gistfile1.txt

 var map = function() { 
	for (i = 0; i < this.items.length; i++) {
		for (j = i + 1; j <= this.items.length; j++) {
			emit({a: this.items[i] ,b: this.items[j] }, 1); // emit each item pair
		}
	}
}

 var reduce = function(key, values) 
                      { return Array.sum(values); } // Sum all occurrences of each item pair

 db.lastDayOrders.mapReduce(map, reduce, 
  {out: { replace: "pairs", sharded: true }) // persist the output to a collection