lualzw

A relatively fast LZW compression algorithm in pure lua

encoding and decoding

Lossless compression for any text. The more repetition in the text, the better.

16 bit encoding is used. So each 8 bit character is encoded as 16 bit. This means that the dictionary size is 65280.

Any special characters like äöå that are represented with multiple characters are supported. The special characters are split up into single characters that are then encoded and decoded.

While compressing, the algorithm checks if the result size gets over the input. If it does, then the input is not compressed and the algorithm returns the input prematurely as the compressed result.

The zeros branch contains a version that does not add additional null \0 characters to the input when encoding. Any existing null characters in input string are preserved as nulls however so make sure your input does not contain nulls.

usage

local lualzw = require("lualzw")

local input = "foofoofoofoofoofoofoofoofoo"
local compressed = assert(lualzw.compress(input))
local decompressed = assert(lualzw.decompress(compressed))
assert(input == decompressed)

errors

Returns nil and an error message when the algorithm fails to compress or decompress.

speed

Times are in seconds. Both have the same generated input. The values are an average of 10 tries.

Note that compressing random generated inputs results usually in bigger result than original. In these cases the algorithms do not compress and return input instead and thus compression result is 100% of input.

lualzw is at an advantage in cases where compression cannot be done as it stops prematurely and LibCompress does not. Also lualzw is at an advantage in cases where compression can be done as it has a larger dictionary in use.

Input: 1000000 random generated bytes converted into string

algorithm	compress	decompress	result % of input
lualzw	0.6622	0.0003	100
LibCompress	2.1983	0.0024	100

Input: 1000000 random generated bytes in ASCII range converted into string

algorithm	compress	decompress	result % of input
lualzw	0.812	0.0022	100
LibCompress	1.782	0.0007	100

Input: 1000000 random generated repeating bytes converted into string

algorithm	compress	decompress	result % of input
lualzw	0.3975	0.0262	4.5001
LibCompress	0.3907	0.0264	6.6997

Input: 1000000 of same character

algorithm	compress	decompress	result % of input
lualzw	0.7045	0.0026	0.2829
LibCompress	0.6418	0.0038	0.4241

Input: "ymn32h8hm8ekrwjkrn9f" repeated 50000 times. In total 1000000 bytes

algorithm	compress	decompress	result % of input
lualzw	0.4788	0.0088	1.2629
LibCompress	0.4426	0.0093	1.8905