programming-examples/js/Algorithms/knuth-morris-pratt.js

(function (exports) {
  'use strict';

  var kmp = (function () {
    function builtKMPTable(str) {
      var res = [];
      var len;
      var front;
      var end;
      var found;
      for (var i = 1; i <= str.length; i += 1) {
        front = Math.max(1, i - ((res[i - 2] || 0) + 1));
        end = Math.min(i - 1, (res[i - 2] || 0) + 1);
        found = false;
        len = 0;
        while (end >= 1 && front <= i && !found) {
          if (str.substring(0, end) === str.substring(front, i)) {
            found = true;
            len = end;
          } else {
            end -= 1;
            front += 1;
          }
        }
        res[i - 1] = len;
      }
      return res;
    }

    /**
     * Knuth–Morris–Pratt algorithm. Searches for the position of
     * the first occurrence of a specified value in a string.
     *
     * @example
     *
     * var indexOf = require('path-to-algorithm/src/searching/'+
     * 'knuth-morris-pratt').kmp;
     * console.log(indexOf('hello', 'll')); // 2
     *
     * @public
     * @module searching/knuth-morris-pratt
     * @param {String} str String.
     * @param {String} substr Substring.
     * @return {Number} A Number, representing the position
     * where the specified substring occurs for the first
     * time, or -1 if it never occurs.
     */
    function indexOf(str, substr) {
      if (str === substr) {
        return 0;
      }
      var table = builtKMPTable(substr);
      var i = 0;
      var j = 0;
      while (i < str.length) {
        if (str[i] === substr[j]) {
          i += 1;
          j += 1;
        }
        if (j === substr.length) {
          return i - j;
        }
        if (i < str.length && str[i] !== substr[j]) {
          if (j > 0 && table[j - 1] !== 0) {
            j = table[j - 1];
          } else {
            i += 1;
            j = 0;
          }
        }
      }
      return -1;
    }
    return indexOf;
  }());

  exports.kmp = kmp;

})(typeof window === 'undefined' ? module.exports : window);