The ring data structure

gistfile1.js

var Ring = (function() {
  function Ring(list) {
    this._list = [].slice.call(list, 0);
    this._length = this._list.length;
    this._curr = -1;
  };
  Ring.prototype = {
    rollTo: function(idx) {
      this._curr = idx % this._length;
      if (this._curr < 0) this._curr += this._length;
      return this._list[this._curr];
    }, 
    next: function() {
      return this.rollTo(this._curr + 1); 
    },
    prev: function() {
      return this.rollTo(this._curr - 1);
    }
  };
  return function(list) {
    return new Ring(list);
  };
}());