var Expression = module.exports = (function(){
	// CONSTRUCTOR
	/** A base class for all pipeline expressions; Performs common expressions within an Op.
	| NOTE:
	| An object expression can take any of the following forms:
	|	f0: {f1: ..., f2: ..., f3: ...}
	|	f0: {$operator:[operand1, operand2, ...]}
	**/
	var klass = module.exports = Expression = function Expression(opts){
		if(arguments.length !== 0) throw new Error("zero args expected");
	}, base = Object, proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});


	// NESTED CLASSES
	/** Utility class for parseObject() below. isDocumentOk indicates that it is OK to use a Document in the current context. **/
	var ObjectCtx = Expression.ObjectCtx = (function(){
		// CONSTRUCTOR
		var klass = function ObjectCtx(opts /*= {isDocumentOk:..., isTopLevel:..., isInclusionOk:...}*/){
			if(!(opts instanceof Object && opts.constructor == Object)) throw new Error("opts is required and must be an Object containing named args");
			for (var k in opts) { // assign all given opts to self so long as they were part of klass.prototype as undefined properties
				if (opts.hasOwnProperty(k) && proto.hasOwnProperty(k) && proto[k] === undefined) this[k] = opts[k];
			}
		}, base = Object, proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

		proto.isDocumentOk =
		proto.isTopLevel =
		proto.isInclusionOk = undefined;
		return klass;
	})();

	/** Decribes how and when to create an Op instance **/
	var OpDesc = (function(){
		// CONSTRUCTOR
		var klass = function OpDesc(name, factory, flags, argCount){
			if (arguments[0] instanceof Object && arguments[0].constructor == Object) { //TODO: using this?
				var opts = arguments[0];
				for (var k in opts) { // assign all given opts to self so long as they were part of klass.prototype as undefined properties
					if (opts.hasOwnProperty(k) && proto.hasOwnProperty(k) && proto[k] === undefined) this[k] = opts[k];
				}
			} else {
				this.name = name;
				this.factory = factory;
				this.flags = flags || 0;
				this.argCount = argCount || 0;
			}
		}, base = Object, proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

		// STATIC MEMBERS
		klass.FIXED_COUNT = 1;
		klass.OBJECT_ARG = 2;

		// PROTOTYPE MEMBERS

		proto.name =
		proto.factory =
		proto.flags =
		proto.argCount = undefined;

		proto.cmp = function cmp(that) {
			return this.name < that.name ? -1 : this.name > that.name ? 1 : 0;
		};

		return klass;
	})();

	var kinds = {
		UNKNOWN: "UNKNOWN",
		OPERATOR: "OPERATOR",
		NOT_OPERATOR: "NOT_OPERATOR"
	};


	// STATIC MEMBERS
	/** Enumeration of comparison operators.  These are shared between a few expression implementations, so they are factored out here. **/
	klass.CmpOp = {
		EQ: "$eq",		// return true for a == b, false otherwise
		NE: "$ne",		// return true for a != b, false otherwise
		GT: "$gt",		// return true for a > b, false otherwise
		GTE: "$gte",	// return true for a >= b, false otherwise
		LT: "$lt",		// return true for a < b, false otherwise
		LTE: "$lte",	// return true for a <= b, false otherwise
		CMP: "$cmp"		// return -1, 0, 1 for a < b, a == b, a > b
	};

	// DEPENDENCIES (later in this file as compared to others to ensure that statics are setup first)
	var FieldPathExpression = require("./FieldPathExpression"),
		ObjectExpression = require("./ObjectExpression"),
		ConstantExpression = require("./ConstantExpression"),
		CompareExpression = require("./CompareExpression");

	// DEFERRED DEPENDENCIES
	/** Expressions, as exposed to users **/
	process.nextTick(function(){ // Even though `opMap` is deferred, force it to load early rather than later to prevent even *more* potential silliness
		Object.defineProperty(klass, "opMap", {value:klass.opMap});
	});
	Object.defineProperty(klass, "opMap", {	//NOTE: deferred requires using a getter to allow circular requires (to maintain the ported API)
		configurable: true,
		/**
		* Autogenerated docs! Please modify if you you touch this method
		*
		* @method get
		**/
		get: function getOpMapOnce() {
			return Object.defineProperty(klass, "opMap", {
				value: [	//NOTE: rather than OpTable because it gets converted to a dict via OpDesc#name in the Array#reduce() below
					new OpDesc("$add", require("./AddExpression"), 0),
					new OpDesc("$and", require("./AndExpression"), 0),
					new OpDesc("$cmp", CompareExpression.bind(null, Expression.CmpOp.CMP), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$cond", require("./CondExpression"), OpDesc.FIXED_COUNT, 3),
			//		$const handled specially in parseExpression
					new OpDesc("$dayOfMonth", require("./DayOfMonthExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$dayOfWeek", require("./DayOfWeekExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$dayOfYear", require("./DayOfYearExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$divide", require("./DivideExpression"), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$eq", CompareExpression.bind(null, Expression.CmpOp.EQ), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$gt", CompareExpression.bind(null, Expression.CmpOp.GT), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$gte", CompareExpression.bind(null, Expression.CmpOp.GTE), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$hour", require("./HourExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$ifNull", require("./IfNullExpression"), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$lt", CompareExpression.bind(null, Expression.CmpOp.LT), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$lte", CompareExpression.bind(null, Expression.CmpOp.LTE), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$minute", require("./MinuteExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$mod", require("./ModExpression"), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$month", require("./MonthExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$multiply", require("./MultiplyExpression"), 0),
					new OpDesc("$ne", CompareExpression.bind(null, Expression.CmpOp.NE), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$not", require("./NotExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$or", require("./OrExpression"), 0),
					new OpDesc("$second", require("./SecondExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$strcasecmp", require("./StrcasecmpExpression"), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$substr", require("./SubstrExpression"), OpDesc.FIXED_COUNT, 3),
					new OpDesc("$subtract", require("./SubtractExpression"), OpDesc.FIXED_COUNT, 2),
					new OpDesc("$toLower", require("./ToLowerExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$toUpper", require("./ToUpperExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$week", require("./WeekExpression"), OpDesc.FIXED_COUNT, 1),
					new OpDesc("$year", require("./YearExpression"), OpDesc.FIXED_COUNT, 1)
				].reduce(function(r,o){r[o.name]=o; return r;}, {})
			}).opMap;
		}
	});

	/**
	* Parse an Object.  The object could represent a functional expression or a Document expression.
	*
	* @param obj	the element representing the object
	* @param ctx	a MiniCtx representing the options above
	* @returns the parsed Expression
	*
	* An object expression can take any of the following forms:
	*	f0: {f1: ..., f2: ..., f3: ...}
	*	f0: {$operator:[operand1, operand2, ...]}
	**/
	klass.parseObject = function parseObject(obj, ctx){
		if(!(ctx instanceof ObjectCtx)) throw new Error("ctx must be ObjectCtx");
		var kind = kinds.UNKNOWN,
			expr, // the result
			exprObj; // the alt result
		if (obj === undefined) return new ObjectExpression();
		var fieldNames = Object.getOwnPropertyNames(obj);
		for (var fc = 0, n = fieldNames.length; fc < n; ++fc) {
			var fn = fieldNames[fc];
			if (fn[0] === "$") {
				if (fc !== 0) throw new Error("the operator must be the only field in a pipeline object (at '" + fn + "'.; code 16410");
				if(ctx.isTopLevel) throw new Error("$expressions are not allowed at the top-level of $project; code 16404");
				kind = kinds.OPERATOR;	//we've determined this "object" is an operator expression
				expr = Expression.parseExpression(fn, obj[fn]);
			} else {
				if (kind === kinds.OPERATOR) throw new Error("this object is already an operator expression, and can't be used as a document expression (at '" + fn + "'.; code 15990");
				if (!ctx.isTopLevel && fn.indexOf(".") != -1) throw new Error("dotted field names are only allowed at the top level; code 16405");
				if (expr === undefined) { // if it's our first time, create the document expression
					if (!ctx.isDocumentOk) throw new Error("document not allowed in this context"); // CW TODO error: document not allowed in this context
					expr = exprObj = new ObjectExpression();
					kind = kinds.NOT_OPERATOR;	//this "object" is not an operator expression
				}
				var fv = obj[fn];
				switch (typeof(fv)) {
				case "object":
					// it's a nested document
					var subCtx = new ObjectCtx({
						isDocumentOk: ctx.isDocumentOk,
						isInclusionOk: ctx.isInclusionOk
					});
					exprObj.addField(fn, Expression.parseObject(fv, subCtx));
					break;
				case "string":
					// it's a renamed field		// CW TODO could also be a constant
					var pathExpr = new FieldPathExpression(Expression.removeFieldPrefix(fv));
					exprObj.addField(fn, pathExpr);
					break;
				case "boolean":
				case "number":
					// it's an inclusion specification
					if (fv) {
						if (!ctx.isInclusionOk) throw new Error("field inclusion is not allowed inside of $expressions; code 16420");
						exprObj.includePath(fn);
					} else {
						if (!(ctx.isTopLevel && fn == "_id")) throw new Error("The top-level _id field is the only field currently supported for exclusion; code 16406");
						exprObj.excludeId(true);
					}
					break;
				default:
					throw new Error("disallowed field type " + (fv ? fv.constructor.name + ":" : "") + typeof(fv) + " in object expression (at '" + fn + "')");
				}
			}
		}
		return expr;
	};

	/**
	* Parse a BSONElement Object which has already been determined to be functional expression.
	*
	* @param opName	the name of the (prefix) operator
	* @param obj	the BSONElement to parse
	* @returns the parsed Expression
	**/
	klass.parseExpression = function parseExpression(opName, obj) {
		// look for the specified operator
		if (opName === "$const") return new ConstantExpression(obj); //TODO: createFromBsonElement was here, not needed since this isn't BSON?
		var op = klass.opMap[opName];
		if (!(op instanceof OpDesc)) throw new Error("invalid operator " + opName + "; code 15999");

		// make the expression node
		var IExpression = op.factory,	//TODO: should this get renamed from `factory` to `ctor` or something?
			expr = new IExpression();

		// add the operands to the expression node
		if (op.flags & OpDesc.FIXED_COUNT && op.argCount > 1 && !(obj instanceof Array)) throw new Error("the " + op.name + " operator requires an array of " + op.argCount + " operands; code 16019");
		var operand; // used below
		if (obj.constructor === Object) { // the operator must be unary and accept an object argument
			if (!(op.flags & OpDesc.OBJECT_ARG)) throw new Error("the " + op.name + " operator does not accept an object as an operand");
			operand = Expression.parseObject(obj, new ObjectCtx({isDocumentOk: 1}));
			expr.addOperand(operand);
		} else if (obj instanceof Array) { // multiple operands - an n-ary operator
			if (op.flags & OpDesc.FIXED_COUNT && op.argCount !== obj.length) throw new Error("the " + op.name + " operator requires " + op.argCount + " operand(s); code 16020");
			for (var i = 0, n = obj.length; i < n; ++i) {
				operand = Expression.parseOperand(obj[i]);
				expr.addOperand(operand);
			}
		} else { //assume it's an atomic operand
			if (op.flags & OpDesc.FIXED_COUNT && op.argCount != 1) throw new Error("the " + op.name + " operator requires an array of " + op.argCount + " operands; code 16022");
			operand = Expression.parseOperand(obj);
			expr.addOperand(operand);
		}

        return expr;
	};

	/**
	* Parse a BSONElement which is an operand in an Expression.
	*
	* @param pBsonElement the expected operand's BSONElement
	* @returns the parsed operand, as an Expression
	**/
	klass.parseOperand = function parseOperand(obj){
		var t = typeof(obj);
		if (t === "string" && obj[0] == "$") { //if we got here, this is a field path expression
			var path = Expression.removeFieldPrefix(obj);
			return new FieldPathExpression(path);
		}
		else if (t === "object" && obj.constructor === Object) return Expression.parseObject(obj, new ObjectCtx({isDocumentOk: true}));
		else return new ConstantExpression(obj);
	};

	/**
	* Produce a field path string with the field prefix removed.
	* Throws an error if the field prefix is not present.
	*
	* @param prefixedField the prefixed field
	* @returns the field path with the prefix removed
	**/
	klass.removeFieldPrefix = function removeFieldPrefix(prefixedField) {
		if (prefixedField.indexOf("\0") != -1) throw new Error("field path must not contain embedded null characters; code 16419");
		if (prefixedField[0] !== "$") throw new Error("field path references must be prefixed with a '$' ('" + prefixedField + "'); code 15982");
		return prefixedField.substr(1);
	};

	/** @returns the sign of a number; -1, 1, or 0 **/
	klass.signum = function signum(i) {
		if (i < 0) return -1;
		if (i > 0) return 1;
		return 0;
	};


	// PROTOTYPE MEMBERS
	/***
	* Evaluate the Expression using the given document as input.
	*
	* @returns the computed value
	***/
	proto.evaluate = function evaluate(obj) {
		throw new Error("WAS NOT IMPLEMENTED BY INHERITOR!");
	};

	/**
	* Optimize the Expression.
	*
	* This provides an opportunity to do constant folding, or to collapse nested
	*  operators that have the same precedence, such as $add, $and, or $or.
	*
	* The Expression should be replaced with the return value, which may or may
	*  not be the same object.  In the case of constant folding, a computed
	*  expression may be replaced by a constant.
	*
	* @returns the optimized Expression
	**/
	proto.optimize = function optimize() {
		throw new Error("WAS NOT IMPLEMENTED BY INHERITOR!");
	};

	/**
	* Add this expression's field dependencies to the set Expressions are trees, so this is often recursive.
	*
	* Top-level ExpressionObject gets pointer to empty vector.
	* If any other Expression is an ancestor, or in other cases where {a:1} inclusion objects aren't allowed, they get NULL.
	*
	* @param deps	output parameter
	* @param path	path to self if all ancestors are ExpressionObjects.
	**/
	proto.addDependencies = function addDependencies(deps, path) {
		throw new Error("WAS NOT IMPLEMENTED BY INHERITOR!");
	};

	/** simple expressions are just inclusion exclusion as supported by ExpressionObject **/
	proto.getIsSimple = function getIsSimple() {
		return false;
	};

	return klass;
})();