luaur-vm 0.1.0

The Luau register virtual machine and standard library (Rust).
Documentation 
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use luaur_common::macros::luau_assert::LUAU_ASSERT;
use luaur_common::macros::luau_unlikely::LUAU_UNLIKELY;

use crate::functions::printexp::printexp;
use crate::functions::printspecial::printspecial;
use crate::functions::printunsignedrev::printunsignedrev;
use crate::functions::schubfach::schubfach;
use crate::functions::trimzero::trimzero;
use crate::macros::fastmemcpy::fastmemcpy;
use crate::macros::fastmemset::fastmemset;
use crate::records::decimal::Decimal;

#[allow(non_snake_case)]
pub unsafe fn luai_num_2_str(mut buf: *mut core::ffi::c_char, n: f64) -> *mut core::ffi::c_char {
    // IEEE-754
    let bits = n.to_bits();
    let sign = (bits >> 63) as i32;
    let exponent = ((bits >> 52) & 2047) as i32;
    let fraction = bits & ((1u64 << 52) - 1);

    // specials
    if LUAU_UNLIKELY!(exponent == 0x7ff) {
        return printspecial(buf, sign, fraction);
    }

    // sign bit
    *buf = '-' as core::ffi::c_char;
    buf = buf.add(sign as usize);

    // zero
    if exponent == 0 && fraction == 0 {
        *buf = '0' as core::ffi::c_char;
        return buf.add(1);
    }

    // convert binary to decimal using Schubfach
    let d: Decimal = schubfach(exponent, fraction);
    LUAU_ASSERT!(d.s < 1e17 as u64);

    // print the decimal to a temporary buffer; we'll need to insert the decimal point and figure out the format
    let mut decbuf = [0i8; 40];
    let decend = decbuf.as_mut_ptr().add(20) as *mut core::ffi::c_char; // significand needs at most 17 digits
    let dec = printunsignedrev(decend, d.s);

    let declen = decend.offset_from(dec) as i32;
    LUAU_ASSERT!(declen <= 17);

    let dot = declen + d.k;

    // the limits are somewhat arbitrary but changing them may require changing fastmemset/fastmemcpy sizes below
    if dot >= -5 && dot <= 21 {
        // fixed point format
        if dot <= 0 {
            *buf = '0' as core::ffi::c_char;
            *buf.add(1) = '.' as core::ffi::c_char;

            fastmemset!(buf.add(2), '0', -dot, 5);
            fastmemcpy!(buf.add(2 + (-dot) as usize), dec, declen, 17);

            return trimzero(buf.add(2 + (-dot) as usize + declen as usize));
        } else if dot == declen {
            // no dot
            fastmemcpy!(buf, dec, dot, 17);

            return buf.add(dot as usize);
        } else if dot < declen {
            // dot in the middle
            fastmemcpy!(buf, dec, dot, 16);

            *buf.add(dot as usize) = '.' as core::ffi::c_char;

            fastmemcpy!(
                buf.add(dot as usize + 1),
                dec.add(dot as usize),
                declen - dot,
                16
            );

            return trimzero(buf.add(declen as usize + 1));
        } else {
            // no dot, zero padding
            fastmemcpy!(buf, dec, declen, 17);
            fastmemset!(buf.add(declen as usize), '0', dot - declen, 8);

            return buf.add(dot as usize);
        }
    } else {
        // scientific format
        *buf = *dec;
        *buf.add(1) = '.' as core::ffi::c_char;
        fastmemcpy!(buf.add(2), dec.add(1), declen - 1, 16);

        let mut exp = trimzero(buf.add(declen as usize + 1));

        if *exp.sub(1) == '.' as core::ffi::c_char {
            exp = exp.sub(1);
        }

        return printexp(exp, dot - 1);
    }
}

#[no_mangle]
#[allow(non_snake_case)]
pub unsafe fn luai_num2str(buf: *mut core::ffi::c_char, n: f64) -> *mut core::ffi::c_char {
    luai_num_2_str(buf, n)
}