v86/src/rust/leb.rs

pub fn write_leb_i32(buf: &mut Vec<u8>, mut v: i32) {
    // Super complex stuff. See the following:
    // https://en.wikipedia.org/wiki/LEB128#Encode_signed_integer
    // http://llvm.org/doxygen/LEB128_8h_source.html#l00048

    let mut more = true;
    let negative = v < 0;
    let size = 32;
    while more {
        let mut byte = (v & 0b1111111) as u8; // get last 7 bits
        v >>= 7; // shift them away from the value
        if negative {
            v |= (!0 as i32) << (size - 7); // extend sign
        }
        let sign_bit = byte & (1 << 6);
        if (v == 0 && sign_bit == 0) || (v == -1 && sign_bit != 0) {
            more = false;
        }
        else {
            byte |= 0b10000000; // turn on MSB
        }
        buf.push(byte);
    }
}

pub fn write_leb_u32(buf: &mut Vec<u8>, mut v: u32) {
    loop {
        let mut byte = v as u8 & 0b01111111; // get last 7 bits
        v >>= 7; // shift them away from the value
        if v != 0 {
            byte |= 0b10000000; // turn on MSB
        }
        buf.push(byte);
        if v == 0 {
            break;
        }
    }
}

pub fn write_fixed_leb16_at_idx(vec: &mut Vec<u8>, idx: usize, x: u16) {
    dbg_assert!(x < (1 << 14)); // we have 14 bits of available space in 2 bytes for leb
    vec[idx] = ((x & 0b1111111) | 0b10000000) as u8;
    vec[idx + 1] = (x >> 7) as u8;
}

pub fn write_fixed_leb32_at_idx(vec: &mut Vec<u8>, idx: usize, x: u32) {
    dbg_assert!(x < (1 << 28)); // we have 28 bits of available space in 4 bytes for leb
    vec[idx] = (x & 0b1111111) as u8 | 0b10000000;
    vec[idx + 1] = (x >> 7 & 0b1111111) as u8 | 0b10000000;
    vec[idx + 2] = (x >> 14 & 0b1111111) as u8 | 0b10000000;
    vec[idx + 3] = (x >> 21 & 0b1111111) as u8;
}