src/libserialize/leb128.rs

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#[inline]
fn write_to_vec(vec: &mut Vec<u8>, position: usize, byte: u8) {
    if position == vec.len() {
        vec.push(byte);
    } else {
        vec[position] = byte;
    }
}

#[inline]
/// encodes an integer using unsigned leb128 encoding and stores
/// the result using a callback function.
///
/// The callback `write` is called once for each position
/// that is to be written to with the byte to be encoded
/// at that position.
pub fn write_unsigned_leb128_to<W>(mut value: u128, mut write: W) -> usize
    where W: FnMut(usize, u8)
{
    let mut position = 0;
    loop {
        let mut byte = (value & 0x7F) as u8;
        value >>= 7;
        if value != 0 {
            byte |= 0x80;
        }

        write(position, byte);
        position += 1;

        if value == 0 {
            break;
        }
    }

    position
}

pub fn write_unsigned_leb128(out: &mut Vec<u8>, start_position: usize, value: u128) -> usize {
    write_unsigned_leb128_to(value, |i, v| write_to_vec(out, start_position+i, v))
}

#[inline]
pub fn read_unsigned_leb128(data: &[u8], start_position: usize) -> (u128, usize) {
    let mut result = 0;
    let mut shift = 0;
    let mut position = start_position;
    loop {
        let byte = data[position];
        position += 1;
        result |= ((byte & 0x7F) as u128) << shift;
        if (byte & 0x80) == 0 {
            break;
        }
        shift += 7;
    }

    (result, position - start_position)
}

#[inline]
/// encodes an integer using signed leb128 encoding and stores
/// the result using a callback function.
///
/// The callback `write` is called once for each position
/// that is to be written to with the byte to be encoded
/// at that position.
pub fn write_signed_leb128_to<W>(mut value: i128, mut write: W) -> usize
    where W: FnMut(usize, u8)
{
    let mut position = 0;

    loop {
        let mut byte = (value as u8) & 0x7f;
        value >>= 7;
        let more = !((((value == 0) && ((byte & 0x40) == 0)) ||
                      ((value == -1) && ((byte & 0x40) != 0))));

        if more {
            byte |= 0x80; // Mark this byte to show that more bytes will follow.
        }

        write(position, byte);
        position += 1;

        if !more {
            break;
        }
    }
    position
}

pub fn write_signed_leb128(out: &mut Vec<u8>, start_position: usize, value: i128) -> usize {
    write_signed_leb128_to(value, |i, v| write_to_vec(out, start_position+i, v))
}

#[inline]
pub fn read_signed_leb128(data: &[u8], start_position: usize) -> (i128, usize) {
    let mut result = 0;
    let mut shift = 0;
    let mut position = start_position;
    let mut byte;

    loop {
        byte = data[position];
        position += 1;
        result |= ((byte & 0x7F) as i128) << shift;
        shift += 7;

        if (byte & 0x80) == 0 {
            break;
        }
    }

    if (shift < 64) && ((byte & 0x40) != 0) {
        // sign extend
        result |= -(1 << shift);
    }

    (result, position - start_position)
}

#[test]
fn test_unsigned_leb128() {
    let mut stream = Vec::with_capacity(10000);

    for x in 0..62 {
        let pos = stream.len();
        let bytes_written = write_unsigned_leb128(&mut stream, pos, 3 << x);
        assert_eq!(stream.len(), pos + bytes_written);
    }

    let mut position = 0;
    for x in 0..62 {
        let expected = 3 << x;
        let (actual, bytes_read) = read_unsigned_leb128(&stream, position);
        assert_eq!(expected, actual);
        position += bytes_read;
    }
    assert_eq!(stream.len(), position);
}

#[test]
fn test_signed_leb128() {
    let values: Vec<_> = (-500..500).map(|i| i * 0x12345789ABCDEF).collect();
    let mut stream = Vec::new();
    for &x in &values {
        let pos = stream.len();
        let bytes_written = write_signed_leb128(&mut stream, pos, x);
        assert_eq!(stream.len(), pos + bytes_written);
    }
    let mut pos = 0;
    for &x in &values {
        let (value, bytes_read) = read_signed_leb128(&mut stream, pos);
        pos += bytes_read;
        assert_eq!(x, value);
    }
    assert_eq!(pos, stream.len());
}