78 lines
2.0 KiB
JavaScript
78 lines
2.0 KiB
JavaScript
var lzw = (function() {
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function setBits(array, value, nbits) {
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for (var i = 0; i < nbits; i++) {
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array.push(value%2);
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value = Math.floor(value / 2);
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}
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}
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function lzwc(str) {
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var dictionary = {};
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var word = "";
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var result = [];
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var dictionarySize = 256;
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var bits = 9;
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for (var i = 0; i < 256; i++) {
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dictionary[String.fromCharCode(i)] = i;
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}
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for (i = 0; i < str.length; i++) {
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c = str.charAt(i);
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var newWord = word + c;
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if (dictionary.hasOwnProperty(newWord)) {
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word = newWord;
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} else {
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//result.push(dictionary[word]);
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setBits(result, dictionary[word], bits);
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dictionary[newWord] = dictionarySize++;
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if (dictionarySize == Math.pow(2, bits)) {
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bits++;
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}
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word = String(c);
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}
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}
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if (word !== '') {
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setBits(result, dictionary[word], bits)
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//result.push(dictionary[word]);
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}
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return result;
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}
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function getBits(array, index, nbits) {
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value = 0;
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for (i = index + nbits - 1; i >= index; i--) {
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value = (value << 1) + array[i];
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}
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return value;
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}
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function lzwd(str, strlen) {
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var dictionary = {};
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var dictionarySize = 256;
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var entry = '';
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for (var i = 0; i < 256; i++) {
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dictionary[i] = String.fromCharCode(i);
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}
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var word = String.fromCharCode(getBits(str, 0, 8));
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var result = word;
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var bits = 9;
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for (i = 9; i < strlen; i+=bits) {
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if (dictionarySize == Math.pow(2, bits) - 1) {
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bits++;
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}
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k = getBits(str, i, bits);
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if (dictionary.hasOwnProperty(k)) {
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entry = dictionary[k];
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} else {
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if (k === dictionarySize) {
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entry = word + word.charAt(0);
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} else {
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throw 'LZW decompress: Incorrect value';
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}
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}
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result += entry;
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dictionary[dictionarySize++] = word + entry.charAt(0);
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word = entry;
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}
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return result;
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}
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return { lzwc: lzwc, lzwd: lzwd }
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})(); |