LZW Encoding/Decoding

filipefalcaos · filipefalcaos · commit 25dc6aabe5e1 · 2015-05-11T01:31:22.000-03:00
Lempel–Ziv–Welch (LZW) is a universal lossless data compression
algorithm. It is an improved implementation of the LZ78 algorithm.
diff --git a/src/compression/LZW/LZW.js b/src/compression/LZW/LZW.js
@@ -0,0 +1,95 @@
+/**
+ * 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);
