kmp/kmp.js

Knuth-Morris-Pratt string searching algorithm.

(function(root) {
  'use strict';

  /**
   * kmp
   * @desc Search for occurances of word in string.
   * @param {string} string - text string
   * @param {string} word - search word
   */
  function kmp(s, w) {
    if (kmp.isNumber(w)) w = ''+w;
    if (kmp.isNumber(s)) s = ''+s;
    if (!(kmp.isString(s) || kmp.isArray(s))) return -1;
    if (!(kmp.isString(w) || kmp.isArray(w))) return -1;
    var m = 0; // start of s
    var i = 0; // start of w
    var t = kmp.table(w);

    while (m + i < s.length) {
      if (w[i] === s[m+i]) {
        if (i === w.length - 1) {
          return m;
        }
        i = i + 1;
      } else {
        if (t[i] > -1) {
          m = m+i - t[i];
          i = t[i];
        } else {
          i = 0;
          m = m + 1;
        }
      }
    }

    return -1;
  }

  /**
   * table
   * @desc Returns "Partial match" table
   * @param {string} word - search word
   */
  kmp.table = function kmpTable(w) {
    var t = [];
    var pos = 2; // current position in T
    var cnd = 0; // index in w of the next character of the current candidate substring

    t[0] = -1;
    t[1] = 0;

    while (pos < w.length) {
      // substring continues
      if (w[pos-1] === w[cnd]) {
        cnd = cnd + 1;
        t[pos] = cnd;
        pos = pos + 1;
      // it doesn't but we can fall back
      } else if (cnd > 0) {
        cnd = t[cnd];
      // we have run out of candidates
      } else {
        t[pos] = 0;
        pos = pos + 1;
      }
    }

    return t;
  };

  kmp.isArray = function(v) {
    return Array.isArray(v);
  };

  kmp.isString = function(v) {
    return typeof v === 'string' || v instanceof String;
  };

  kmp.isDefined = function(v) {
    return !(typeof v === 'undefined' || typeof v === null);
  };

  kmp.isNumber = function(v) {
    return !isNaN(v);
  };

  if (typeof exports !== 'undefined') {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports = kmp;
    }
    exports.kmp = kmp;
  } else if (typeof define === 'function' && define.amd) {
    define([], function() {
      return kmp;
    });
  } else {
    root.kmp = kmp;
  }

})(this);