"use strict";

/** 
 * A base class for all pipeline accumulators. Uses NaryExpression as a base class.
 *
 * @class Accumulator
 * @namespace mungedb-aggregate.pipeline.accumulators
 * @module mungedb-aggregate
 * @constructor
 **/
var Accumulator = module.exports = function Accumulator(){
	if (arguments.length !== 0) throw new Error("zero args expected");
	base.call(this);
}, klass = Accumulator, base = require("../expressions/NaryExpression"), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

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

proto.getFactory = function getFactory(){
	return klass;	// using the ctor rather than a separate .create() method
};

/**
 * Adds the operand after checking the current limit
 * The equal is there because it checks *before* adding the requested argument.
 * Cannot use checkArgLimit because Accumulator must return a different error code.
 *
 * @param expr the operand to add
 **/
proto.addOperand = function addOperand(expr) {
	if (this.operands.length >= 1) throw new Error("code 15943; group accumulator " + this.getOpName() + " only accepts one operand");
	base.prototype.addOperand.call(this, expr);
};

proto.toJSON = function toJSON(isExpressionRequired){
	var rep = {};
	rep[this.getOpName()] = this.operands[0].toJSON(isExpressionRequired);
	return rep;
};

/**
 * Convenience method for doing this for accumulators.  The pattern
 * is always the same, so a common implementation works, but requires
 * knowing the operator name.
 *
 * @param {Object} pBuilder the builder to add to
 * @param {String} fieldName the projected name
 * @param {String} opName the operator name
 * @param {Boolean} requireExpression pass down if the expression is needed
 **/
//	proto.opToBson = function opToBson(pBuilder, opName, fieldName, requireExpression) {
//		if (this.operands.length == 1) throw new Error("this should never happen");
//		var builder = new BSONObjBuilder();
//		this.operands[0].addToBsonObj(builder, opName, requireExpression);
//		pBuilder.append(fieldName, builder.done());
//	};

/**
 * Wrapper around opToBson
 *
 * @param {Object} pBuilder the builder to add to
 * @param {String} fieldName the projected name
 * @param {Boolean} requireExpression pass down if the expression is needed
 **/
//	proto.addToBsonObj = function addToBsonObj(pBuilder, fieldName, requireExpression) {
//		this.opToBson(pBuilder, this.getOpName(), fieldName, requireExpression);
//	};

/**
 * Make sure that nobody adds an accumulator to an array
 *
 * @param {Object} pBuilder the builder to add to
 **/
proto.addToBsonArray = function addToBsonArray(pBuilder) {
	if (false) throw new Error("this should never happen"); // these can't appear in arrays
};

/** 
 * If this function is not overridden in the sub classes,
 * then throw an error
 *
 **/
proto.getValue = function getValue() {
	throw new Error("You need to define this function on your accumulator");
};