String ranking algorithm

// qs_score - Quicksilver Score
//
// A port of the Quicksilver string ranking algorithm
//
// "hello world".score("axl") //=> 0.0
// "hello world".score("ow") //=> 0.6
// "hello world".score("hello world") //=> 1.0
//
// Tested in Firefox 2 and Safari 3
//
// The Quicksilver code is available here
// http://code.google.com/p/blacktree-alchemy/
// http://blacktree-alchemy.googlecode.com/svn/trunk/Crucible/Code/NSString+BLTRRanking.m
//
// The MIT License
//
// Copyright (c) 2008 Lachie Cox
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
String.prototype.score = function (abbreviation, offset) {

  offset = offset || 0; // TODO: I think this is unused... remove

  if (abbreviation.length == 0) {
    return 0.9;
  }

  if (abbreviation.length > this.length) {
    return 0.0;
  }

  for (var i = abbreviation.length; i > 0; i--) {
    var sub_abbreviation = abbreviation.substring(0, i);
    var index = this.indexOf(sub_abbreviation);

    if (index < 0) continue;
    if (index + abbreviation.length > this.length + offset) continue;

    var next_string = this.substring(index + sub_abbreviation.length);
    var next_abbreviation = null;

    if (i >= abbreviation.length) {
      next_abbreviation = '';
    } else {
      next_abbreviation = abbreviation.substring(i);
    }

    var remaining_score = next_string.score(next_abbreviation, offset + index);

    if (remaining_score > 0) {
      var score = this.length - next_string.length;

      if (index != 0) {
        var j = 0;
        var c = this.charCodeAt(index - 1);

        if (c == 32 || c == 9) {
          for (j = (index - 2); j >= 0; j--) {
            c = this.charCodeAt(j);
            score -= ((c == 32 || c == 9) ? 1 : 0.15);
          }
        } else {
          score -= index;
        }
      }

      score += remaining_score * next_string.length;
      score /= this.length;

      return score;
    }
  }

  return 0.0;
};