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curriculum/challenges/english/10-coding-interview-prep/rosetta-code/lzw-compression.md

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+---
+id: 5ea2815e364d9a2222ea55f8
+title: LZW compression
+challengeType: 5
+forumTopicId: 385288
+---
+
+## Description
+<section id='description'>
+The Lempel-Ziv-Welch (LZW) algorithm provides loss-less data compression.
+You can read a complete description of it in the <a href="https://en.wikipedia.org/wiki/Lempel-Ziv-Welch" target="_blank">Wikipedia article</a> on the subject.
+</section>
+
+## Instructions
+<section id='instructions'>
+Write a function that takes two parameters. The first parameter is a boolean where `true` indicates compress and `false` indicates decompress. The second parameter is either a string or an array to be processed. If it is a string to be compressed, return an array of numbers. If it's an array of numbers to be decompressed, return a string.
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>LZW</code> should be a function.
+    testString: assert(typeof LZW === 'function');
+  - text: <code>LZW(true, "TOBEORNOTTOBEORTOBEORNOT")</code> should return a array.
+    testString: assert(Array.isArray(LZW(true, "TOBEORNOTTOBEORTOBEORNOT")));
+  - text: <code>LZW(false, [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263])</code> should return a string.
+    testString: assert(typeof LZW(false, [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]) === 'string');
+  - text: <code>LZW(true, "TOBEORNOTTOBEORTOBEORNOT")</code> should return <code>[84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]</code>.
+    testString: assert.deepEqual(LZW(true, "TOBEORNOTTOBEORTOBEORNOT"), [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]);
+  - text: <code>LZW(false, [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263])</code> should return <code>"TOBEORNOTTOBEORTOBEORNOT"</code>.
+    testString: assert.equal(LZW(false, [84, 79, 66, 69, 79, 82, 78, 79, 84, 256, 258, 260, 265, 259, 261, 263]), "TOBEORNOTTOBEORTOBEORNOT");
+  - text: <code>LZW(true, "0123456789")</code> should return <code>[48, 49, 50, 51, 52, 53, 54, 55, 56, 57]</code>.
+    testString: assert.deepEqual(LZW(true, "0123456789"), [48, 49, 50, 51, 52, 53, 54, 55, 56, 57]);
+  - text: <code>LZW(false, [48, 49, 50, 51, 52, 53, 54, 55, 56, 57])</code> should return <code>"0123456789"</code>.
+    testString: assert.equal(LZW(false, [48, 49, 50, 51, 52, 53, 54, 55, 56, 57]), "0123456789");
+  - text: <code>LZW(true, "BABAABAAA")</code> should return <code>[66, 65, 256, 257, 65, 260]</code>.
+    testString: assert.deepEqual(LZW(true, "BABAABAAA"), [66, 65, 256, 257, 65, 260]);
+  - text: <code>LZW(false, [66, 65, 256, 257, 65, 260])</code> should return <code>"BABAABAAA"</code>.
+    testString: assert.equal(LZW(false, [66, 65, 256, 257, 65, 260]), "BABAABAAA");
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function LZW (compressData, input) {
+
+}
+```
+
+</div>
+
+</section>
+
+## Solution
+<section id='solution'>
+
+```js
+function LZW (compressData, input) {
+  function compress(uncompressed) {
+    // Build the dictionary.
+    var i,
+      dictionary = {},
+      c,
+      wc,
+      w = "",
+      result = [],
+      dictSize = 256;
+    for (i = 0; i < 256; i += 1) {
+      dictionary[String.fromCharCode(i)] = i;
+    }
+
+    for (i = 0; i < uncompressed.length; i += 1) {
+      c = uncompressed.charAt(i);
+      wc = w + c;
+      //Do not use dictionary[wc] because javascript arrays
+      //will return values for array['pop'], array['push'] etc
+      // if (dictionary[wc]) {
+      if (dictionary.hasOwnProperty(wc)) {
+        w = wc;
+      } else {
+        result.push(dictionary[w]);
+        // Add wc to the dictionary.
+        dictionary[wc] = dictSize++;
+        w = String(c);
+      }
+    }
+
+    // Output the code for w.
+    if (w !== "") {
+      result.push(dictionary[w]);
+    }
+    return result;
+  }
+
+
+  function decompress(compressed) {
+    // Build the dictionary.
+    var i,
+      dictionary = [],
+      w,
+      result,
+      k,
+      entry = "",
+      dictSize = 256;
+    for (i = 0; i < 256; i += 1) {
+      dictionary[i] = String.fromCharCode(i);
+    }
+
+    w = String.fromCharCode(compressed[0]);
+    result = w;
+    for (i = 1; i < compressed.length; i += 1) {
+      k = compressed[i];
+      if (dictionary[k]) {
+        entry = dictionary[k];
+      } else {
+        if (k === dictSize) {
+          entry = w + w.charAt(0);
+        } else {
+          return null;
+        }
+      }
+
+      result += entry;
+
+      // Add w+entry[0] to the dictionary.
+      dictionary[dictSize++] = w + entry.charAt(0);
+
+      w = entry;
+    }
+    return result;
+  }
+
+  if(compressData){
+    return compress(input)
+  }else{
+    return decompress(input)
+  }
+}
+```
+
+</section>