"use strict";

/** 
 * A $subtract pipeline expression.
 * @see evaluateInternal
 * @class SubtractExpression
 * @namespace mungedb-aggregate.pipeline.expressions
 * @module mungedb-aggregate
 * @constructor
 **/
var SubtractExpression = module.exports = function SubtractExpression() {
		this.fixedAirty(2);
		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"),
		Expression = require("./Expression");

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

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

/** Register Expression */
Expression.registerExpression("$subtract", SubtractExpression.parse);