Replaces url entities in a string received from Twitter API

gistfile1.js

var linkify = (function() {
  var replaceSubstr = function(text, i, j, substr) {
    return text.substr(0, i) + substr + text.substr(j);
  }

  var mergeByIndices = function(a, b) {
    var i = 0, j = 0, result = [];
    while (i < a.length || j < b.length) {
      if (i < a.length && (j >= b.length || a[i].indices[0] < b[j].indices[0]))
        result.push(a[i++]);
      else
        result.push(b[j++]);
    }
    return result;
  }

  var linkEntity = function(entity) {
    if (entity.name) // user mention
      return "<a href=\"http://twitter.com/"+entity.screen_name+"\" class=\"user-mention\">@"+entity.screen_name+"</a>";
    else if (entity.url) // url
      return "<a href=\""+entity.url+"\" class=\"url\">"+entity.display_url+"</a>";
    else // hashtag
      return "<a href=\"http://twitter.com/search/%23"+entity.text+"\" class=\"hashtag\">#"+entity.text+"</a>";
  }

  var linkify = function(post) {
    var text = post.text, offset = 0;
    var entities = mergeByIndices(mergeByIndices(post.entities.hashtags, post.entities.urls), post.entities.user_mentions);
    $.each(entities, function(i, entity) {
      var new_substr = linkEntity(entity);
      text = replaceSubstr(text, entity.indices[0] + offset, entity.indices[1] + offset, new_substr);
      offset += new_substr.length - (entity.indices[1] - entity.indices[0]);
    });
    return text;
  }
  return linkify;
})();