"use strict";

/**
 * An $hour pipeline expression.
 * @see evaluateInternal
 * @class HourExpression
 * @namespace mungedb-aggregate.pipeline.expressions
 * @module mungedb-aggregate
 * @constructor
 **/
var HourExpression = module.exports = function HourExpression(){
	base.call(this);
}, klass = HourExpression,
	FixedArityExpression = require("./FixedArityExpressionT")(klass, 1),
	base = FixedArityExpression,
	proto = klass.prototype = Object.create(base.prototype, {
		constructor:{
			value:klass
		}
	});

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

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

/**
 * Takes a date and returns the hour between 0 and 23.
 * @method evaluateInternal
 **/
proto.evaluateInternal = function evaluateInternal(vars){
	var date = this.operands[0].evaluateInternal(vars);
	return date.getUTCHours();
};


/** Register Expression */
Expression.registerExpression("$hour",base.parse);