mungedb-aggregate/lib/pipeline/accumulators/SumAccumulator.js

var SumAccumulator = module.exports = (function(){

	// Constructor
	/** 
	* Accumulator for summing a field across documents
	*
	* @class SumAccumulator
	* @namespace munge.pipeline.accumulators
	* @module munge
	* @constructor
	**/
	var klass = module.exports = function SumAccumulator(){
		this.total = 0;
		this.count = 0;
		this.totalIsANumber = true;
		base.call(this);
	}, Accumulator = require("./Accumulator"), base = Accumulator, proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});


	proto.evaluate = function evaluate(doc){
		if(this.operands.length != 1) throw new Error("this should never happen");
		var v = this.operands[0].evaluate(doc);

		if(typeof v !== "number"){ // do nothing with non-numeric types
			return 0;
		}
		else {
			this.totalIsANumber = true;
			this.total += v;
		}
		this.count++;
	
		return 0;
	};

	proto.getValue = function getValue(){
		if(this.totalIsANumber)
			return this.total;
		else
			throw new Error("$sum resulted in a non-numeric type");
	};

	proto.getOpName = function getOpName(){
		return "$sum";
	};

	return klass;

})();