luaur-vm 0.1.3

The Luau register virtual machine and standard library (Rust).
Documentation 
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use crate::functions::lua_l_checkinteger::lua_l_checkinteger;
use crate::functions::lua_l_checklstring::lua_l_checklstring;
use crate::functions::lua_l_error_l::lua_l_error_l;
use crate::functions::lua_l_optinteger::lua_l_optinteger;
use crate::functions::lua_pushinteger::lua_pushinteger;
use crate::functions::lua_pushnil::lua_pushnil;
use crate::functions::u_posrelat::u_posrelat;
use crate::macros::iscont::iscont;
use crate::macros::lua_l_argcheck::luaL_argcheck;
use crate::type_aliases::lua_state::lua_State;
use core::ffi::c_int;

#[no_mangle]
pub unsafe fn byteoffset(L: *mut lua_State) -> c_int {
    let mut len: usize = 0;
    let s = lua_l_checklstring(L, 1, &mut len);
    let mut n = lua_l_checkinteger(L, 2);
    let mut posi = if n >= 0 { 1 } else { len as i32 + 1 };
    posi = u_posrelat(lua_l_optinteger(L, 3, posi), len);
    luaL_argcheck!(
        L,
        1 <= posi && posi <= len as i32 + 1,
        3,
        "position out of range"
    );
    posi -= 1;

    if n == 0 {
        // find beginning of current byte sequence
        while posi > 0 && iscont(s.add(posi as usize)) {
            posi -= 1;
        }
    } else {
        if iscont(s.add(posi as usize)) {
            lua_l_error_l(
                L,
                c"initial position is a continuation byte".as_ptr(),
                core::format_args!("initial position is a continuation byte"),
            );
        }
        if n < 0 {
            while n < 0 && posi > 0 {
                // move back
                loop {
                    // find beginning of previous character
                    posi -= 1;
                    if !(posi > 0 && iscont(s.add(posi as usize))) {
                        break;
                    }
                }
                n += 1;
            }
        } else {
            n -= 1; // do not move for 1st character
            while n > 0 && posi < len as i32 {
                loop {
                    // find beginning of next character
                    posi += 1;
                    if !iscont(s.add(posi as usize)) {
                        break;
                    }
                } // (cannot pass final '\0')
                n -= 1;
            }
        }
    }

    if n == 0 {
        // did it find given character?
        lua_pushinteger(L, posi + 1);
    } else {
        // no such character
        lua_pushnil(L);
    }
    1
}