prefix-varint 0.0.1-alpha.6

#![feature(cursor_remaining)]

use std::{
    io::{self, Read, Write},
    mem::size_of,
    slice,
};

#[inline]
pub fn read_prefix(byte: u8) -> (u64, usize) {
    let size = byte.trailing_zeros() as usize;

    ((u64::from(byte) >> (size + 1)), size)
}

#[inline]
pub fn read_varint<R>(mut rdr: R) -> io::Result<u64>
where
    R: Read,
{
    let prefix_byte = {
        let mut byte = 0;
        rdr.read_exact(slice::from_mut(&mut byte))?;

        byte
    };

    let (mut int, size) = read_prefix(prefix_byte);

    let mut rest = [0; size_of::<u64>()];
    rdr.read_exact(&mut rest[..size])?;

    let pad = (size + 1).min(8) as u8;

    for (i, byte) in (1_u8..).zip(rest) {
        int |= u64::from(byte) << ((i * 8) - pad)
    }

    Ok(int)
}

#[inline]
fn calc_varint_size(int: u64) -> usize {
    let bits = match int.checked_next_power_of_two() {
        Some(pow) => match pow {
            pow if pow > int => pow,
            pow => pow + pow,
        }
        .trailing_zeros(),
        None => u64::BITS - 1,
    } as usize;

    1.max(bits / 7 + usize::from(bits % 7 > 0))
}

#[inline]
pub fn write_varint<W: Write>(mut int: u64, mut f: W) -> io::Result<usize> {
    let size = calc_varint_size(int) as u8;

    // number of bits packed into first byte (excluding bits needed for marker)
    let packed_bits = 8_u8.saturating_sub(size);

    let prefix = {
        let int = if packed_bits == 0 {
            0
        } else {
            ((int & ((2_u64.pow(u32::from(packed_bits + 1))) - 1)) << size) as u8
        };

        int | (1_u8.checked_shl(u32::from(size) - 1).unwrap_or(0))
    };

    int >>= packed_bits;

    f.write_all(&[prefix])?;

    for _ in 0..(size - 1) {
        let byte: u8 = (int & u64::from(u8::MAX)) as u8;
        f.write_all(&[byte])?;

        int >>= 8;
    }

    Ok(size.into())
}

#[cfg(test)]
mod tests;