"use strict";

/** 
 * A $subtract pipeline expression.
 * @see evaluate 
 * @class SubtractExpression
 * @namespace mungedb-aggregate.pipeline.expressions
 * @module mungedb-aggregate
 * @constructor
 **/
var SubtractExpression = module.exports = function SubtractExpression(){
	if (arguments.length !== 0) throw new Error("zero args expected");
	base.call(this);
}, klass = SubtractExpression, base = require("./NaryExpression"), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

// DEPENDENCIES
var Value = require("../Value");

// PROTOTYPE MEMBERS
proto.getOpName = function getOpName(){
	return "$subtract";
};

proto.addOperand = function addOperand(expr) {
	this.checkArgLimit(2);
	base.prototype.addOperand.call(this, expr);
};

/** 
* Takes an array that contains a pair of numbers and subtracts the second from the first, returning their difference. 
**/
proto.evaluate = function evaluate(doc) {
	this.checkArgCount(2);
	var left = this.operands[0].evaluate(doc),
		right = this.operands[1].evaluate(doc);
	if (left instanceof Date || right instanceof Date) throw new Error("$subtract does not support dates; code 16376");
	return left - right;
};