &filename, b"r")?;
        state.replace(1)?;
        true
    };

    aux_lines(state, toclose)?;

    if toclose {
        state.push(LuaValue::Nil); // state
        state.push(LuaValue::Nil); // control
        state.push_value_at(1);    // file as to-be-closed variable (4th result)
        Ok(4)
    } else {
        Ok(1)
    }
}

/// Iteration function created by `aux_lines`. C: `io_readline`.
///
/// Upvalue layout matches what `aux_lines` creates:
///   upvalue 1: file handle (userdata)
///   upvalue 2: n (number of read-format args)
///   upvalue 3: toclose flag
///   upvalue 4..n+3: format arguments
fn io_readline(state: &mut LuaState) -> Result<usize, LuaError> {
    let n = match state.value_at(crate::state_stub::upvalue_index(2)) {
        LuaValue::Int(i) => i as usize,
        _ => 0,
    };

    let p_rc = lstream_from_upvalue(state, 1)?;

    if p_rc.borrow().is_closed() {
        return Err(LuaError::runtime(format_args!("file is already closed")));
    }

    lua_vm::api::set_top(state, 1)?;
    state.ensure_stack(n as i32, "too many arguments")?;

    for i in 1..=n {
        let uv = state.value_at(crate::state_stub::upvalue_index(3 + i as i32));
        state.push(uv);
    }

    let result_n: usize = g_read(state, &p_rc, 2)?;

    debug_assert!(result_n > 0, "g_read should return at least one value");

    let top = state.top_idx().get() as i32;
    let first_result_idx = top - result_n as i32;
    let first_truthy = !matches!(
        state.stack_at(first_result_idx),
        LuaValue::Nil | LuaValue::Bool(false)
    );
    if first_truthy {
        return Ok(result_n);
    }

    if result_n > 1 {
        let err_val = state.stack_at(first_result_idx + 1).clone();
        return Err(LuaError::from_value(err_val));
    }

    let toclose = !matches!(
        state.value_at(crate::state_stub::upvalue_index(3)),
        LuaValue::Nil | LuaValue::Bool(false)
    );
    if toclose {
        lua_vm::api::set_top(state, 0)?;
        state.push_upvalue(1)?;
        aux_close(state)?;
    }

    Ok(0)
}

// ── Module registration ──────────────────────────────────────────────────────

/// Create the file-handle metatable in the registry. C: `createmeta(L)`.
fn create_meta(state: &mut LuaState) -> Result<(), LuaError> {
    state.new_metatable(LUA_FILE_HANDLE)?;
    state.set_funcs(FILE_METAMETHODS, 0)?;
    state.new_lib_table(FILE_METHODS)?;
    state.set_funcs(FILE_METHODS, 0)?;
    state.set_field(-2, b"__index")?;
    state.pop_n(1);
    Ok(())
}

/// Register stdin, stdout, or stderr as a Lua file handle. C: `createstdfile`.
fn create_std_file(
    state: &mut LuaState,
    std_kind: StdFileKind,
    registry_key: Option<&[u8]>,
    field_name: &[u8],
) -> Result<(), LuaError> {
    let cell = new_pre_file(state)?;
    {
        let mut p = cell.borrow_mut();
        p.file = Some(Box::new(StdStreamHandle::new(std_kind)));
        p.close_fn = Some(io_noclose);
    }
    if let Some(key) = registry_key {
        state.push_value_at(-1);
        state.registry_set(key)?;
    }
    state.set_field(-2, field_name)?;
    Ok(())
}

/// Open the `io` library and return 1 (the library table). C: `luaopen_io`.
pub fn luaopen_io(state: &mut LuaState) -> Result<usize, LuaError> {
    state.new_lib(IO_LIB)?;
    create_meta(state)?;
    create_std_file(state, StdFileKind::Stdin, Some(IO_INPUT_KEY), b"stdin")?;
    create_std_file(state, StdFileKind::Stdout, Some(IO_OUTPUT_KEY), b"stdout")?;
    create_std_file(state, StdFileKind::Stderr, None, b"stderr")?;
    Ok(1)
}

// ────────────────────────────────────────────────────────────────────────────
// PORT STATUS
//   source:        src/liolib.c  (841 lines, ~35 functions)
//   target_crate:  lua-stdlib
//   confidence:    medium
//   todos:         62
//   port_notes:    2
//   unsafe_blocks: 0   (must be 0 outside explicit unsafe-budget crates)
//   notes:         Logic faithfully translated. Phase F closed the io_readline
//                  is_closed/g_read stubs via lstream_from_upvalue (looks up
//                  the LStream side-table from the GcRef<LuaUserData> sitting
//                  at upvalue 1). io.popen is now wired through a new
//                  GlobalState::popen_hook (mirrors file_open_hook): the
//                  lua-cli backend spawns /bin/sh -c <cmd> and wraps the
//                  resulting ChildStdout/ChildStdin in a PopenFile so the
//                  existing LStream read/write/close path Just Works. With
//                  no hook registered (sandboxed embeddings) io.popen
//                  returns nil, errmsg, errno via file_result rather than
//                  panicking. Remaining systemic Phase B blockers:
//                  (1) All concrete LuaFileOps implementations need std::fs or
//                  std::process, both banned outside lua-cli by PORTING.md; the
//                  architecture must grant an exemption for lua-stdlib/src/io_lib.rs
//                  or introduce a thin IO-abstraction crate.
//                  (2) The borrow checker prevents holding &mut dyn LuaFileOps
//                  (extracted from LuaUserData) and &mut LuaState simultaneously;
//                  fix via RefCell<Box<dyn LuaFileOps>> inside LStream, plus
//                  restructure g_read/g_write to accept StackIdx not a raw borrow.
//                  (3) C's %.14g (significant-digit float format) has no direct
//                  Rust equivalent; a custom formatter is needed for faithful
//                  number serialisation. The typed-userdata API (needed to cast
//                  raw LuaUserData bytes to LStream) must also land in Phase B.
//                  rustc self-check shows only expected E0432/E0433 import errors.
// ────────────────────────────────────────────────────────────────────────────