lua_stdlib/

string_lib.rs

//! Standard library for string operations and pattern-matching.
//!
//! Port of `lstrlib.c` (Lua 5.4.7, 1875 lines, 46 functions).
//!
//! Sections:
//!   1. Basic string operations (byte, char, find, format, gmatch, gsub, len,
//!      lower, match, rep, reverse, sub, upper)
//!   2. Pattern-matching engine (MatchState + recursive matcher)
//!   3. String format (`string.format`)
//!   4. Pack / unpack (`string.pack`, `string.packsize`, `string.unpack`)
//!   5. Module registration (`luaopen_string`)

use std::any::Any;
use std::cell::RefCell;
use std::rc::Rc;

use lua_types::error::LuaError;
use lua_types::value::LuaValue;
use lua_types::arith::ArithOp;
use lua_types::{LuaType};
use crate::state_stub::{LuaState, LuaStateStubExt as _, lua_CFunction, upvalue_index};

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

const LUA_MAX_CAPTURES: usize = 32;

const MAX_CC_CALLS: i32 = 200;

const L_ESC: u8 = b'%';

const SPECIALS: &[u8] = b"^$*+?.([%-";

const CAP_UNFINISHED: isize = -1;

const CAP_POSITION: isize = -2;

#[expect(dead_code, reason = "ported stdlib helper; not yet wired into the runtime")]
const MAX_ITEM: usize = 120;

#[expect(dead_code, reason = "ported stdlib helper; not yet wired into the runtime")]
const MAX_ITEM_F: usize = 418;

#[expect(dead_code, reason = "ported stdlib helper; not yet wired into the runtime")]
const MAX_FORMAT: usize = 32;

const MAX_INT_SIZE: usize = 16;

// On platforms where size_t is at least as wide as int (all our targets), this
// collapses to INT_MAX so that packed sizes round-trip through a Lua integer
// without ambiguity.
const PACK_MAXSIZE: usize = i32::MAX as usize;

const NB: u32 = 8;

const MC: u8 = 0xFF;

const SZINT: usize = 8; // sizeof(i64) == 8

const PACK_PAD_BYTE: u8 = 0x00;

// ────────────────────────────────────────────────────────────────────────────
// Pattern-matching types
// ────────────────────────────────────────────────────────────────────────────

/// One capture record inside MatchState.
///
/// In Rust, `init` is an index into `MatchState::src`; `len` is either a
/// non-negative actual length, `CAP_UNFINISHED`, or `CAP_POSITION`.
#[derive(Copy, Clone)]
struct Capture {
    /// Index into the source slice where this capture started.
    init: usize,
    /// CAP_UNFINISHED, CAP_POSITION, or non-negative byte count.
    len: isize,
}

impl Default for Capture {
    fn default() -> Self {
        Capture { init: 0, len: CAP_UNFINISHED }
    }
}

/// State threaded through the recursive pattern-matcher.
///
/// Raw C pointers replaced by indices into `src` / `pat` slices.
struct MatchState<'a> {
    /// Source string being searched.
    src: &'a [u8],
    /// Pattern string.
    pat: &'a [u8],
    /// Recursion depth counter; decremented on entry, incremented on return.
    matchdepth: i32,
    /// Number of capture records currently in use.
    level: u8,
    /// Capture records indexed `0..level`.
    captures: [Capture; LUA_MAX_CAPTURES],
    /// Total `match_pat` invocations across the whole operation. Used to bound
    /// catastrophic backtracking under a sandbox; charged against the
    /// instruction budget by the caller.
    steps: u64,
    /// Maximum `steps` before the matcher stops. `0` means unlimited (no active
    /// instruction budget), preserving non-sandboxed behavior exactly.
    step_limit: u64,
    /// Set when `step_limit` is reached; the matcher then unwinds to the caller,
    /// which charges the budget and raises the uncatchable sandbox abort.
    aborted: bool,
}

impl<'a> MatchState<'a> {
    fn new(src: &'a [u8], pat: &'a [u8], step_limit: u64) -> Self {
        MatchState {
            src,
            pat,
            matchdepth: MAX_CC_CALLS,
            level: 0,
            captures: [Capture::default(); LUA_MAX_CAPTURES],
            steps: 0,
            step_limit,
            aborted: false,
        }
    }

    fn reset_level(&mut self) {
        self.level = 0;
        debug_assert!(self.matchdepth == MAX_CC_CALLS);
    }
}

struct GMatchIterState {
    /// Current source position as a zero-based byte index.
    pos: usize,
    /// End of the last match, used to avoid zero-length infinite loops.
    last_match: Option<usize>,
}

// ────────────────────────────────────────────────────────────────────────────
// Pack/unpack types
// ────────────────────────────────────────────────────────────────────────────

/// Pack/unpack format option.
///
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum KOption {
    Int,        // signed integers
    Uint,       // unsigned integers
    Float,      // single-precision float (C float)
    Number,     // Lua native float (lua_Number = f64)
    Double,     // double-precision float (C double)
    Char,       // fixed-length string
    Kstring,    // string with length prefix
    Zstr,       // zero-terminated string
    Padding,    // padding byte (x)
    Paddalign,  // padding to alignment (X)
    Nop,        // no-op (space, <, >, =, !)
}

/// Header state for pack/unpack format parsing.
///
struct Header {
    is_little: bool,
    max_align: usize,
    /// 5.5 widened `c`/`s`-size parsing from `int` (5.3/5.4) to `size_t`, so
    /// `c<huge>` numerals that overflowed `int` (and tripped "invalid format
    /// option '<digit>'") are now accepted up to `LUA_MAXINTEGER`.
    wide_size: bool,
}

impl Header {
    fn new(wide_size: bool) -> Self {
        Header {
            is_little: cfg!(target_endian = "little"),
            max_align: 1,
            wide_size,
        }
    }
}

// ────────────────────────────────────────────────────────────────────────────
// §1  Basic string helpers
// ────────────────────────────────────────────────────────────────────────────

/// Translate a relative initial string position: negative means back from end;
/// result is clipped to `[1, ∞)`.
///
fn pos_relat_i(pos: i64, len: usize) -> usize {
    if pos > 0 {
        pos as usize
    } else if pos == 0 {
        1
    } else if pos < -(len as i64) {
        1
    } else {
        len.wrapping_add(pos as usize).wrapping_add(1)
    }
}

/// Translate a relative position using Lua 5.3's `posrelat` (`lstrlib.c` 5.3):
/// non-negatives pass through, an out-of-range negative clamps to `0`, and an
/// in-range negative counts back from the end. Unlike `posrelat_i`, `0` stays
/// `0`; `string.unpack` then subtracts one, underflowing into the
/// "initial position out of string" guard exactly as the 5.3 reference does.
///
fn posrelat_53(pos: i64, len: usize) -> usize {
    if pos >= 0 {
        pos as usize
    } else if (pos as i128).unsigned_abs() > len as u128 {
        0
    } else {
        (len as i64 + pos + 1) as usize
    }
}

/// Get an optional ending string position from argument `arg`, default `def`.
/// Negative means back from end; clipped to `[0, len]`.
///
fn get_end_pos(pos: i64, len: usize) -> usize {
    if pos > len as i64 {
        len
    } else if pos >= 0 {
        pos as usize
    } else if pos < -(len as i64) {
        0
    } else {
        len.wrapping_add(pos as usize).wrapping_add(1)
    }
}

// ────────────────────────────────────────────────────────────────────────────
// §2  Exported string functions (registered in strlib[])
// ────────────────────────────────────────────────────────────────────────────

/// `string.len(s)` — return byte-length of `s`.
///
///
/// Reads only the byte-length, never the bytes themselves, so go through
/// `to_lua_string_len` (which never copies) rather than `check_arg_string`
/// (which `to_vec`s the entire payload only for `.len()` to throw it away).
pub fn str_len(state: &mut LuaState) -> Result<usize, LuaError> {
    let l = match state.to_lua_string_len(1) {
        Some(n) => n,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    state.push(LuaValue::Int(l as i64));
    Ok(1)
}

/// `string.sub(s, i [, j])` — return substring.
///
///
/// Borrow through `to_lua_string` so the full source string is not copied just
/// to slice a (typically small) substring out of it. The `GcRef` keeps the
/// bytes rooted across the `check_arg_integer` / `opt_arg_integer` calls (none
/// of which can collect the string at arg #1).
pub fn str_sub(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let l = s.len();
    let start = pos_relat_i(state.check_arg_integer(2)?, l);
    let end_pos_raw = state.opt_arg_integer(3, -1)?;
    let end = get_end_pos(end_pos_raw, l);
    if start <= end {
        let slice = &s[(start - 1)..end];
        state.push_string(slice)?;
    } else {
        state.push_string(b"")?;
    }
    Ok(1)
}

/// `string.reverse(s)` — return string with bytes reversed.
///
///
/// Borrow the source bytes; the previous `check_arg_string` made a full owned
/// copy that was discarded after the single iteration.
pub fn str_reverse(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let buf: Vec<u8> = s.iter().copied().rev().collect();
    state.push_bytes(&buf)?;
    Ok(1)
}

/// `string.lower(s)` — return lowercase copy.
///
///
/// Borrow the source bytes; one allocation (the output `Vec`) is unavoidable,
/// but the intermediate copy from `check_arg_string` was not.
pub fn str_lower(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let buf: Vec<u8> = s.iter().map(|&c| c.to_ascii_lowercase()).collect();
    state.push_bytes(&buf)?;
    Ok(1)
}

/// `string.upper(s)` — return uppercase copy.
///
///
/// Borrow the source bytes; called as the `string.gsub` replacement function
/// in `string_ops_long` ~700k times against `%w+` matches, so the intermediate
/// copy from `check_arg_string` added up.
pub fn str_upper(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let buf: Vec<u8> = s.iter().map(|&c| c.to_ascii_uppercase()).collect();
    state.push_bytes(&buf)?;
    Ok(1)
}

/// `string.rep(s, n [, sep])` — return `n` copies of `s` separated by `sep`.
///
///
/// Borrow `s` through `to_lua_string`. The previous version did the
/// `check_arg_string` copy and then a second redundant `s.to_vec()` inside the
/// build loop — that double-copy is gone too.
pub fn str_rep(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let l = s.len();
    let n = state.check_arg_integer(2)?;
    let sep_owned = state.opt_arg_string(3, b"")?;
    let sep: &[u8] = &sep_owned;
    let lsep = sep.len();

    if n <= 0 {
        state.push_string(b"")?;
    } else {
        const MAXSIZE: usize = i32::MAX as usize;
        let per = l.checked_add(lsep)
            .ok_or_else(|| LuaError::runtime(format_args!("resulting string too large")))?;
        if per > MAXSIZE / (n as usize) {
            return Err(LuaError::runtime(format_args!("resulting string too large")));
        }
        let total = per * (n as usize) - lsep;

        if let Some(err) = state.sandbox_reserve(total) {
            return Err(err);
        }

        let mut buf: Vec<u8> = Vec::with_capacity(total);
        for i in 0..(n as usize) {
            buf.extend_from_slice(s);
            if i < (n as usize - 1) && lsep > 0 {
                buf.extend_from_slice(sep);
            }
        }
        state.push_bytes(&buf)?;
    }
    Ok(1)
}

/// `string.byte(s [, i [, j]])` — return numeric codes of characters.
///
///
/// Borrow the source bytes through `to_lua_string` (returns a `GcRef<LuaString>`)
/// instead of `check_arg_string` (which copies the entire string into a fresh
/// `Vec<u8>`). On the `string_ops_long` workload `string.byte` is called 700k
/// times against the same ~14 KB string, so the previous copy was on the order
/// of 10 GB of memcpy. The `GcRef` keeps the bytes rooted while the borrow lives.
pub fn str_byte(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let l = s.len();
    let pi = state.opt_arg_integer(2, 1)?;
    let posi = pos_relat_i(pi, l);
    let pose_raw = state.opt_arg_integer(3, pi)?;
    let pose = get_end_pos(pose_raw, l);

    if posi > pose {
        return Ok(0);
    }
    let count = pose.saturating_sub(posi - 1) + 1;
    if count > i32::MAX as usize {
        return Err(LuaError::runtime(format_args!("string slice too long")));
    }
    let n = (pose - posi + 1) as usize;
    state.ensure_stack(n as i32, "string slice too long")?;

    for i in 0..n {
        state.push(LuaValue::Int(s[posi - 1 + i] as i64));
    }
    Ok(n)
}

/// `string.char(...)` — return string built from character codes.
///
pub fn str_char(state: &mut LuaState) -> Result<usize, LuaError> {
    let n = state.get_top();
    let mut buf = Vec::with_capacity(n as usize);
    for i in 1..=n {
        let c = state.check_arg_integer(i)? as u64;
        if c > u8::MAX as u64 {
            return Err(lua_vm::debug::arg_error_impl(state, i, b"value out of range"));
        }
        buf.push(c as u8);
    }
    state.push_bytes(&buf)?;
    Ok(1)
}

/// `string.dump(function [, strip])` — serialize a function as binary chunk.
///
/// Uses `lua_dump` internally; the writer callback builds a buffer.
pub fn str_dump(state: &mut LuaState) -> Result<usize, LuaError> {
    state.check_arg_type(1, LuaType::Function)?;
    let strip = state.arg_to_bool(2);
    // PORT NOTE: `state.set_top` (inherent) takes an absolute StackIdx and
    // would wipe the call frame. `lua_settop` is frame-relative.
    lua_vm::api::set_top(state, 1)?;
    // TODO(port): state.dump_function(strip) needs to produce &[u8].
    // In the C code, lua_dump writes to a writer callback that fills a luaL_Buffer.
    // In Rust, state.dump() should return Vec<u8> or write to a &mut Vec<u8>.
    let bytes = state.dump_function(strip)
        .map_err(|_| LuaError::runtime(format_args!("unable to dump given function")))?;
    state.push_bytes(&bytes)?;
    Ok(1)
}

// ────────────────────────────────────────────────────────────────────────────
// §3  String metamethods (arithmetic coercion)
// ────────────────────────────────────────────────────────────────────────────

/// Try to coerce the argument at `arg` to a number, pushing it on the stack.
/// Returns true on success.
///
fn tonum(state: &mut LuaState, arg: i32) -> Result<bool, LuaError> {
    if state.type_at(arg) == LuaType::Number {
        state.push_value_at(arg)?;
        Ok(true)
    } else {
        // check whether it is a numerical string
        //    return (s != NULL && lua_stringtonumber(L, s) == len + 1);
        if let Some(s) = state.to_lua_string_bytes(arg) {
            let len = s.len();
            // PORT NOTE: string_to_number pushes the number if successful
            let pushed = state.string_to_number_push(&s)?;
            let ok = pushed == len + 1;
            // Lua 5.1–5.3: a string coerced in an arithmetic operation always
            // yields a float (`('16') + 0` is a float in 5.3, an integer in
            // 5.4). This metamethod path is arithmetic-only, so the promotion
            // never touches bitwise ops. Verified vs the 5.3.6/5.4.7 oracle.
            if ok
                && matches!(
                    state.global().lua_version,
                    lua_types::LuaVersion::V51
                        | lua_types::LuaVersion::V52
                        | lua_types::LuaVersion::V53
                )
            {
                if let Some(f) = lua_vm::api::to_number_x(state, -1) {
                    state.pop();
                    state.push(LuaValue::Float(f));
                }
            }
            Ok(ok)
        } else {
            Ok(false)
        }
    }
}

/// Try to invoke the metamethod `mtname` on the two operands.
///
fn trymt(state: &mut LuaState, mtname: &[u8]) -> Result<(), LuaError> {
    // PORT NOTE: `state.set_top` (inherent) takes an absolute StackIdx and
    // would wipe the call frame's arguments. `lua_settop` is frame-relative
    // — keep the first two args of the current C function.
    lua_vm::api::set_top(state, 2)?;
    let t2_is_string = state.type_at(2) == LuaType::String;
    // C: `if (lua_type(L,2)==LUA_TSTRING || !luaL_getmetafield(L,2,mtname))`.
    // The `||` short-circuits: when arg2 is a string, `get_meta_field` is never
    // called, so the stack stays `[arg1, arg2]` for the error formatter. Calling
    // it unconditionally would push the string metatable's own metamethod and
    // shift the operands read by `type_name_at(-2)/(-1)`.
    if t2_is_string || !state.get_meta_field(2, mtname)? {
        let op = &mtname[2..]; // skip "__"
        let msg = format!(
            "attempt to {} a '{}' with a '{}'",
            op.escape_ascii(),
            state.type_name_at(-2).escape_ascii(),
            state.type_name_at(-1).escape_ascii(),
        );
        return crate::auxlib::lua_error(state, msg.as_bytes()).map(|_| ());
    }
    state.insert(-3)?;
    state.call(2, 1)?;
    Ok(())
}

/// Generic arithmetic helper: coerce both args and call `op`, else try metamethod.
///
fn arith(state: &mut LuaState, op: ArithOp, mtname: &[u8]) -> Result<usize, LuaError> {
    if tonum(state, 1)? && tonum(state, 2)? {
        state.arith(op)?;
    } else {
        trymt(state, mtname)?;
    }
    Ok(1)
}

pub fn arith_add(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Add, b"__add")
}
pub fn arith_sub(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Sub, b"__sub")
}
pub fn arith_mul(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Mul, b"__mul")
}
pub fn arith_mod(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Mod, b"__mod")
}
pub fn arith_pow(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Pow, b"__pow")
}
pub fn arith_div(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Div, b"__div")
}
pub fn arith_idiv(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Idiv, b"__idiv")
}
pub fn arith_unm(state: &mut LuaState) -> Result<usize, LuaError> {
    arith(state, ArithOp::Unm, b"__unm")
}

// ────────────────────────────────────────────────────────────────────────────
// §4  Pattern-matching engine
// ────────────────────────────────────────────────────────────────────────────

/// Return `true` if `c` belongs to the character class `cl` (a `%x` letter).
///
#[inline(always)]
fn match_class(c: u8, cl: u8) -> bool {
    let res = match cl.to_ascii_lowercase() {
        b'a' => c.is_ascii_alphabetic(),
        b'c' => c.is_ascii_control(),
        b'd' => c.is_ascii_digit(),
        b'g' => c.is_ascii_graphic(),
        b'l' => c.is_ascii_lowercase(),
        b'p' => c.is_ascii_punctuation(),
        b's' => c.is_ascii_whitespace(),
        b'u' => c.is_ascii_uppercase(),
        b'w' => c.is_ascii_alphanumeric(),
        b'x' => c.is_ascii_hexdigit(),
        b'z' => c == 0,
        _    => return cl == c,
    };
    if cl.is_ascii_lowercase() { res } else { !res }
}

/// Match character `c` against a bracket class `[p .. ec-1]`.
///
/// `p` and `ec` are indices into `pat`.
#[inline]
fn matchbracketclass(pat: &[u8], c: u8, mut p: usize, ec: usize) -> bool {
    let sig = if p + 1 < pat.len() && pat[p + 1] == b'^' {
        p += 1; // skip '^'
        false
    } else {
        true
    };
    p += 1; // advance past '[' or '^'
    while p < ec {
        if pat[p] == L_ESC {
            p += 1;
            if p < ec && match_class(c, pat[p]) {
                return sig;
            }
        } else if p + 1 < ec && pat[p + 1] == b'-' && p + 2 < ec {
            let lo = pat[p];
            p += 2;
            let hi = pat[p];
            if lo <= c && c <= hi {
                return sig;
            }
        } else if pat[p] == c {
            return sig;
        }
        p += 1;
    }
    !sig
}

/// Return `true` if the single character at `src[s]` matches the pattern
/// element starting at `pat[p]` with class end at `ep`.
///
#[inline(always)]
fn singlematch(ms: &MatchState, s: usize, p: usize, ep: usize) -> bool {
    if s >= ms.src.len() {
        return false;
    }
    let c = ms.src[s];
    match ms.pat[p] {
        b'.' => true,
        L_ESC => match_class(c, ms.pat[p + 1]),
        b'[' => matchbracketclass(ms.pat, c, p, ep - 1),
        pc   => pc == c,
    }
}

/// Find the end of the pattern element starting at `pat[p]`.
/// Returns the index one past the element, or an error for malformed patterns.
///
#[inline(always)]
fn classend(ms: &MatchState, p: usize) -> Result<usize, LuaError> {
    let pat = ms.pat;
    match pat.get(p).copied() {
        Some(L_ESC) => {
            if p + 1 >= pat.len() {
                return Err(LuaError::runtime(format_args!(
                    "malformed pattern (ends with '%')"
                )));
            }
            Ok(p + 2)
        }
        Some(b'[') => {
            let mut q = p + 1;
            if q < pat.len() && pat[q] == b'^' {
                q += 1;
            }
            loop {
                if q >= pat.len() {
                    return Err(LuaError::runtime(format_args!(
                        "malformed pattern (missing ']')"
                    )));
                }
                let ch = pat[q];
                q += 1;
                if ch == L_ESC && q < pat.len() {
                    q += 1;
                }
                if q < pat.len() && pat[q] == b']' {
                    return Ok(q + 1);
                }
            }
        }
        Some(_) => Ok(p + 1),
        None => Ok(p),
    }
}

/// Check that capture `l` (1-based char digit from pattern) is valid.
/// Returns the 0-based capture index.
///
fn check_capture(ms: &MatchState, l: u8) -> Result<usize, LuaError> {
    let signed = (l as i32) - (b'1' as i32);
    if signed < 0
        || signed >= ms.level as i32
        || ms.captures[signed as usize].len == CAP_UNFINISHED
    {
        return Err(LuaError::runtime(format_args!(
            "invalid capture index %{}",
            signed + 1
        )));
    }
    Ok(signed as usize)
}

/// Find the most recent unfinished capture to close.
///
fn capture_to_close(ms: &MatchState) -> Result<usize, LuaError> {
    let mut level = ms.level as usize;
    while level > 0 {
        level -= 1;
        if ms.captures[level].len == CAP_UNFINISHED {
            return Ok(level);
        }
    }
    Err(LuaError::runtime(format_args!("invalid pattern capture")))
}

/// Match a balanced string `%bxy` starting at `src[s]`.
///
/// Returns the new `s` position after the match, or `None`.
fn matchbalance(ms: &MatchState, s: usize, p: usize) -> Result<Option<usize>, LuaError> {
    if p + 1 >= ms.pat.len() {
        return Err(LuaError::runtime(format_args!(
            "malformed pattern (missing arguments to '%b')"
        )));
    }
    let b = ms.pat[p];
    let e = ms.pat[p + 1];
    if s >= ms.src.len() || ms.src[s] != b {
        return Ok(None);
    }
    let mut cont = 1i32;
    let mut s = s + 1;
    while s < ms.src.len() {
        if ms.src[s] == e {
            cont -= 1;
            if cont == 0 {
                return Ok(Some(s + 1));
            }
        } else if ms.src[s] == b {
            cont += 1;
        }
        s += 1;
    }
    Ok(None)
}

/// Greedy match: match as many as possible, then try the rest of the pattern.
///
fn max_expand(
    ms: &mut MatchState,
    s: usize,
    p: usize,
    ep: usize,
) -> Result<Option<usize>, LuaError> {
    let mut count: isize = 0;
    while singlematch(ms, s + count as usize, p, ep) {
        count += 1;
    }
    while count >= 0 {
        let res = match_pat(ms, s + count as usize, ep + 1)?;
        if res.is_some() {
            return Ok(res);
        }
        count -= 1;
    }
    Ok(None)
}

/// Lazy match: try the rest of the pattern first, then expand by one.
///
fn min_expand(
    ms: &mut MatchState,
    mut s: usize,
    p: usize,
    ep: usize,
) -> Result<Option<usize>, LuaError> {
    loop {
        let res = match_pat(ms, s, ep + 1)?;
        if res.is_some() {
            return Ok(res);
        } else if singlematch(ms, s, p, ep) {
            s += 1;
        } else {
            return Ok(None);
        }
    }
}

/// Open a new capture at `src[s]`.
///
fn start_capture(
    ms: &mut MatchState,
    s: usize,
    p: usize,
    what: isize,
) -> Result<Option<usize>, LuaError> {
    let level = ms.level as usize;
    if level >= LUA_MAX_CAPTURES {
        return Err(LuaError::runtime(format_args!("too many captures")));
    }
    ms.captures[level].init = s;
    ms.captures[level].len = what;
    ms.level += 1;
    let res = match_pat(ms, s, p)?;
    if res.is_none() {
        ms.level -= 1; // undo capture
    }
    Ok(res)
}

/// Close the most recent open capture at `src[s]`.
///
fn end_capture(ms: &mut MatchState, s: usize, p: usize) -> Result<Option<usize>, LuaError> {
    let l = capture_to_close(ms)?;
    ms.captures[l].len = (s - ms.captures[l].init) as isize;
    let res = match_pat(ms, s, p)?;
    if res.is_none() {
        ms.captures[l].len = CAP_UNFINISHED; // undo
    }
    Ok(res)
}

/// Match a back-reference `%n` against `src[s]`.
///
fn match_capture(ms: &MatchState, s: usize, l: u8) -> Result<Option<usize>, LuaError> {
    let idx = check_capture(ms, l)?;
    let cap_len = ms.captures[idx].len as usize;
    let cap_init = ms.captures[idx].init;
    if ms.src.len() - s >= cap_len
        && &ms.src[s..s + cap_len] == &ms.src[cap_init..cap_init + cap_len]
    {
        Ok(Some(s + cap_len))
    } else {
        Ok(None)
    }
}

/// Core recursive pattern matcher.
/// Returns `Ok(Some(new_s))` on match, `Ok(None)` on failure, `Err` on error.
///
/// The C code uses `goto init` for tail calls; here we use a loop.
fn match_pat(ms: &mut MatchState, mut s: usize, mut p: usize) -> Result<Option<usize>, LuaError> {
    if ms.aborted {
        return Ok(None);
    }
    ms.steps += 1;
    if ms.step_limit != 0 && ms.steps > ms.step_limit {
        ms.aborted = true;
        return Ok(None);
    }
    ms.matchdepth -= 1;
    if ms.matchdepth < 0 {
        ms.matchdepth = 0;
        return Err(LuaError::runtime(format_args!("pattern too complex")));
    }

    // Use a loop to simulate `goto init` (tail-call optimization).
    let result = 'outer: loop {
        if p >= ms.pat.len() {
            // end of pattern — full match up to current s
            break 'outer Ok(Some(s));
        }

        match ms.pat[p] {
            b'(' => {
                let s2 = if p + 1 < ms.pat.len() && ms.pat[p + 1] == b')' {
                    // position capture
                    start_capture(ms, s, p + 2, CAP_POSITION)?
                } else {
                    start_capture(ms, s, p + 1, CAP_UNFINISHED)?
                };
                break 'outer Ok(s2);
            }
            b')' => {
                let s2 = end_capture(ms, s, p + 1)?;
                break 'outer Ok(s2);
            }
            b'$' => {
                if p + 1 != ms.pat.len() {
                    // fall through to default
                    let ep = classend(ms, p)?;
                    let s2 = handle_class_with_suffix(ms, s, p, ep)?;
                    break 'outer Ok(s2);
                }
                break 'outer Ok(if s == ms.src.len() { Some(s) } else { None });
            }
            L_ESC => {
                match ms.pat.get(p + 1).copied().unwrap_or(0) {
                    b'b' => {
                        let s2 = matchbalance(ms, s, p + 2)?;
                        if let Some(ns) = s2 {
                            s = ns;
                            p += 4;
                            continue 'outer; // tail call: match(ms, s, p+4)
                        }
                        break 'outer Ok(None);
                    }
                    b'f' => {
                        p += 2;
                        if ms.pat.get(p).copied() != Some(b'[') {
                            return Err(LuaError::runtime(format_args!(
                                "missing '[' after '%f' in pattern"
                            )));
                        }
                        let ep = classend(ms, p)?;
                        let previous = if s == 0 { 0u8 } else { ms.src[s - 1] };
                        let current = ms.src.get(s).copied().unwrap_or(0);
                        if !matchbracketclass(ms.pat, previous, p, ep - 1)
                            && matchbracketclass(ms.pat, current, p, ep - 1)
                        {
                            p = ep;
                            continue 'outer; // tail call: match(ms, s, ep)
                        }
                        break 'outer Ok(None);
                    }
                    c @ b'0'..=b'9' => {
                        let s2 = match_capture(ms, s, c)?;
                        if let Some(ns) = s2 {
                            s = ns;
                            p += 2;
                            continue 'outer; // tail call: match(ms, s, p+2)
                        }
                        break 'outer Ok(None);
                    }
                    _ => {
                        // fall through to default class handling
                        let ep = classend(ms, p)?;
                        let s2 = handle_class_with_suffix(ms, s, p, ep)?;
                        break 'outer Ok(s2);
                    }
                }
            }
            _ => {
                // default: pattern class plus optional suffix
                let ep = classend(ms, p)?;
                let s2 = handle_class_with_suffix(ms, s, p, ep)?;
                break 'outer Ok(s2);
            }
        }
    };

    ms.matchdepth += 1;
    result
}

/// Handle a pattern class element with an optional repetition suffix (`*`, `+`, `?`, `-`).
///
/// PORT NOTE: Factored out from `match_pat`'s `default/dflt` label to share
/// code between the ESC-default and plain-default paths.
#[inline(always)]
fn handle_class_with_suffix(
    ms: &mut MatchState,
    s: usize,
    p: usize,
    ep: usize,
) -> Result<Option<usize>, LuaError> {
    let matched_once = singlematch(ms, s, p, ep);
    if !matched_once {
        //    else s = NULL;
        match ms.pat.get(ep).copied() {
            Some(b'*') | Some(b'?') | Some(b'-') => {
                // Accept zero occurrences: tail-call match(ms, s, ep+1)
                // We can't do a tail call into match_pat because we're returning
                // from handle_class_with_suffix, but we can call it directly.
                return match_pat(ms, s, ep + 1);
            }
            _ => return Ok(None),
        }
    }

    // Matched at least once
    match ms.pat.get(ep).copied() {
        Some(b'?') => {
            // Optional: try matching with s+1, fall back to ep+1
            let res = match_pat(ms, s + 1, ep + 1)?;
            if res.is_some() {
                Ok(res)
            } else {
                match_pat(ms, s, ep + 1)
            }
        }
        Some(b'+') => {
            // 1 or more: greedy from s+1
            max_expand(ms, s + 1, p, ep)
        }
        Some(b'*') => {
            // 0 or more: greedy from s
            max_expand(ms, s, p, ep)
        }
        Some(b'-') => {
            // 0 or more: lazy from s
            min_expand(ms, s, p, ep)
        }
        _ => {
            // No suffix: match one, advance both s and p
            match_pat(ms, s + 1, ep)
        }
    }
}

// ────────────────────────────────────────────────────────────────────────────
// §5  Pattern-matching public API helpers
// ────────────────────────────────────────────────────────────────────────────

/// Find `needle` in `haystack` using a plain memmem-style search.
///
/// Returns the byte-offset of the first occurrence, or `None`.
fn lmemfind(haystack: &[u8], needle: &[u8]) -> Option<usize> {
    if needle.is_empty() {
        return Some(0);
    }
    if needle.len() > haystack.len() {
        return None;
    }
    let first = needle[0];
    let rest = &needle[1..];
    let limit = haystack.len() - rest.len();
    let mut s = 0;
    while s <= limit {
        if let Some(pos) = haystack[s..].iter().position(|&b| b == first) {
            let pos = s + pos;
            if pos + 1 + rest.len() <= haystack.len()
                && &haystack[pos + 1..pos + 1 + rest.len()] == rest
            {
                return Some(pos);
            }
            s = pos + 1;
        } else {
            break;
        }
    }
    None
}

fn required_start_byte(pat: &[u8]) -> Option<u8> {
    let (byte, ep) = match pat.first().copied()? {
        L_ESC => {
            let escaped = *pat.get(1)?;
            if escaped.is_ascii_alphanumeric() {
                return None;
            }
            (escaped, 2)
        }
        c if !SPECIALS.contains(&c) => (c, 1),
        _ => return None,
    };
    match pat.get(ep).copied() {
        Some(b'*') | Some(b'?') | Some(b'-') => None,
        _ => Some(byte),
    }
}

fn next_start_with_byte(src: &[u8], pos: usize, byte: u8) -> Option<usize> {
    src.get(pos..)?
        .iter()
        .position(|&c| c == byte)
        .map(|offset| pos + offset)
}

/// Check whether the pattern `pat` has no special characters (for plain search).
///
fn nospecials(pat: &[u8]) -> bool {
    !pat.iter().any(|b| SPECIALS.contains(b))
}

/// Information about one capture result.
enum CaptureInfo<'a> {
    /// A position capture; value is 1-based index.
    Position(i64),
    /// A string capture (slice of source).
    Bytes(&'a [u8]),
}

/// Get information about the `i`-th capture.
/// If there are no captures and `i == 0`, returns the whole match `s..e`.
///
fn get_one_capture<'a>(
    ms: &'a MatchState,
    i: usize,
    s: usize,
    e: usize,
) -> Result<CaptureInfo<'a>, LuaError> {
    if i >= ms.level as usize {
        if i != 0 {
            return Err(LuaError::runtime(format_args!(
                "invalid capture index %{}",
                i + 1
            )));
        }
        // Return whole match
        return Ok(CaptureInfo::Bytes(&ms.src[s..e]));
    }
    let cap = &ms.captures[i];
    if cap.len == CAP_UNFINISHED {
        return Err(LuaError::runtime(format_args!("unfinished capture")));
    }
    if cap.len == CAP_POSITION {
        return Ok(CaptureInfo::Position((cap.init + 1) as i64));
    }
    let len = cap.len as usize;
    Ok(CaptureInfo::Bytes(&ms.src[cap.init..cap.init + len]))
}

/// Push all captures onto the stack, returning the number of values pushed.
///
/// `span` mirrors upstream's `const char *s` argument: `Some((s, e))` means a
/// whole-match span is available (so a zero-capture pattern pushes the whole
/// match), while `None` mirrors a `NULL s` and pushes nothing when there are no
/// explicit captures. Upstream guard: `nlevels = (ms->level == 0 && s) ? 1 : ms->level`.
///
fn push_captures(
    state: &mut LuaState,
    ms: &MatchState,
    span: Option<(usize, usize)>,
) -> Result<usize, LuaError> {
    let nlevels = if ms.level == 0 && span.is_some() {
        1
    } else {
        ms.level as usize
    };
    state.ensure_stack(nlevels as i32, "too many captures")?;
    let (s, e) = span.unwrap_or((0, 0));
    for i in 0..nlevels {
        match get_one_capture(ms, i, s, e)? {
            CaptureInfo::Position(n) => state.push(LuaValue::Int(n)),
            CaptureInfo::Bytes(b) => state.push_bytes(b)?,
        }
    }
    Ok(nlevels)
}

// ────────────────────────────────────────────────────────────────────────────
// §6  str_find / str_match / gmatch / gsub
// ────────────────────────────────────────────────────────────────────────────

/// Shared implementation of `string.find` and `string.match`.
///
fn str_find_aux(state: &mut LuaState, find: bool) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let p_ref = match state.to_lua_string(2) {
        Some(r) => r,
        None => {
            state.check_arg_string(2)?;
            unreachable!("check_arg_string raises when arg #2 is not a string");
        }
    };
    let s: &[u8] = s_ref.as_bytes();
    let p: &[u8] = p_ref.as_bytes();
    let ls = s.len();
    let lp = p.len();
    let init_raw = state.opt_arg_integer(3, 1)?;
    let init = pos_relat_i(init_raw, ls).saturating_sub(1);

    if init > ls {
        state.push(LuaValue::Nil);
        return Ok(1);
    }

    if find && (state.arg_to_bool(4) || nospecials(p)) {
        // plain search
        if let Some(pos) = lmemfind(&s[init..], p) {
            let abs = init + pos;
            state.push(LuaValue::Int((abs + 1) as i64));
            state.push(LuaValue::Int((abs + lp) as i64));
            return Ok(2);
        }
    } else {
        let step_limit = state.sandbox_match_step_limit();
        let mut ms = MatchState::new(s, p, step_limit);
        let anchor = p.first() == Some(&b'^');
        let p_slice = if anchor { &p[1..] } else { p };
        ms.pat = p_slice;
        let start_byte = if anchor { None } else { required_start_byte(ms.pat) };

        let mut s1 = init;
        let mut matched: Option<usize> = None;
        loop {
            if let Some(byte) = start_byte {
                let Some(next) = next_start_with_byte(ms.src, s1, byte) else {
                    break;
                };
                s1 = next;
            }
            ms.reset_level();
            if let Some(res) = match_pat(&mut ms, s1, 0)? {
                matched = Some(res);
                break;
            }
            if ms.aborted || s1 >= ms.src.len() || anchor {
                break;
            }
            s1 += 1;
        }

        if let Some(err) = state.sandbox_charge(ms.steps) {
            return Err(err);
        }

        if let Some(res) = matched {
            if find {
                state.push(LuaValue::Int((s1 + 1) as i64));
                state.push(LuaValue::Int(res as i64));
                let nc = push_captures(state, &ms, None)?;
                return Ok(nc + 2);
            } else {
                return push_captures(state, &ms, Some((s1, res)));
            }
        }
    }

    state.push(LuaValue::Nil);
    Ok(1)
}

/// `string.find(s, pattern [, init [, plain]])` — find pattern in `s`.
///
pub fn str_find(state: &mut LuaState) -> Result<usize, LuaError> {
    str_find_aux(state, true)
}

/// `string.match(s, pattern [, init])` — match pattern against `s`.
///
pub fn str_match(state: &mut LuaState) -> Result<usize, LuaError> {
    str_find_aux(state, false)
}

/// Continuation function for `string.gmatch` iterator closure.
///
///
/// PORT NOTE: C stores source, pattern, and `GMatchState` as three C-closure
/// upvalues. The Rust port mirrors that shape: upvalues 1 and 2 are traced Lua
/// strings, and upvalue 3 is a full userdata whose host payload stores only the
/// mutable byte positions.
pub fn gmatch_aux(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_val = state.value_at(upvalue_index(1));
    let p_val = state.value_at(upvalue_index(2));
    let (LuaValue::Str(s_str), LuaValue::Str(p_str)) = (&s_val, &p_val) else {
        return Ok(0);
    };
    let iter_val = state.value_at(upvalue_index(3));
    let LuaValue::UserData(iter_ud) = iter_val else {
        return Ok(0);
    };
    let Some(host) = iter_ud.host_value() else {
        return Ok(0);
    };
    let Ok(iter_state) = host.downcast::<RefCell<GMatchIterState>>() else {
        return Ok(0);
    };

    let s: &[u8] = s_str.as_bytes();
    let p: &[u8] = p_str.as_bytes();
    let (start_pos, last_match) = {
        let iter = iter_state.borrow();
        (iter.pos, iter.last_match)
    };

    let ls = s.len();

    let step_limit = state.sandbox_match_step_limit();
    let mut ms = MatchState::new(s, p, step_limit);
    let start_byte = required_start_byte(p);

    let mut src = start_pos;
    let mut hit: Option<(usize, usize)> = None;
    while src <= ls {
        if let Some(byte) = start_byte {
            let Some(next) = next_start_with_byte(s, src, byte) else {
                break;
            };
            src = next;
        }
        ms.reset_level();
        if let Some(e) = match_pat(&mut ms, src, 0)? {
            if Some(e) != last_match {
                hit = Some((src, e));
                break;
            }
        }
        if ms.aborted {
            break;
        }
        src += 1;
    }

    if let Some(err) = state.sandbox_charge(ms.steps) {
        return Err(err);
    }

    if let Some((src, e)) = hit {
        {
            let mut iter = iter_state.borrow_mut();
            iter.pos = e;
            iter.last_match = Some(e);
        }
        return push_captures(state, &ms, Some((src, e)));
    }

    Ok(0)
}

/// `string.gmatch(s, pattern [, init])` — return an iterator for all matches.
///
///
/// PORT NOTE: C uses `lua_newuserdatauv` for the GMatchState plus a 3-upvalue
/// C closure. The port stores the two strings as traced closure upvalues and
/// the mutable byte positions in the userdata host payload.
pub fn gmatch(state: &mut LuaState) -> Result<usize, LuaError> {
    let s_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let ls = s_ref.len();
    match state.to_lua_string(2) {
        Some(_) => {}
        None => {
            state.check_arg_string(2)?;
            unreachable!("check_arg_string raises when arg #2 is not a string");
        }
    };
    let init_raw = state.opt_arg_integer(3, 1)?;
    let mut init = pos_relat_i(init_raw, ls).saturating_sub(1);
    if init > ls {
        init = ls + 1;
    }

    lua_vm::api::set_top(state, 2)?;

    state.push_value_at(1)?;
    state.push_value_at(2)?;
    let iter_ud = state.new_userdata_typed(b"string.gmatch.state", 0, 0)?;
    let iter_state: Rc<dyn Any> = Rc::new(RefCell::new(GMatchIterState {
        pos: init,
        last_match: None,
    }));
    iter_ud.set_host_value(Some(iter_state));

    state.push_c_closure(gmatch_aux, 3)?;
    Ok(1)
}

/// Add a replacement string with `%n` capture references to `buf`.
///
fn add_s(
    state: &mut LuaState,
    ms: &MatchState,
    buf: &mut Vec<u8>,
    s: usize,
    e: usize,
) -> Result<(), LuaError> {
    let news_bytes = state.to_lua_string_bytes(3).unwrap_or_default();
    let mut i = 0usize;
    while i < news_bytes.len() {
        if news_bytes[i] != L_ESC {
            buf.push(news_bytes[i]);
            i += 1;
        } else {
            i += 1; // skip ESC
            if i >= news_bytes.len() {
                break;
            }
            let c = news_bytes[i];
            if c == L_ESC {
                buf.push(L_ESC);
            } else if c == b'0' {
                buf.extend_from_slice(&ms.src[s..e]);
            } else if c.is_ascii_digit() {
                match get_one_capture(ms, (c - b'1') as usize, s, e)? {
                    CaptureInfo::Position(n) => {
                        // push position then pop into buf
                        let formatted = format!("{}", n).into_bytes();
                        buf.extend_from_slice(&formatted);
                    }
                    CaptureInfo::Bytes(b) => {
                        buf.extend_from_slice(b);
                    }
                }
            } else {
                return Err(LuaError::runtime(format_args!(
                    "invalid use of '{}' in replacement string",
                    L_ESC as char
                )));
            }
            i += 1;
        }
    }
    Ok(())
}

/// Add the replacement value (string, table lookup, or function call) to `buf`.
/// Returns `true` if the original text was changed.
///
fn add_value(
    state: &mut LuaState,
    ms: &MatchState,
    buf: &mut Vec<u8>,
    s: usize,
    e: usize,
    tr: LuaType,
) -> Result<bool, LuaError> {
    match tr {
        LuaType::Function => {
            state.push_value_at(3)?;
            let n = push_captures(state, ms, Some((s, e)))?;
            state.call(n as i32, 1)?;
        }
        LuaType::Table => {
            match get_one_capture(ms, 0, s, e)? {
                CaptureInfo::Position(n) => state.push(LuaValue::Int(n)),
                CaptureInfo::Bytes(b) => state.push_bytes(b)?,
            }
            state.get_table(3)?;
        }
        _ => {
            // LUA_TNUMBER or LUA_TSTRING: add replacement string directly
            add_s(state, ms, buf, s, e)?;
            return Ok(true);
        }
    }

    let top_bool = state.arg_to_bool(-1);
    if !top_bool {
        state.pop_n(1);
        buf.extend_from_slice(&ms.src[s..e]);
        return Ok(false);
    }
    if state.type_at(-1) != LuaType::String {
        let tname = state.type_name_at(-1).to_owned();
        return Err(LuaError::runtime(format_args!(
            "invalid replacement value (a {})", tname.escape_ascii()
        )));
    }
    let v = state.to_bytes(-1).unwrap_or_default();
    state.pop();
    buf.extend_from_slice(&v);
    Ok(true)
}

/// `string.gsub(s, pattern, repl [, n])` — global substitution.
///
pub fn str_gsub(state: &mut LuaState) -> Result<usize, LuaError> {
    let src_ref = match state.to_lua_string(1) {
        Some(r) => r,
        None => {
            state.check_arg_string(1)?;
            unreachable!("check_arg_string raises when arg #1 is not a string");
        }
    };
    let pat_ref = match state.to_lua_string(2) {
        Some(r) => r,
        None => {
            state.check_arg_string(2)?;
            unreachable!("check_arg_string raises when arg #2 is not a string");
        }
    };
    let src: &[u8] = src_ref.as_bytes();
    let pat: &[u8] = pat_ref.as_bytes();
    let src_len = src.len();
    let max_s = state.opt_arg_integer(4, (src_len + 1) as i64)?;
    let tr = state.type_at(3);

    if !matches!(tr, LuaType::Number | LuaType::String | LuaType::Function | LuaType::Table) {
        let v = state.arg(3);
        return Err(LuaError::type_arg_error(3, "string/function/table", &v));
    }

    let anchor = pat.first() == Some(&b'^');
    let pat_slice = if anchor { &pat[1..] } else { pat };

    let step_limit = state.sandbox_match_step_limit();
    let mut ms = MatchState::new(src, pat_slice, step_limit);
    let start_byte = if anchor { None } else { required_start_byte(ms.pat) };
    let mut buf: Vec<u8> = Vec::with_capacity(src_len);
    let mut src_pos = 0usize;
    let mut last_match: Option<usize> = None;
    let mut n: i64 = 0;
    let mut changed = false;

    while n < max_s {
        if let Some(byte) = start_byte {
            let Some(next) = next_start_with_byte(ms.src, src_pos, byte) else {
                buf.extend_from_slice(&ms.src[src_pos..]);
                src_pos = ms.src.len();
                break;
            };
            if next > src_pos {
                buf.extend_from_slice(&ms.src[src_pos..next]);
                src_pos = next;
            }
        }
        ms.reset_level();
        let maybe_e = match_pat(&mut ms, src_pos, 0)?;
        if let Some(e) = maybe_e {
            if last_match != Some(e) {
                n += 1;
                let delta = add_value(state, &ms, &mut buf, src_pos, e, tr)?;
                changed |= delta;
                src_pos = e;
                last_match = Some(e);
            } else if src_pos < ms.src.len() {
                buf.push(ms.src[src_pos]);
                src_pos += 1;
            } else {
                break;
            }
        } else if src_pos < ms.src.len() {
            buf.push(ms.src[src_pos]);
            src_pos += 1;
        } else {
            break;
        }
        if ms.aborted || anchor {
            break;
        }
    }

    if let Some(err) = state.sandbox_charge(ms.steps) {
        return Err(err);
    }

    if !changed {
        state.push_value_at(1)?;
    } else {
        buf.extend_from_slice(&ms.src[src_pos..]);
        state.push_bytes(&buf)?;
    }
    state.push(LuaValue::Int(n));
    Ok(2)
}

// ────────────────────────────────────────────────────────────────────────────
// §7  String format (`string.format`)
// ────────────────────────────────────────────────────────────────────────────

/// Add a hex-float digit to buffer and return the fractional remainder.
///
fn adddigit(buf: &mut Vec<u8>, x: f64) -> f64 {
    let dd = x.floor();
    let d = dd as i32;
    let c = if d < 10 { b'0' + d as u8 } else { b'a' + (d - 10) as u8 };
    buf.push(c);
    x - dd
}

/// Convert a float to a hex-float string body (digits only, no sign, no `0x` prefix).
///
/// Returns `(frac_digits, exponent_string)` for use by `format_hex_float`.
///
fn num2straux(x: f64) -> Vec<u8> {
    format_hex_float(x, None)
}

/// Produce a hex-float string for `x` with optional precision (digits after the point).
///
/// When `precision` is `None` the minimum number of digits needed for a round-trip
/// is emitted (C's default `%a` behaviour). When `precision` is `Some(p)` exactly `p`
/// digits follow the radix point; trailing zeros are added as needed, and excess
/// digits are discarded (C truncates rather than rounds, matching the C `printf`
/// behaviour on the tested platforms).
fn format_hex_float(x: f64, precision: Option<usize>) -> Vec<u8> {
    if x.is_nan() {
        return b"nan".to_vec();
    }
    if x.is_infinite() {
        return if x < 0.0 { b"-inf".to_vec() } else { b"inf".to_vec() };
    }
    if x == 0.0 {
        let sign: &[u8] = if x.is_sign_negative() { b"-" } else { b"" };
        return match precision {
            None => [sign, b"0x0p+0"].concat(),
            Some(0) => [sign, b"0x0p+0"].concat(),
            Some(p) => {
                let zeros = "0".repeat(p);
                [sign, b"0x0.", zeros.as_bytes(), b"p+0"].concat()
            }
        };
    }

    let (m_raw, exp) = frexp(x);
    let mut buf: Vec<u8> = Vec::new();
    let mut m = m_raw;
    if m < 0.0 {
        buf.push(b'-');
        m = -m;
    }
    buf.extend_from_slice(b"0x");

    let nbfd = 1;
    m = adddigit(&mut buf, m * (1 << nbfd) as f64);
    let e = exp - nbfd;

    match precision {
        None => {
            if m > 0.0 {
                buf.push(b'.');
                while m > 0.0 {
                    m = adddigit(&mut buf, m * 16.0);
                }
            }
        }
        Some(0) => {}
        Some(p) => {
            buf.push(b'.');
            for _ in 0..p {
                if m > 0.0 {
                    m = adddigit(&mut buf, m * 16.0);
                } else {
                    buf.push(b'0');
                }
            }
        }
    }

    let exp_str = format!("p{:+}", e);
    buf.extend_from_slice(exp_str.as_bytes());
    buf
}

/// Decompose `x` into mantissa in `[-1.0, -0.5] ∪ [0.5, 1.0)` and exponent.
///
/// Equivalent to C's `frexp`. The sign of `x` is preserved in the returned mantissa
/// so that `num2straux` can emit the leading `-` correctly for negative inputs.
fn frexp(x: f64) -> (f64, i32) {
    if x == 0.0 || x.is_nan() || x.is_infinite() {
        return (x, 0);
    }
    let bits = x.to_bits();
    let sign_bit = bits & 0x8000_0000_0000_0000u64;
    let exp_bits = ((bits >> 52) & 0x7FF) as i32;
    if exp_bits == 0 {
        let (m, e) = frexp(x * (1u64 << 52) as f64);
        return (m, e - 52);
    }
    let exp = exp_bits - 1022;
    let mantissa_bits = sign_bit | (bits & 0x000F_FFFF_FFFF_FFFF) | 0x3FE0_0000_0000_0000;
    (f64::from_bits(mantissa_bits), exp)
}

/// Convert float `n` to a Lua-readable literal (hex or special representation).
///
fn quotefloat(n: f64) -> Vec<u8> {
    if n == f64::INFINITY {
        return b"1e9999".to_vec();
    } else if n == f64::NEG_INFINITY {
        return b"-1e9999".to_vec();
    } else if n.is_nan() {
        return b"(0/0)".to_vec();
    }
    // hex float, ensuring dot separator
    let buf = num2straux(n);
    if !buf.contains(&b'.') && !buf.contains(&b'p') {
        // try to find locale decimal point and replace with '.'
        // PORT NOTE: We always produce '.' so this branch is not taken.
    }
    buf
}

/// Add a quoted Lua string literal to `buf`.
///
fn addquoted(buf: &mut Vec<u8>, s: &[u8]) {
    buf.push(b'"');
    for (idx, &c) in s.iter().enumerate() {
        if c == b'"' || c == b'\\' || c == b'\n' {
            buf.push(b'\\');
            buf.push(c);
        } else if c.is_ascii_control() {
            let next_is_digit = s.get(idx + 1).map_or(false, |n| n.is_ascii_digit());
            let formatted = if next_is_digit {
                format!("\\{:03}", c)
            } else {
                format!("\\{}", c)
            };
            buf.extend_from_slice(formatted.as_bytes());
        } else {
            buf.push(c);
        }
    }
    buf.push(b'"');
}

/// Add a Lua literal representation of arg `n` to `buf`.
///
fn addliteral(state: &mut LuaState, buf: &mut Vec<u8>, arg: i32) -> Result<(), LuaError> {
    match state.type_at(arg) {
        LuaType::String => {
            let s = state.check_arg_string(arg)?.to_vec();
            addquoted(buf, &s);
        }
        LuaType::Number => {
            if state.is_integer(arg) {
                let n = state.to_integer(arg).unwrap_or(0);
                let formatted = if n == i64::MIN {
                    format!("0x{:016x}", n as u64)
                } else {
                    format!("{}", n)
                };
                buf.extend_from_slice(formatted.as_bytes());
            } else {
                let n = state.to_number(arg).unwrap_or(0.0);
                let hex = quotefloat(n);
                buf.extend_from_slice(&hex);
            }
        }
        LuaType::Nil => {
            buf.extend_from_slice(b"nil");
        }
        LuaType::Boolean => {
            buf.extend_from_slice(if state.to_boolean(arg) { b"true" } else { b"false" });
        }
        _ => {
            return Err(LuaError::arg_error(arg, "value has no literal form"));
        }
    }
    Ok(())
}


/// Flags allowed per conversion type (matches lstrlib.c constants).
const FMT_FLAGS_F: &[u8] = b"-+#0 ";
const FMT_FLAGS_X: &[u8] = b"-#0";
const FMT_FLAGS_I: &[u8] = b"-+0 ";
const FMT_FLAGS_U: &[u8] = b"-0";
const FMT_FLAGS_C: &[u8] = b"-";

/// Validate a format specifier against allowed flags and width/precision digit counts.
///
/// `form` is the full specifier slice including the leading `%` and the trailing
/// conversion character (e.g. `b"%100.3d"`). `flags` is the allowed-flags byte set for
/// this conversion type. `allow_precision` is false for conversions that forbid `.`.
///
/// Mirrors C `checkformat` in lstrlib.c: consumes flags, then up to 2 width digits,
/// then (if allowed) `.` + up to 2 precision digits, then asserts we are at the
/// conversion character. Returns `Err("invalid conversion specification")` on failure.
fn check_conv_spec(
    state: &mut LuaState,
    form: &[u8],
    flags: &[u8],
    allow_precision: bool,
) -> Result<(), LuaError> {
    let mut i = 1usize; // skip '%'
    while i < form.len() && flags.contains(&form[i]) {
        i += 1;
    }
    if i < form.len() && form[i] == b'0' {
        return Err(invalid_conv_spec(state, form));
    }
    if i < form.len() && form[i].is_ascii_digit() {
        i += 1;
        if i < form.len() && form[i].is_ascii_digit() {
            i += 1;
        }
    }
    if allow_precision && i < form.len() && form[i] == b'.' {
        i += 1;
        if i < form.len() && form[i].is_ascii_digit() {
            i += 1;
            if i < form.len() && form[i].is_ascii_digit() {
                i += 1;
            }
        }
    }
    if i != form.len() - 1 {
        return Err(invalid_conv_spec(state, form));
    }
    Ok(())
}

/// Build the version-appropriate "invalid conversion specification" error,
/// prefixed with the calling location like reference `luaL_error`.
///
/// Lua 5.3 `scanformat` raises `invalid format (width or precision too long)`
/// with no offending spec; Lua 5.4/5.5 `checkformat` raises
/// `invalid conversion specification: '<form>'`.
fn invalid_conv_spec(state: &mut LuaState, form: &[u8]) -> LuaError {
    let msg: Vec<u8> = if state.global().lua_version == lua_types::LuaVersion::V53 {
        b"invalid format (width or precision too long)".to_vec()
    } else {
        let mut m = b"invalid conversion specification: '".to_vec();
        m.extend_from_slice(form);
        m.push(b'\'');
        m
    };
    lua_vm::debug::c_api_runtime(state, msg)
}

/// Parsed printf-style format specifier (flags, width, precision).
#[derive(Default)]
struct FmtSpec {
    left_align: bool,
    plus_sign: bool,
    space_sign: bool,
    alt_form: bool,
    zero_pad: bool,
    width: usize,
    precision: Option<usize>,
}

fn parse_fmt_spec(spec: &[u8]) -> FmtSpec {
    let mut s = FmtSpec::default();
    let mut i = 0;
    while i < spec.len() {
        match spec[i] {
            b'-' => s.left_align = true,
            b'+' => s.plus_sign = true,
            b' ' => s.space_sign = true,
            b'#' => s.alt_form = true,
            b'0' => s.zero_pad = true,
            _ => break,
        }
        i += 1;
    }
    while i < spec.len() && spec[i].is_ascii_digit() {
        s.width = s.width * 10 + (spec[i] - b'0') as usize;
        i += 1;
    }
    if i < spec.len() && spec[i] == b'.' {
        i += 1;
        let mut p = 0usize;
        while i < spec.len() && spec[i].is_ascii_digit() {
            p = p * 10 + (spec[i] - b'0') as usize;
            i += 1;
        }
        s.precision = Some(p);
    }
    s
}

fn pad_str(buf: &mut Vec<u8>, body: &[u8], spec: &FmtSpec) {
    let body = match spec.precision {
        Some(p) if body.len() > p => &body[..p],
        _ => body,
    };
    if body.len() >= spec.width {
        buf.extend_from_slice(body);
        return;
    }
    let pad = spec.width - body.len();
    if spec.left_align {
        buf.extend_from_slice(body);
        for _ in 0..pad { buf.push(b' '); }
    } else {
        for _ in 0..pad { buf.push(b' '); }
        buf.extend_from_slice(body);
    }
}

fn pad_int(buf: &mut Vec<u8>, sign_prefix: &[u8], digits: &[u8], spec: &FmtSpec) {
    let min_digits = spec.precision.unwrap_or(0);
    let zeroes_for_prec = if digits.len() < min_digits { min_digits - digits.len() } else { 0 };
    let core_len = sign_prefix.len() + zeroes_for_prec + digits.len();
    if core_len >= spec.width {
        buf.extend_from_slice(sign_prefix);
        for _ in 0..zeroes_for_prec { buf.push(b'0'); }
        buf.extend_from_slice(digits);
        return;
    }
    let pad = spec.width - core_len;
    let use_zero_pad = spec.zero_pad && !spec.left_align && spec.precision.is_none();
    if spec.left_align {
        buf.extend_from_slice(sign_prefix);
        for _ in 0..zeroes_for_prec { buf.push(b'0'); }
        buf.extend_from_slice(digits);
        for _ in 0..pad { buf.push(b' '); }
    } else if use_zero_pad {
        buf.extend_from_slice(sign_prefix);
        for _ in 0..pad { buf.push(b'0'); }
        for _ in 0..zeroes_for_prec { buf.push(b'0'); }
        buf.extend_from_slice(digits);
    } else {
        for _ in 0..pad { buf.push(b' '); }
        buf.extend_from_slice(sign_prefix);
        for _ in 0..zeroes_for_prec { buf.push(b'0'); }
        buf.extend_from_slice(digits);
    }
}

fn signed_int_parts(n: i64, spec: &FmtSpec) -> (Vec<u8>, Vec<u8>) {
    if n == 0 && spec.precision == Some(0) {
        return (Vec::new(), Vec::new());
    }
    let (sign, abs_digits) = if n < 0 {
        (b"-".to_vec(), {
            let u = (n as i128).unsigned_abs();
            format!("{}", u).into_bytes()
        })
    } else {
        let s: Vec<u8> = if spec.plus_sign {
            b"+".to_vec()
        } else if spec.space_sign {
            b" ".to_vec()
        } else {
            Vec::new()
        };
        (s, format!("{}", n).into_bytes())
    };
    (sign, abs_digits)
}

fn unsigned_int_parts(n: u64, base: u32, upper: bool, spec: &FmtSpec) -> (Vec<u8>, Vec<u8>) {
    let digits = if n == 0 && spec.precision == Some(0) {
        Vec::new()
    } else {
        match base {
            8 => format!("{:o}", n).into_bytes(),
            16 if upper => format!("{:X}", n).into_bytes(),
            16 => format!("{:x}", n).into_bytes(),
            _ => format!("{}", n).into_bytes(),
        }
    };
    let prefix: Vec<u8> = if spec.alt_form && n != 0 {
        match base {
            8 => b"0".to_vec(),
            16 if upper => b"0X".to_vec(),
            16 => b"0x".to_vec(),
            _ => Vec::new(),
        }
    } else {
        Vec::new()
    };
    (prefix, digits)
}

fn format_float(n: f64, conv: u8, spec: &FmtSpec) -> Vec<u8> {
    let prec = spec.precision.unwrap_or(6);
    if n.is_nan() {
        return if conv.is_ascii_uppercase() { b"NAN".to_vec() } else { b"nan".to_vec() };
    }
    if n.is_infinite() {
        let s: &[u8] = if conv.is_ascii_uppercase() {
            if n < 0.0 { b"-INF" } else { b"INF" }
        } else if n < 0.0 { b"-inf" } else { b"inf" };
        return s.to_vec();
    }
    match conv {
        b'f' | b'F' => {
            let mut result = format!("{:.*}", prec, n).into_bytes();
            if spec.alt_form && !result.contains(&b'.') {
                result.push(b'.');
            }
            result
        }
        b'e' => format_exp(n, prec, false, spec.alt_form),
        b'E' => {
            let mut v = format_exp(n, prec, false, spec.alt_form);
            for b in v.iter_mut() { if *b == b'e' { *b = b'E'; } }
            v
        }
        b'g' | b'G' => {
            let p = if prec == 0 { 1 } else { prec };
            let v = format_g(n, p, spec.alt_form);
            if conv == b'G' {
                v.into_iter().map(|b| if b == b'e' { b'E' } else { b }).collect()
            } else { v }
        }
        _ => format!("{}", n).into_bytes(),
    }
}

fn format_exp(n: f64, prec: usize, _upper: bool, alt: bool) -> Vec<u8> {
    if n == 0.0 {
        let mantissa: String = if prec == 0 {
            if alt { "0.".to_string() } else { "0".to_string() }
        } else {
            format!("0.{}", "0".repeat(prec))
        };
        return format!("{}e+00", mantissa).into_bytes();
    }
    let abs = n.abs();
    let exp = abs.log10().floor() as i32;
    let mantissa = n / 10f64.powi(exp);
    let mantissa_str = format!("{:.*}", prec, mantissa);
    let (mant_final, exp_final) = if let Some(dot_pos) = mantissa_str.find('.') {
        let int_part = &mantissa_str[..dot_pos];
        let abs_int = int_part.trim_start_matches('-');
        if abs_int.len() > 1 {
            let new_mant = if prec == 0 {
                mantissa_str[..mantissa_str.len()-1].to_string()
            } else {
                let neg = if int_part.starts_with('-') { "-" } else { "" };
                let frac = &mantissa_str[dot_pos+1..];
                format!("{}{}.{}{}", neg, &abs_int[..1], &abs_int[1..], frac)
            };
            (new_mant, exp + (abs_int.len() as i32 - 1))
        } else {
            (mantissa_str, exp)
        }
    } else if mantissa_str.trim_start_matches('-').len() > 1 {
        let neg = if mantissa_str.starts_with('-') { "-" } else { "" };
        let body = mantissa_str.trim_start_matches('-');
        let bumped = format!("{}{}.{}", neg, &body[..1], &body[1..]);
        (bumped, exp + (body.len() as i32 - 1))
    } else {
        (mantissa_str, exp)
    };
    let sign = if exp_final < 0 { '-' } else { '+' };
    let mant_out = if alt && !mant_final.contains('.') {
        format!("{}.", mant_final)
    } else { mant_final };
    format!("{}e{}{:02}", mant_out, sign, exp_final.abs()).into_bytes()
}

fn format_g(n: f64, prec: usize, alt: bool) -> Vec<u8> {
    if n == 0.0 {
        return if alt { format!("0.{}", "0".repeat(prec.saturating_sub(1))).into_bytes() } else { b"0".to_vec() };
    }
    let abs = n.abs();
    let exp = abs.log10().floor() as i32;
    if exp < -4 || exp >= prec as i32 {
        let ep = if prec == 0 { 0 } else { prec - 1 };
        let mut v = format_exp(n, ep, false, alt);
        if !alt {
            v = strip_trailing_zeros_exp(&v);
        }
        v
    } else {
        let dec_places = (prec as i32 - 1 - exp).max(0) as usize;
        let mut v = format!("{:.*}", dec_places, n).into_bytes();
        if !alt {
            v = strip_trailing_zeros_fixed(&v);
        }
        v
    }
}

fn strip_trailing_zeros_fixed(s: &[u8]) -> Vec<u8> {
    if !s.contains(&b'.') { return s.to_vec(); }
    let mut end = s.len();
    while end > 0 && s[end-1] == b'0' { end -= 1; }
    if end > 0 && s[end-1] == b'.' { end -= 1; }
    s[..end].to_vec()
}

fn strip_trailing_zeros_exp(s: &[u8]) -> Vec<u8> {
    let e_pos = match s.iter().position(|&b| b == b'e' || b == b'E') {
        Some(p) => p,
        None => return s.to_vec(),
    };
    let mantissa = &s[..e_pos];
    let exp_part = &s[e_pos..];
    if !mantissa.contains(&b'.') {
        let mut out = mantissa.to_vec();
        out.extend_from_slice(exp_part);
        return out;
    }
    let mut end = mantissa.len();
    while end > 0 && mantissa[end-1] == b'0' { end -= 1; }
    if end > 0 && mantissa[end-1] == b'.' { end -= 1; }
    let mut out = mantissa[..end].to_vec();
    out.extend_from_slice(exp_part);
    out
}

/// `string.format(fmt, ...)` — C-style string formatting.
///
/// Fetch the integer argument for a `%d`/`%i`/`%u`/`%o`/`%x`/`%X` conversion.
///
/// On the dual-number versions (5.3+) an integer is required and a non-integral
/// number raises "number has no integer representation". On the float-only
/// versions (5.1/5.2) there is no integer subtype, so `string.format` truncates
/// the number toward zero — `("%d"):format(3.5)` is `3`, `(-3.5)` is `-3` —
/// matching lua5.2.4. A value outside the `lua_Integer` range (including inf/nan)
/// raises "number has no integer representation", which lua5.2.4 phrases as
/// "not a number in proper range"; the harness battery checks the truncation
/// cases (the out-of-range message text is a separate 5.2 error-format gap).
fn format_int_arg(state: &mut LuaState, arg: i32) -> Result<i64, LuaError> {
    if state.global().lua_version.number_model() != lua_types::NumberModel::FloatOnly {
        return state.check_arg_integer(arg);
    }
    let n = state.check_arg_number(arg)?;
    let t = n.trunc();
    if t.is_finite() && (-9223372036854775808.0..=9223372036854775808.0).contains(&t) {
        Ok(t as i64)
    } else {
        Err(LuaError::arg_error(arg, "number has no integer representation"))
    }
}

pub fn str_format(state: &mut LuaState) -> Result<usize, LuaError> {
    let top = state.get_top();
    let mut arg = 1i32;
    let fmt_bytes = state.check_arg_string(1)?.to_vec();
    let mut buf: Vec<u8> = Vec::new();
    let mut i = 0usize;

    while i < fmt_bytes.len() {
        let c = fmt_bytes[i];
        if c != L_ESC {
            buf.push(c);
            i += 1;
            continue;
        }
        i += 1;
        if i >= fmt_bytes.len() {
            break;
        }
        if fmt_bytes[i] == L_ESC {
            buf.push(L_ESC);
            i += 1;
            continue;
        }

        // Parse a format specifier
        arg += 1;
        if arg > top {
            return Err(lua_vm::debug::arg_error_impl(state, arg, b"no value"));
        }

        // Collect flags, width, precision
        let spec_start = i - 1; // includes the initial '%'
        // Skip flags: -, +, #, 0, space
        while i < fmt_bytes.len() && b"-+#0 ".contains(&fmt_bytes[i]) {
            i += 1;
        }
        // Lua 5.3 `scanformat`: the flags buffer is `FLAGS = "-+ #0"`, so a flags
        // run of `sizeof(FLAGS) == 6` or more characters is "repeated flags".
        // 5.4/5.5 fold this into the single "(too long)" check below.
        if state.global().lua_version == lua_types::LuaVersion::V53
            && i - (spec_start + 1) >= 6
        {
            return Err(lua_vm::debug::c_api_runtime(
                state,
                b"invalid format (repeated flags)".to_vec(),
            ));
        }
        // Skip width digits
        if i < fmt_bytes.len() && fmt_bytes[i] != b'0' {
            while i < fmt_bytes.len() && fmt_bytes[i].is_ascii_digit() {
                i += 1;
            }
        }
        // Skip precision
        if i < fmt_bytes.len() && fmt_bytes[i] == b'.' {
            i += 1;
            while i < fmt_bytes.len() && fmt_bytes[i].is_ascii_digit() {
                i += 1;
            }
        }

        if i >= fmt_bytes.len() {
            let form: Vec<u8> = fmt_bytes[spec_start..].to_vec();
            return Err(invalid_conv_spec(state, &form));
        }

        let conv = fmt_bytes[i];
        i += 1;

        let spec_slice = &fmt_bytes[spec_start + 1..i - 1];
        let form = &fmt_bytes[spec_start..i];

        // Must check before parse_fmt_spec to avoid overflow on huge widths.
        if spec_slice.len() + 1 >= 22 {
            return Err(lua_vm::debug::c_api_runtime(state, b"invalid format (too long)".to_vec()));
        }

        let spec = parse_fmt_spec(spec_slice);

        match conv {
            b'c' => {
                check_conv_spec(state, form, FMT_FLAGS_C, false)?;
                let n = state.check_arg_integer(arg)?;
                let body = vec![n as u8];
                pad_str(&mut buf, &body, &spec);
            }
            b'd' | b'i' => {
                check_conv_spec(state, form, FMT_FLAGS_I, true)?;
                let n = format_int_arg(state, arg)?;
                let (sign, digits) = signed_int_parts(n, &spec);
                pad_int(&mut buf, &sign, &digits, &spec);
            }
            b'u' => {
                check_conv_spec(state, form, FMT_FLAGS_U, true)?;
                let n = format_int_arg(state, arg)? as u64;
                let (prefix, digits) = unsigned_int_parts(n, 10, false, &spec);
                pad_int(&mut buf, &prefix, &digits, &spec);
            }
            b'o' => {
                check_conv_spec(state, form, FMT_FLAGS_X, true)?;
                let n = format_int_arg(state, arg)? as u64;
                let (prefix, digits) = unsigned_int_parts(n, 8, false, &spec);
                pad_int(&mut buf, &prefix, &digits, &spec);
            }
            b'x' => {
                check_conv_spec(state, form, FMT_FLAGS_X, true)?;
                let n = format_int_arg(state, arg)? as u64;
                let (prefix, digits) = unsigned_int_parts(n, 16, false, &spec);
                pad_int(&mut buf, &prefix, &digits, &spec);
            }
            b'X' => {
                check_conv_spec(state, form, FMT_FLAGS_X, true)?;
                let n = format_int_arg(state, arg)? as u64;
                let (prefix, digits) = unsigned_int_parts(n, 16, true, &spec);
                pad_int(&mut buf, &prefix, &digits, &spec);
            }
            b'a' | b'A' => {
                check_conv_spec(state, form, FMT_FLAGS_F, true)?;
                let n = state.check_arg_number(arg)?;
                let body = format_hex_float(n, spec.precision);
                let body: Vec<u8> = if conv == b'A' {
                    body.into_iter().map(|b| b.to_ascii_uppercase()).collect()
                } else {
                    body
                };
                let (sign, digits): (Vec<u8>, Vec<u8>) =
                    if !body.is_empty() && (body[0] == b'-' || body[0] == b'+') {
                        (vec![body[0]], body[1..].to_vec())
                    } else if spec.plus_sign {
                        (b"+".to_vec(), body)
                    } else if spec.space_sign {
                        (b" ".to_vec(), body)
                    } else {
                        (Vec::new(), body)
                    };
                let no_prec_spec = FmtSpec {
                    left_align: spec.left_align,
                    plus_sign: spec.plus_sign,
                    space_sign: spec.space_sign,
                    alt_form: spec.alt_form,
                    zero_pad: spec.zero_pad,
                    width: spec.width,
                    precision: None,
                };
                pad_int(&mut buf, &sign, &digits, &no_prec_spec);
            }
            b'f' | b'e' | b'E' | b'g' | b'G' => {
                check_conv_spec(state, form, FMT_FLAGS_F, true)?;
                let n = state.check_arg_number(arg)?;
                let body = format_float(n, conv, &spec);
                let (sign, digits): (Vec<u8>, Vec<u8>) = if !body.is_empty() && (body[0] == b'-' || body[0] == b'+') {
                    (vec![body[0]], body[1..].to_vec())
                } else if n >= 0.0 && spec.plus_sign {
                    (b"+".to_vec(), body)
                } else if n >= 0.0 && spec.space_sign {
                    (b" ".to_vec(), body)
                } else {
                    (Vec::new(), body)
                };
                let no_prec_spec = FmtSpec {
                    left_align: spec.left_align,
                    plus_sign: spec.plus_sign,
                    space_sign: spec.space_sign,
                    alt_form: spec.alt_form,
                    zero_pad: spec.zero_pad,
                    width: spec.width,
                    precision: None,
                };
                pad_int(&mut buf, &sign, &digits, &no_prec_spec);
            }
            b'p' => {
                check_conv_spec(state, form, FMT_FLAGS_C, false)?;
                let s: Vec<u8> = match lua_vm::api::to_pointer(state, arg) {
                    Some(p) => format!("0x{:x}", p).into_bytes(),
                    None => b"(null)".to_vec(),
                };
                pad_str(&mut buf, &s, &FmtSpec { precision: None, ..spec });
            }
            b'q' => {
                if form.len() > 2 {
                    return Err(LuaError::runtime(format_args!(
                        "specifier '%q' cannot have modifiers"
                    )));
                }
                addliteral(state, &mut buf, arg)?;
            }
            b's' => {
                check_conv_spec(state, form, FMT_FLAGS_C, true)?;
                let s = state.to_display_string(arg)?;
                let has_modifiers = spec.width != 0 || spec.precision.is_some();
                if has_modifiers && s.contains(&0u8) {
                    return Err(lua_vm::debug::arg_error_impl(state, arg, b"string contains zeros"));
                }
                pad_str(&mut buf, &s, &spec);
                state.pop_n(1);
            }
            _ => {
                let verb: &[u8] = if state.global().lua_version == lua_types::LuaVersion::V53 {
                    b"option"
                } else {
                    b"conversion"
                };
                let mut msg = b"invalid ".to_vec();
                msg.extend_from_slice(verb);
                msg.extend_from_slice(b" '");
                msg.extend_from_slice(form);
                msg.extend_from_slice(b"' to 'format'");
                return Err(lua_vm::debug::c_api_runtime(state, msg));
            }
        }
    }

    state.push_bytes(&buf)?;
    Ok(1)
}

// ────────────────────────────────────────────────────────────────────────────
// §8  Pack / unpack
// ────────────────────────────────────────────────────────────────────────────

/// Return `true` if `c` is an ASCII digit.
fn is_digit(c: u8) -> bool {
    c.is_ascii_digit()
}

/// Read an optional integer from the format string, returning `df` if absent.
///
/// `wide` selects the accumulator width: 5.3/5.4 used `int` (cap `i32::MAX`);
/// 5.5 uses `size_t` (cap the host pointer width). The reference stops consuming
/// digits once another `*10 + 9` would overflow, leaving the rest to be read as
/// the next option — which is why `c<int-overflow>` yields "invalid format
/// option '<digit>'" on 5.3/5.4 but parses cleanly on 5.5.
fn getnum(fmt: &[u8], pos: &mut usize, df: i64, wide: bool) -> i64 {
    if *pos >= fmt.len() || !is_digit(fmt[*pos]) {
        return df;
    }
    let cap: i64 = if wide { i64::MAX } else { i32::MAX as i64 };
    let mut a = 0i64;
    while *pos < fmt.len() && is_digit(fmt[*pos]) {
        a = a * 10 + (fmt[*pos] - b'0') as i64;
        *pos += 1;
        if a > (cap - 9) / 10 {
            break;
        }
    }
    a
}

/// Read an integer from the format string, error if out of `[1, MAXINTSIZE]`.
///
fn getnumlimit(fmt: &[u8], pos: &mut usize, df: i64) -> Result<usize, LuaError> {
    let sz = getnum(fmt, pos, df, false);
    if sz > MAX_INT_SIZE as i64 || sz <= 0 {
        return Err(LuaError::runtime(format_args!(
            "integral size ({}) out of limits [1,{}]",
            sz, MAX_INT_SIZE
        )));
    }
    Ok(sz as usize)
}

/// Read and classify the next pack format option, filling `size`.
///
fn getoption(h: &mut Header, fmt: &[u8], pos: &mut usize, size: &mut usize) -> Result<KOption, LuaError> {
    // In Rust, the native max-align of a union of f64/void*/size_t is 8 on 64-bit.
    const NATIVE_MAX_ALIGN: usize = std::mem::align_of::<f64>();

    if *pos >= fmt.len() {
        return Ok(KOption::Nop);
    }
    let opt = fmt[*pos];
    *pos += 1;
    *size = 0;

    match opt {
        b'b' => { *size = 1; Ok(KOption::Int) }
        b'B' => { *size = 1; Ok(KOption::Uint) }
        b'h' => { *size = 2; Ok(KOption::Int) }
        b'H' => { *size = 2; Ok(KOption::Uint) }
        b'l' => { *size = 8; Ok(KOption::Int) }  // sizeof(long) on 64-bit
        b'L' => { *size = 8; Ok(KOption::Uint) }
        b'j' => { *size = SZINT; Ok(KOption::Int) }
        b'J' => { *size = SZINT; Ok(KOption::Uint) }
        b'T' => { *size = std::mem::size_of::<usize>(); Ok(KOption::Uint) }
        b'f' => { *size = 4; Ok(KOption::Float) }
        b'n' => { *size = 8; Ok(KOption::Number) }  // sizeof(lua_Number) = sizeof(f64) = 8
        b'd' => { *size = 8; Ok(KOption::Double) }  // sizeof(double) = 8
        b'i' => { *size = getnumlimit(fmt, pos, 4)?; Ok(KOption::Int) }
        b'I' => { *size = getnumlimit(fmt, pos, 4)?; Ok(KOption::Uint) }
        b's' => { *size = getnumlimit(fmt, pos, std::mem::size_of::<usize>()  as i64)?; Ok(KOption::Kstring) }
        b'c' => {
            let n = getnum(fmt, pos, -1, h.wide_size);
            if n == -1 {
                return Err(LuaError::runtime(format_args!("missing size for format option 'c'")));
            }
            *size = n as usize;
            Ok(KOption::Char)
        }
        b'z' => Ok(KOption::Zstr),
        b'x' => { *size = 1; Ok(KOption::Padding) }
        b'X' => Ok(KOption::Paddalign),
        b' ' => Ok(KOption::Nop),
        b'<' => { h.is_little = true; Ok(KOption::Nop) }
        b'>' => { h.is_little = false; Ok(KOption::Nop) }
        b'=' => { h.is_little = cfg!(target_endian = "little"); Ok(KOption::Nop) }
        b'!' => {
            let n = getnum(fmt, pos, NATIVE_MAX_ALIGN as i64, false);
            h.max_align = getnumlimit(fmt, pos, n)?;
            Ok(KOption::Nop)
        }
        _ => Err(LuaError::runtime(format_args!("invalid format option '{}'", opt as char)))
    }
}

/// Get full details about the next format option, including alignment padding.
///
fn getdetails(
    state: &mut LuaState,
    h: &mut Header,
    total_size: usize,
    fmt: &[u8],
    pos: &mut usize,
    psize: &mut usize,
    ntoalign: &mut usize,
) -> Result<KOption, LuaError> {
    let opt = getoption(h, fmt, pos, psize)?;
    let mut align = *psize;

    if opt == KOption::Paddalign {
        if *pos >= fmt.len() {
            return Err(lua_vm::debug::arg_error_impl(state, 1, b"invalid next option for option 'X'"));
        }
        let mut dummy_size = 0usize;
        let next_opt = getoption(h, fmt, pos, &mut dummy_size)?;
        align = dummy_size;
        if next_opt == KOption::Char || align == 0 {
            return Err(lua_vm::debug::arg_error_impl(state, 1, b"invalid next option for option 'X'"));
        }
    }

    if align <= 1 || opt == KOption::Char {
        *ntoalign = 0;
    } else {
        if align > h.max_align {
            align = h.max_align;
        }
        if (align & (align - 1)) != 0 {
            return Err(lua_vm::debug::arg_error_impl(state, 1, b"format asks for alignment not power of 2"));
        }
        *ntoalign = (align - (total_size & (align - 1))) & (align - 1);
    }
    Ok(opt)
}

/// Pack integer `n` with `size` bytes into `buf` with given endianness.
///
fn packint(buf: &mut Vec<u8>, mut n: u64, is_little: bool, size: usize, neg: bool) {
    let start = buf.len();
    buf.resize(start + size, 0);
    let slice = &mut buf[start..start + size];
    // Write LSB first (little-endian), then swap if big-endian
    for i in 0..size {
        slice[if is_little { i } else { size - 1 - i }] = (n & MC as u64) as u8;
        n >>= NB;
    }
    // Sign extension for negative numbers larger than lua_Integer
    if neg && size > SZINT {
        for i in SZINT..size {
            slice[if is_little { i } else { size - 1 - i }] = MC;
        }
    }
}

/// Copy bytes with endianness correction.
///
fn copywithendian(dest: &mut [u8], src: &[u8], is_little: bool) {
    debug_assert_eq!(dest.len(), src.len());
    if is_little == cfg!(target_endian = "little") {
        dest.copy_from_slice(src);
    } else {
        for (d, s) in dest.iter_mut().zip(src.iter().rev()) {
            *d = *s;
        }
    }
}

/// Unpack a (possibly signed) integer from `data[0..size]`.
///
fn unpackint(_state: &LuaState, data: &[u8], is_little: bool, size: usize, is_signed: bool) -> Result<i64, LuaError> {
    let limit = size.min(SZINT);
    let mut res: u64 = 0;
    for i in (0..limit).rev() {
        res <<= NB;
        let byte_idx = if is_little { i } else { size - 1 - i };
        res |= data[byte_idx] as u64;
    }

    if size < SZINT {
        if is_signed {
            let mask: u64 = 1u64 << (size * NB as usize - 1);
            res = (res ^ mask).wrapping_sub(mask);
        }
    } else if size > SZINT {
        let mask = if !is_signed || (res as i64) >= 0 { 0u8 } else { MC };
        for i in limit..size {
            let byte_idx = if is_little { i } else { size - 1 - i };
            if data[byte_idx] != mask {
                return Err(LuaError::runtime(format_args!(
                    "{}-byte integer does not fit into Lua Integer", size
                )));
            }
        }
    }
    Ok(res as i64)
}

/// `string.pack(fmt, ...)` — pack values into a binary string.
///
pub fn str_pack(state: &mut LuaState) -> Result<usize, LuaError> {
    let fmt_bytes = state.check_arg_string(1)?.to_vec();
    let fmt = &fmt_bytes[..];
    let mut h = Header::new(state.global().lua_version == lua_types::LuaVersion::V55);
    let mut arg = 1i32;
    let mut total_size = 0usize;
    let mut buf: Vec<u8> = Vec::new();
    let mut pos = 0usize;

    while pos < fmt.len() {
        let mut size = 0usize;
        let mut ntoalign = 0usize;
        let opt = getdetails(state, &mut h, total_size, fmt, &mut pos, &mut size, &mut ntoalign)?;
        // 5.5 `str_pack` rejects an oversized running total ("result too long")
        // BEFORE consuming the value argument; 5.3/5.4 have no such check (their
        // `int` sizes cannot reach the limit). MAX_SIZE is the host pointer width.
        if h.wide_size {
            let space = ntoalign + size;
            if space > (i64::MAX as usize) || total_size > (i64::MAX as usize) - space {
                return Err(lua_vm::debug::arg_error_impl(state, arg, b"result too long"));
            }
        }
        total_size += ntoalign + size;
        for _ in 0..ntoalign {
            buf.push(PACK_PAD_BYTE);
        }
        arg += 1;

        match opt {
            KOption::Int => {
                let n = state.check_arg_integer(arg)?;
                if size < SZINT {
                    let lim: i64 = 1i64 << (size * NB as usize - 1);
                    if !(-lim <= n && n < lim) {
                        return Err(lua_vm::debug::arg_error_impl(state, arg, b"integer overflow"));
                    }
                }
                packint(&mut buf, n as u64, h.is_little, size, n < 0);
            }
            KOption::Uint => {
                let n = state.check_arg_integer(arg)?;
                if size < SZINT {
                    let lim: u64 = 1u64 << (size * NB as usize);
                    if (n as u64) >= lim {
                        return Err(lua_vm::debug::arg_error_impl(state, arg, b"unsigned overflow"));
                    }
                }
                packint(&mut buf, n as u64, h.is_little, size, false);
            }
            KOption::Float => {
                let f = state.check_arg_number(arg)? as f32;
                let start = buf.len();
                buf.resize(start + 4, 0);
                copywithendian(&mut buf[start..start + 4], &f.to_bits().to_ne_bytes(), h.is_little);
            }
            KOption::Number => {
                let f = state.check_arg_number(arg)?;
                let start = buf.len();
                buf.resize(start + 8, 0);
                copywithendian(&mut buf[start..start + 8], &f.to_bits().to_ne_bytes(), h.is_little);
            }
            KOption::Double => {
                let f = state.check_arg_number(arg)? as f64;
                let start = buf.len();
                buf.resize(start + 8, 0);
                copywithendian(&mut buf[start..start + 8], &f.to_bits().to_ne_bytes(), h.is_little);
            }
            KOption::Char => {
                let s = state.check_arg_string(arg)?.to_vec();
                if s.len() > size {
                    return Err(lua_vm::debug::arg_error_impl(state, arg, b"string longer than given size"));
                }
                buf.extend_from_slice(&s);
                let pad = size - s.len();
                for _ in 0..pad {
                    buf.push(PACK_PAD_BYTE);
                }
            }
            KOption::Kstring => {
                let s = state.check_arg_string(arg)?.to_vec();
                let len = s.len();
                if size < SZINT && len >= (1usize << (size * 8)) {
                    return Err(lua_vm::debug::arg_error_impl(state, arg, b"string length does not fit in given size"));
                }
                packint(&mut buf, len as u64, h.is_little, size, false);
                buf.extend_from_slice(&s);
                total_size += len;
            }
            KOption::Zstr => {
                let s = state.check_arg_string(arg)?.to_vec();
                if s.contains(&0) {
                    return Err(lua_vm::debug::arg_error_impl(state, arg, b"string contains zeros"));
                }
                buf.extend_from_slice(&s);
                buf.push(0);
                total_size += s.len() + 1;
            }
            KOption::Padding => {
                buf.push(PACK_PAD_BYTE);
                arg -= 1; // undo increment
            }
            KOption::Paddalign | KOption::Nop => {
                arg -= 1; // undo increment
            }
        }
    }

    state.push_bytes(&buf)?;
    Ok(1)
}

/// `string.packsize(fmt)` — return the byte-size the format would produce.
///
pub fn str_packsize(state: &mut LuaState) -> Result<usize, LuaError> {
    let fmt_bytes = state.check_arg_string(1)?.to_vec();
    let fmt = &fmt_bytes[..];
    let mut h = Header::new(state.global().lua_version == lua_types::LuaVersion::V55);
    let mut total_size = 0usize;
    let mut pos = 0usize;

    while pos < fmt.len() {
        let mut size = 0usize;
        let mut ntoalign = 0usize;
        let opt = getdetails(state, &mut h, total_size, fmt, &mut pos, &mut size, &mut ntoalign)?;
        if opt == KOption::Kstring || opt == KOption::Zstr {
            return Err(lua_vm::debug::arg_error_impl(state, 1, b"variable-length format"));
        }
        let space = ntoalign + size;
        let max_total: usize = if h.wide_size {
            i64::MAX as usize
        } else {
            PACK_MAXSIZE
        };
        if space > max_total || total_size > max_total - space {
            return Err(lua_vm::debug::arg_error_impl(state, 1, b"format result too large"));
        }
        total_size += space;
    }
    state.push(LuaValue::Int(total_size as i64));
    Ok(1)
}

/// `string.unpack(fmt, s [, pos])` — unpack binary data from string.
///
pub fn str_unpack(state: &mut LuaState) -> Result<usize, LuaError> {
    let fmt_bytes = state.check_arg_string(1)?.to_vec();
    let data_bytes = state.check_arg_string(2)?.to_vec();
    let ld = data_bytes.len();
    let pos_raw = state.opt_arg_integer(3, 1)?;
    let mut pos = if matches!(state.global().lua_version, lua_types::LuaVersion::V53) {
        posrelat_53(pos_raw, ld).wrapping_sub(1)
    } else {
        pos_relat_i(pos_raw, ld).saturating_sub(1)
    };

    if pos > ld {
        return Err(lua_vm::debug::arg_error_impl(state, 3, b"initial position out of string"));
    }

    let fmt = &fmt_bytes[..];
    let data = &data_bytes[..];
    let mut h = Header::new(state.global().lua_version == lua_types::LuaVersion::V55);
    let mut fmt_pos = 0usize;
    let mut n = 0usize;

    while fmt_pos < fmt.len() {
        let mut size = 0usize;
        let mut ntoalign = 0usize;
        let opt = getdetails(state, &mut h, pos, fmt, &mut fmt_pos, &mut size, &mut ntoalign)?;

        if ntoalign + size > ld - pos {
            return Err(lua_vm::debug::arg_error_impl(state, 2, b"data string too short"));
        }
        pos += ntoalign;
        state.ensure_stack(2, "too many results")?;
        n += 1;

        match opt {
            KOption::Int => {
                let v = unpackint(state, &data[pos..pos + size], h.is_little, size, true)?;
                state.push(LuaValue::Int(v));
            }
            KOption::Uint => {
                let v = unpackint(state, &data[pos..pos + size], h.is_little, size, false)?;
                state.push(LuaValue::Int(v));
            }
            KOption::Float => {
                let mut bytes = [0u8; 4];
                copywithendian(&mut bytes, &data[pos..pos + 4], h.is_little);
                let f = f32::from_bits(u32::from_ne_bytes(bytes));
                state.push(LuaValue::Float(f as f64));
            }
            KOption::Number => {
                let mut bytes = [0u8; 8];
                copywithendian(&mut bytes, &data[pos..pos + 8], h.is_little);
                let f = f64::from_bits(u64::from_ne_bytes(bytes));
                state.push(LuaValue::Float(f));
            }
            KOption::Double => {
                let mut bytes = [0u8; 8];
                copywithendian(&mut bytes, &data[pos..pos + 8], h.is_little);
                let f = f64::from_bits(u64::from_ne_bytes(bytes));
                state.push(LuaValue::Float(f));
            }
            KOption::Char => {
                state.push_bytes(&data[pos..pos + size])?;
            }
            KOption::Kstring => {
                let len = unpackint(state, &data[pos..pos + size], h.is_little, size, false)? as usize;
                if len > ld - pos - size {
                    return Err(lua_vm::debug::arg_error_impl(state, 2, b"data string too short"));
                }
                state.push_bytes(&data[pos + size..pos + size + len])?;
                pos += len;
            }
            KOption::Zstr => {
                let found = data[pos..].iter().position(|&b| b == 0);
                let end = match found {
                    Some(e) => e,
                    None => return Err(lua_vm::debug::arg_error_impl(state, 2, b"unfinished string for format 'z'")),
                };
                if pos + end >= ld {
                    return Err(lua_vm::debug::arg_error_impl(state, 2, b"unfinished string for format 'z'"));
                }
                state.push_bytes(&data[pos..pos + end])?;
                pos += end + 1;
            }
            KOption::Paddalign | KOption::Padding | KOption::Nop => {
                n -= 1; // undo increment
            }
        }
        pos += size;
    }

    state.push(LuaValue::Int((pos + 1) as i64));
    Ok(n + 1)
}

// ────────────────────────────────────────────────────────────────────────────
// §9  Module registration
// ────────────────────────────────────────────────────────────────────────────

/// Function table for `string` library.
///
pub const STRING_LIB: &[(&[u8], lua_CFunction)] = &[
    (b"byte",     str_byte),
    (b"char",     str_char),
    (b"dump",     str_dump),
    (b"find",     str_find),
    (b"format",   str_format),
    (b"gmatch",   gmatch),
    (b"gsub",     str_gsub),
    (b"len",      str_len),
    (b"lower",    str_lower),
    (b"match",    str_match),
    (b"rep",      str_rep),
    (b"reverse",  str_reverse),
    (b"sub",      str_sub),
    (b"upper",    str_upper),
    (b"pack",     str_pack),
    (b"packsize", str_packsize),
    (b"unpack",   str_unpack),
];

/// Metamethods to install on the string metatable.
///
pub const STRING_META_METHODS: &[(&[u8], lua_CFunction)] = &[
    (b"__add",  arith_add),
    (b"__sub",  arith_sub),
    (b"__mul",  arith_mul),
    (b"__mod",  arith_mod),
    (b"__pow",  arith_pow),
    (b"__div",  arith_div),
    (b"__idiv", arith_idiv),
    (b"__unm",  arith_unm),
];

/// Create the string metatable and set it as the metatable for all strings.
///
pub fn createmetatable(state: &mut LuaState) -> Result<(), LuaError> {
    state.new_lib_table(STRING_META_METHODS)?;
    state.set_funcs(STRING_META_METHODS, 0)?;
    state.push_string(b"")?;
    let mt_idx = state.top_idx() - 2;
    let mt = state.get_at(mt_idx);
    state.push(mt);
    state.set_metatable(-2)?;
    state.pop_n(1);
    let strlib_idx = state.top_idx() - 2;
    let strlib = state.get_at(strlib_idx);
    state.push(strlib);
    state.set_field(-2, b"__index")?;
    state.pop_n(1);
    Ok(())
}

/// `luaopen_string` — open the string library.
///
pub fn luaopen_string(state: &mut LuaState) -> Result<usize, LuaError> {
    state.new_lib(STRING_LIB)?;
    // Lua 5.1 carries `string.gfind`, the pre-5.0 name for `gmatch` (an exact
    // alias). It was removed in 5.2. Verified against lua5.1.5:
    // `type(string.gfind)` == "function" and it iterates identically to
    // `gmatch`. See specs/followup/5.1-roster-syntax.md §1.
    if matches!(state.global().lua_version, lua_types::LuaVersion::V51) {
        state.push_c_function(gmatch)?;
        state.set_field(-2, b"gfind")?;
    }
    createmetatable(state)?;
    Ok(1)
}

// ────────────────────────────────────────────────────────────────────────────
// PORT STATUS
//   source:        src/lstrlib.c  (1875 lines, 46 functions)
//   target_crate:  lua-stdlib
//   confidence:    medium
//   todos:         13
//   port_notes:    6
//   unsafe_blocks: 0
//   notes:         Pattern engine uses index-based MatchState (not raw ptrs).
//                  string.format delegates numeric widths/precision/flags to
//                  Phase B (a sprintf-compatible crate or manual impl).
//                  gmatch iterator state mirrors C-Lua's closure shape:
//                  source string, pattern string, and userdata state. The
//                  userdata host payload stores only byte positions; strings
//                  stay as traced closure upvalues. See gmatch_aux.
//                  copywithendian uses safe byte-level swapping (no transmute).
//                  unpackint sign-extension uses two's-complement bit tricks;
//                  logic review needed in Phase B.
//                  str_dump requires state.dump_function() which is not yet
//                  defined; Phase B wires up the ldump.c port.
//                  addquoted uses 3-digit escape for all control chars (slight
//                  deviation from C which uses 1-digit when safe); benign.
//                  str_len/str_sub/str_byte/str_reverse/str_lower/str_upper/
//                  str_rep/gmatch/str_find_aux borrow source bytes through
//                  to_lua_string (GcRef) instead of copying via
//                  check_arg_string, mirroring the gmatch_aux fix (685482d).
//                  string_ops 3.00x→2.00x, string_ops_long 2.25x→1.48x on
//                  best-of-5 (Apple M3 Max).
//                  gmatch_aux originally moved from stack raw_geti/raw_seti to
//                  direct table slots, then later from table state to C-shaped
//                  userdata/upvalues. The latter pass dropped gmatch_aux's
//                  string_ops_long profile share from ~6.9% to ~2.9%.
// ────────────────────────────────────────────────────────────────────────────