96 lines
2.1 KiB
JavaScript
96 lines
2.1 KiB
JavaScript
|
/**
|
|||
|
|
* LZW Encoding/Decoding
|
|||
|
|
*
|
|||
|
|
* Lempel–Ziv–Welch (LZW) is a universal lossless data
|
|||
|
|
* compression algorithm. It is an improved implementation
|
|||
|
|
* of the LZ78 algorithm.
|
|||
|
|
*
|
|||
|
|
* @example
|
|||
|
|
* var lzwModule = require('path-to-algorithms/src/compression'+
|
|||
|
|
* '/LZW/LZW');
|
|||
|
|
* var lzw = new lzwModule.LZW();
|
|||
|
|
*
|
|||
|
|
* var compressed = lzw.compress("ABCABCABCABCABCABC");
|
|||
|
|
* console.log(compressed);
|
|||
|
|
*
|
|||
|
|
* var decompressed = lzw.decompress(compressed);
|
|||
|
|
* console.log(decompressed);
|
|||
|
|
*
|
|||
|
|
* @module compression/LZW/LZW
|
|||
|
|
*/
|
|||
|
|
(function (exports) {
|
|||
|
|
'use strict';
|
|||
|
|
|
|||
|
|
exports.LZW = function () {
|
|||
|
|
this.dictionarySize = 256;
|
|||
|
|
};
|
|||
|
|
|
|||
|
|
exports.LZW.compress = function (data) {
|
|||
|
|
var i;
|
|||
|
|
var dictionary = {};
|
|||
|
|
var character;
|
|||
|
|
var wc;
|
|||
|
|
var w = '';
|
|||
|
|
var result = [];
|
|||
|
|
|
|||
|
|
for (i = 0; i < this.dictionarySize; i = i + 1) {
|
|||
|
|
dictionary[String.fromCharCode(i)] = i;
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
for (i = 0; i < data.length; i = i + 1) {
|
|||
|
|
character = data.charAt(i);
|
|||
|
|
wc = w + character;
|
|||
|
|
if (dictionary.hasOwnProperty(wc)) {
|
|||
|
|
w = wc;
|
|||
|
|
} else {
|
|||
|
|
result.push(dictionary[w]);
|
|||
|
|
dictionary[wc] = this.dictionarySize;
|
|||
|
|
this.dictionarySize = this.dictionarySize + 1;
|
|||
|
|
w = String(character);
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
if (w !== '') {
|
|||
|
|
result.push(dictionary[w]);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
return result;
|
|||
|
|
};
|
|||
|
|
|
|||
|
|
exports.LZW.decompress = function (compressedData) {
|
|||
|
|
var i;
|
|||
|
|
var dictionary = [];
|
|||
|
|
var w;
|
|||
|
|
var result;
|
|||
|
|
var key;
|
|||
|
|
var entry = '';
|
|||
|
|
|
|||
|
|
for (i = 0; i < this.dictionarySize; i = i + 1) {
|
|||
|
|
dictionary[i] = String.fromCharCode(i);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
w = String.fromCharCode(compressedData[0]);
|
|||
|
|
result = w;
|
|||
|
|
|
|||
|
|
for (i = 1; i < compressedData.length; i = i + 1) {
|
|||
|
|
key = compressedData[i];
|
|||
|
|
if (dictionary[key]) {
|
|||
|
|
entry = dictionary[key];
|
|||
|
|
} else {
|
|||
|
|
if (key === this.dictionarySize) {
|
|||
|
|
entry = w + w.charAt(0);
|
|||
|
|
} else {
|
|||
|
|
return null;
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
result += entry;
|
|||
|
|
dictionary[this.dictionarySize] = w + entry.charAt(0);
|
|||
|
|
this.dictionarySize = this.dictionarySize + 1;
|
|||
|
|
w = entry;
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
return result;
|
|||
|
|
};
|
|||
|
|
})(typeof window === 'undefined' ? module.exports : window);
|