lzf 0.3.1

A small wrapper around LibLZF, a very small data compression library
Documentation 
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use super::{LzfResult,LzfError};
use std::{cmp,mem};

const HLOG    : usize = 16;
const HSIZE   : u32 = 1 << HLOG;
const MAX_OFF : usize = 1 << 13;
const MAX_REF : usize = ((1 << 8) + (1 << 3));
const MAX_LIT : i32 = (1 << 5);

fn first(p: &[u8], off: usize) -> u32 {
    ((p[off] as u32) << 8) | p[off+1] as u32
}

fn next(v: u32, p: &[u8], off: usize) -> u32 {
    (v << 8) | p[off+2] as u32
}

fn idx(h: u32) -> usize {
    let h = h as u64;
    (
        // 8 = 3*8-HLOG, but HLOG is constant at 16
        (h.wrapping_shr(8).wrapping_sub(h*5))
        & (HSIZE-1) as u64
    ) as usize
}

fn not(i: i32) -> i32 {
    if i == 0 { 1 } else { 0 }
}

/// Compress the given data, if possible.
/// The return value will be set to the error if compression fails.
///
/// The buffer is always set to the same size as the input buffer.
/// If that is not enough to hold the lzf-compressed data,
/// an error will be returned.
///
/// Example:
///
/// ```rust
/// let data = "foobar";
/// let compressed = lzf::compress(data.as_bytes());
/// ```
pub fn compress(data: &[u8]) -> LzfResult<Vec<u8>> {
    let in_len = data.len();
    let out_buf_len = in_len;
    let mut out = Vec::with_capacity(out_buf_len);
    unsafe { out.set_len(out_buf_len) };

    let mut out_len : i32 = 1; /* start run by default */

    /* This goes against all of Rust's statically verifiable guarantees,
     * but for the below use-case accessing uninitialized memory is ok,
     * as we have other checks to make sure the read memory is not used.
     *
     * The otherwise happening memset slows down the code by a factor of 20-30
     */
    let mut htab : [usize; 1<<HLOG] = unsafe { mem::uninitialized() };

    let mut current_offset = 0;

    if in_len == 0 {
        return Err(LzfError::DataCorrupted);
    }

    let mut lit : i32 = 0;

    let mut hval : u32;
    let mut ref_offset;

    hval = first(data, current_offset);

    while current_offset < in_len-2 {
        hval = next(hval, data, current_offset);
        let hslot_idx = idx(hval);

        ref_offset = htab[hslot_idx];
        htab[hslot_idx] = current_offset;

        let off = current_offset.wrapping_sub(ref_offset).wrapping_sub(1);
        if off < MAX_OFF
            && current_offset+4 < in_len
            && ref_offset > 0
            && data[ref_offset+0] == data[current_offset+0]
            && data[ref_offset+1] == data[current_offset+1]
            && data[ref_offset+2] == data[current_offset+2] {

            let mut len = 2;
            let maxlen = cmp::min(in_len - current_offset - len, MAX_REF);

            /* stop run */
            out[(out_len - lit - 1) as usize] = (lit as u8).wrapping_sub(1);
            out_len -= not(lit); /* undo run if length is zero */

            if out_len as i32 + 3 + 1 >= out_buf_len as i32 {
                return Err(LzfError::NoCompressionPossible);
            }

            loop {
                len += 1;
                while len < maxlen && data[ref_offset+len] == data[current_offset+len] {
                    len += 1;
                }
                break;
            }

            len -= 2; /* len is now #octets - 1 */
            current_offset += 1;

            if len < 7 {
                out[out_len as usize] = (off >> 8) as u8 + (len << 5) as u8;
                out_len += 1;
            } else {
                out[out_len as usize] = (off >> 8) as u8 + (7 << 5);
                out[out_len as usize + 1] = (len as u8).wrapping_sub(7);
                out_len += 2;
            }

            out[out_len as usize] = off as u8;
            out_len += 2; /* start run */
            lit = 0;

            /* we add here, because we later substract from the total length */
            current_offset += len-1;

            if current_offset >= in_len {
                break;
            }

            hval = first(data, current_offset);

            hval = next(hval, data, current_offset);
            htab[idx(hval)] = current_offset;
            current_offset += 1;

            hval = next(hval, data, current_offset);
            htab[idx(hval)] = current_offset;
            current_offset += 1;
        } else {
            /* one more literal byte we must copy */
            if current_offset >= out_buf_len {
                return Err(LzfError::NoCompressionPossible);
            }

            lit += 1;
            out[out_len as usize] = data[current_offset];
            out_len += 1;
            current_offset += 1;

            if lit == MAX_LIT {
                /* stop run */
                out[(out_len - lit - 1) as usize] = (lit as u8).wrapping_sub(1);
                lit = 0;
                out_len += 1; /* start run */
            }
        }
    }

    /* at most 3 bytes can be missing here */
    if out_len + 3 > out_buf_len as i32 {
        return Err(LzfError::NoCompressionPossible);
    }

    while current_offset < in_len {
        lit += 1;
        out[out_len as usize] = data[current_offset];
        out_len += 1;
        current_offset += 1;

        if lit == MAX_LIT {
            /* stop run */
            out[(out_len - lit - 1) as usize] = (lit as u8).wrapping_sub(1);
            lit = 0;
            out_len += 1; /* start run */
        }
    }

    /* end run */
    out[(out_len - lit - 1) as usize] = (lit as u8).wrapping_sub(1);
    out_len -= not(lit); /* undo run if length is zero */

    unsafe { out.set_len(out_len as usize) };

    Ok(out)
}

#[test]
fn test_compress_skips_short() {
    match compress("foo".as_bytes()) {
        Ok(_) => panic!("Compression did _something_, which is wrong for 'foo'"),
        Err(err) => assert_eq!(LzfError::NoCompressionPossible, err)
    }
}

#[test]
fn test_compress_lorem() {
    let lorem = "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.";

    match compress(lorem.as_bytes()) {
        Ok(compressed) => {
            assert_eq!(272, compressed.len())
        }
        Err(err) => panic!("Compression failed with error {:?}", err)
    }
}

#[test]
fn test_compress_decompress_lorem_round() {
    use super::decompress;

    let lorem = "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.";

    let compressed = match compress(lorem.as_bytes()) {
        Ok(c) => c,
        Err(err) => panic!("Compression failed with error {:?}", err)
    };

    match decompress(&compressed, lorem.len()) {
        Ok(decompressed) => {
            assert_eq!(lorem.len(), decompressed.len());
            assert_eq!(lorem.as_bytes(), &decompressed[..]);
        },
        Err(err) => panic!("Decompression failed with error {:?}", err)
    };
}

#[test]
fn test_alice_wonderland_both() {
    let alice = "\r\n\r\n\r\n\r\n                ALICE'S ADVENTURES IN WONDERLAND\r\n";

    let compressed = match compress(alice.as_bytes()) {
        Ok(c) => c,
        Err(err) => panic!("Compression failed with error {:?}", err)
    };

    let c_compressed = match super::compress(alice.as_bytes()) {
        Ok(c) => c,
        Err(err) => panic!("Compression failed with error {:?}", err)
    };

    assert_eq!(&compressed[..], &c_compressed[..]);
}