Added the Burrows-Wheeler algorithm. (mgechev#145)

* Added the Burrows-Wheeler algorithm.

* Added the Burrows-Wheeler algorithm.

Author: brunohadlich

doc-config.json

@@ -5,6 +5,7 @@
     "source": {
         "include": [
             "./src/combinatorics/",
+            "./src/compression/",
             "./src/data-structures/",
             "./src/graphics/",
             "./src/graphs/others/",

src/compression/burrows-wheeler/burrows-wheeler.js

@@ -0,0 +1,84 @@
+(function (exports) {
+  'use strict';
+
+  /**
+   * Burrows Wheeler.
+   *
+   * This algorithm is commonly used as a step in the process of compressing data,
+   * it rearranges a character string into runs of similar characters making algorithms
+   * like move-to-front transform and run-length encoding reach higher compression
+   * rates. This implementation only supports characters with ascii code greater than $(36) as
+   * 36 is used at the process of encode and decode.
+   *
+   * @example
+   * const burrows = require('path-to-algorithms/src/burrows-wheeler/burrows-wheeler').burrowsWheeler;
+   * const s = 'ananabanana';
+   * const encodedStr = burrows.encode(s);
+   * console.log(encodedStr);
+   * const decodedStr = burrows.decode(encodedStr);
+   * console.log(decodedStr);
+   *
+   * @module compression/burrows-wheeler/burrows-wheeler
+   */
+  exports.burrowsWheeler = function() {
+
+  }
+
+  /**
+   * Consumes n^2 space.
+   */
+  exports.burrowsWheeler.encode = function(str) {
+    str = '$' + str;
+    var combinations = [];
+    for (let i = 0; i < str.length; i += 1) {
+      combinations.push(str.substring(i) + str.substring(0, i));
+    }
+    var sorted = combinations.sort();
+    var result = [];
+    for (let i = 0; i < sorted.length; i += 1) {
+      result.push(combinations[i][str.length - 1]);
+    }
+    return result.join('');
+  }
+
+  exports.burrowsWheeler.decode = function(encodedStr) {
+    const sortedCharSequence = encodedStr.split('').sort().join('');
+    const leftSide = {};
+    const rightSide = {};
+    var maxEachCharLeft = {};
+    var maxEachCharRight = {};
+
+    for (let i = 0; i < encodedStr.length; i += 1) {
+      var idLeft = sortedCharSequence[i];
+      if (idLeft in maxEachCharLeft) {
+        maxEachCharLeft[idLeft] = maxEachCharLeft[idLeft] + 1;
+      } else {
+        maxEachCharLeft[idLeft] = 1;
+      }
+      idLeft += String(maxEachCharLeft[idLeft]);
+
+      var idRight = encodedStr[i];
+      if (idRight in maxEachCharRight) {
+        maxEachCharRight[idRight] = maxEachCharRight[idRight] + 1;
+      } else {
+        maxEachCharRight[idRight] = 1;
+      }
+      idRight += String(maxEachCharRight[idRight]);
+
+      leftSide[idLeft] = {char: sortedCharSequence[i], right: idRight};
+      rightSide[idRight] = {char: encodedStr[i], left: idRight};
+    }
+    var result = '';
+    var firstChar = sortedCharSequence[0];
+    var searchChar = firstChar + '1';
+    var endChar = searchChar;
+    while (rightSide[leftSide[searchChar].right].left !== endChar) {
+      result     += leftSide[searchChar].char;
+      searchChar = rightSide[leftSide[searchChar].right].left;
+    }
+    result += leftSide[searchChar].char;
+    result = result.substring(1).split('').reverse().join('');
+    return result;
+  }
+
+}(typeof exports === 'undefined' ? window : exports));

test/compression/burrows-wheeler/burrows-wheeler.spec.js

@@ -0,0 +1,13 @@
+var bw = require('../../../src/compression/burrows-wheeler/burrows-wheeler').burrowsWheeler;
+
+describe('Burrows Wheeler', function () {
+  'use strict';
+
+  it('should return "annnnb$aaaaa" for the entry "ananabanana"', function () {
+    expect(bw.encode('ananabanana')).toEqual('annnnb$aaaaa');
+  });
+
+  it('should return "ananabanana" for the entry "annnnb$aaaaa"', function () {
+    expect(bw.decode('annnnb$aaaaa')).toEqual('ananabanana');
+  });
+});