JCALG1 – Compression Library

Open-Source Compression Library

JCALG1

A small, lossless compression library loosely based on LZSS and written entirely in 32-bit x86 assembly. It compresses very well, though slowly, and decompresses as fast as any other LZ77 derivative.

Download Library and Source

A legacy project, first released in 1999. The download includes the library and its x86 assembly source.

Version: r5.34
License: MIT
Written in: 32-bit x86 assembly
Author: Jeremy Collake
Credits: Joergen Ibsen, the author of apLib, was instrumental in the algorithm’s development; much of the encoding is similar to apLib’s.

Features

Small, Strong and Simple

Strong Compression

A good compression ratio, typically much better than ZIP’s deflate.

Tiny, Fast Decompressor

An extremely small and fast decompressor that needs no memory beyond the output buffer.

High-Entropy Data

A unique ability to handle data that is already compressed or otherwise high in entropy.

Adjustable Window

Choose a smaller window for faster compression, at the cost of compression ratio.

Easy to Integrate

A handful of STDCALL functions that fit into any application.

Free

Open source under the MIT license.

Results

Canterbury Corpus

Compression results with a 128 KB window (c7).

File	Original Size	Compressed Size	Bits Per Byte
bib	111,261	36,594	2.63
book1	768,771	335,310	3.49
book2	610,856	213,402	2.79
geo	102,400	71,022	5.55
news	377,109	142,978	3.03
obj1	21,504	9,982	3.71
obj2	246,814	77,722	2.52
paper1	53,161	20,014	3.01
paper2	82,199	32,534	3.17
pic	513,216	58,338	0.91
progc	39,611	14,194	2.87
progl	71,646	17,174	1.92
progp	49,379	11,674	1.89
trans	93,695	19,486	1.66
Total	3,141,622	1,060,424	2.80 (2.70 weighted)

Documentation

API Reference

All functions use the STDCALL calling convention: Parameters are pushed onto the stack, and the called function adjusts the stack pointer before returning.

DWORD JCALG1_Compress(
    void         *Source,
    DWORD         Length,
    void         *Destination,
    DWORD         WindowSize,
    AllocFunc    *pAlloc,
    DeAllocFunc  *pDealloc,
    CallbackFunc *pCallback,
    BOOLEAN       bDisableChecksum);

Source: The uncompressed data.
Length: The size of the uncompressed data.
Destination: The buffer for the compressed data. It should be at least 4 bytes larger than the uncompressed data; JCALG1_GetNeededBufferSize returns the minimum size.
WindowSize: A nonzero value up to the size of the file. The larger it is, the better the compression ratio, but the slower the compression.
pAlloc, pDealloc: Memory allocation and deallocation functions (prototypes below).
pCallback: Called on every iteration of the main compression loop (prototype below).
bDisableChecksum: If TRUE, no checksum is computed or stored.

Returns: The size of the compressed data, or 0 if the data could not be compressed.

DWORD JCALG1_Decompress_Fast(void *Source, void *Destination);
DWORD JCALG1_Decompress_Small(void *Source, void *Destination);

Source: The compressed data.
Destination: The buffer for the uncompressed data.

Returns: The size of the uncompressed data, or 0 if the block is not a valid JCALG1 stream (it failed the header or checksum tests). The Small version is optimized for size, the Fast version for speed.

DWORD JCALG1_GetUncompressedSizeOfCompressedBlock(void *pBlock);

pBlock: A JCALG1 compressed block.

Returns: The size of the uncompressed block, in bytes.

DWORD JCALG1_GetNeededBufferSize(DWORD nSize);

nSize: The size, in bytes, of a block of uncompressed data.

Returns: The minimum destination buffer size (currently nSize + 4).

void JCALG1_GetInfo(_JCALG1_Info *JCALG1_Info);

typedef struct __JCALG1_Info {
    DWORD MajorRev;
    DWORD MinorRev;
    DWORD FastDecompressorSize;
    DWORD SmallDecompressorSize;
} _JCALG1_Info;

JCALG1_Info: A _JCALG1_Info structure to fill in.

BOOL CallbackFunc(DWORD pCurrentSrc, DWORD pCurrentDest);

pCurrentSrc: The offset of the current position in the uncompressed data.
pCurrentDest: The offset of the current position in the compressed data.

Returns: FALSE to stop compression, TRUE to continue.

void *AllocFunc(DWORD Size);

Size: The number of bytes requested.

Returns: A pointer to the allocated memory block.

BOOL DeallocFunc(void *pMemory);

pMemory: The memory block to free.

Returns: FALSE on failure.

Discover more from Bitsum

Subscribe to get the latest posts sent to your email.