Data commit

Task/LZW-compression/JavaScript/lzw-compression-2.js

@@ -0,0 +1,110 @@
+'use strict';
+/**
+    Namespace for LZW compression and decompression.
+    Methods:
+        LZW.compress(uncompressed)
+        LZW.decompress(compressed)
+*/
+class LZW
+{
+    /**
+        Perform the LZW compression
+        uncompressed - String. The string on which to perform the compression.
+    */
+    static compress(uncompressed)
+    {
+        // Initialize dictionary
+        let dictionary = {};
+        for (let i = 0; i < 256; i++)
+        {
+            dictionary[String.fromCharCode(i)] = i;
+        }
+
+        let word = '';
+        let result = [];
+        let dictSize = 256;
+
+        for (let i = 0, len = uncompressed.length; i < len; i++)
+        {
+            let curChar = uncompressed[i];
+            let joinedWord = word + curChar;
+
+            // Do not use dictionary[joinedWord] because javascript objects
+            // will return values for myObject['toString']
+            if (dictionary.hasOwnProperty(joinedWord))
+            {
+                word = joinedWord;
+            }
+            else
+            {
+                result.push(dictionary[word]);
+                // Add wc to the dictionary.
+                dictionary[joinedWord] = dictSize++;
+                word = curChar;
+            }
+        }
+
+        if (word !== '')
+        {
+            result.push(dictionary[word]);
+        }
+
+        return result;
+    }
+
+    /**
+        Decompress LZW array generated by LZW.compress()
+        compressed - Array. The array that holds LZW compressed data.
+    */
+    static decompress(compressed)
+    {
+        // Initialize Dictionary (inverse of compress)
+        let dictionary = {};
+        for (let i = 0; i < 256; i++)
+        {
+            dictionary[i] = String.fromCharCode(i);
+        }
+
+        let word = String.fromCharCode(compressed[0]);
+        let result = word;
+        let entry = '';
+        let dictSize = 256;
+
+        for (let i = 1, len = compressed.length; i < len; i++)
+        {
+            let curNumber = compressed[i];
+
+            if (dictionary[curNumber] !== undefined)
+            {
+                entry = dictionary[curNumber];
+            }
+            else
+            {
+                if (curNumber === dictSize)
+                {
+                    entry = word + word[0];
+                }
+                else
+                {
+                    throw 'Error in processing';
+                    return null;
+                }
+            }
+
+            result += entry;
+
+            // Add word + entry[0] to dictionary
+            dictionary[dictSize++] = word + entry[0];
+
+            word = entry;
+        }
+
+        return result;
+    }
+}
+
+let comp = LZW.compress('TOBEORNOTTOBEORTOBEORNOT');
+let decomp = LZW.decompress(comp);
+
+console.log(`${comp}
+${decomp}`);