func (this *ExpressionSargable) isConstant(e ast.Expression) bool {
	_, err := e.Accept(this.constantChecker)
	if err == nil {
		return true
	}
	return false
}

func (this *ExpressionSargable) matchesIndex(e ast.Expression) bool {
	_, err := e.Accept(this.equivalenceChecker)
	if err == nil {
		return true
	}
	return false
}

func CanIUseThisIndexForThisWhereClause(index catalog.RangeIndex, where ast.Expression, bucket string) (bool, plan.ScanRanges, ast.Expression, error) {

	// convert the index key to formal notation
	indexKeyFormal, err := IndexKeyInFormalNotation(index.Key(), bucket)
	if err != nil {
		return false, nil, nil, err
	}

	// put the where clause into conjunctive normal form
	ennf := ast.NewExpressionNNF()
	whereNNF, err := where.Accept(ennf)
	if err != nil {
		return false, nil, nil, err
	}
	ecnf := ast.NewExpressionCNF()
	whereCNF, err := whereNNF.Accept(ecnf)
	if err != nil {
		return false, nil, nil, err
	}

	switch whereCNF := whereCNF.(type) {
	case *ast.AndOperator:
		// this is an and, we can try to satisfy individual operands
		found := false
		rranges := plan.ScanRanges{}
		for _, oper := range whereCNF.Operands {
			// see if the where clause expression is sargable with respect to the index key
			es := NewExpressionSargable(indexKeyFormal[0])
			oper.Accept(es)
			if es.IsSargable() {
				found = true
				for _, ran := range es.ScanRanges() {
					rranges = MergeRanges(rranges, ran)
					clog.To(planner.CHANNEL, "now ranges are: %v", rranges)
				}
			}
		}
		if found {
			return true, rranges, nil, nil
		}
	default:
		// not an and, we must satisfy the whole expression
		// see if the where clause expression is sargable with respect to the index key
		es := NewExpressionSargable(indexKeyFormal[0])
		whereCNF.Accept(es)
		if es.IsSargable() {
			return true, es.ScanRanges(), nil, nil
		}
	}

	// cannot use this index
	return false, nil, nil, nil
}

// inorder traversal of the AST to get JS expression out of it
func (this *JsStatement) Visit(e ast.Expression) (ast.Expression, error) {
	switch expr := e.(type) {

	case *ast.DotMemberOperator:
		if this.js.Len() == 0 {
			this.js.WriteString("doc.")
		}
		_, err := expr.Left.Accept(this)
		if err != nil {
			return nil, err
		}
		this.js.WriteString(".")
		_, err = expr.Right.Accept(this)
		if err != nil {
			return nil, err
		}

	case *ast.BracketMemberOperator:
		if this.js.Len() == 0 {
			this.js.WriteString("doc.")
		}
		_, err := expr.Left.Accept(this)
		if err != nil {
			return nil, err
		}
		this.js.WriteString("[")
		_, err = expr.Right.Accept(this)
		if err != nil {
			return nil, err
		}
		this.js.WriteString("]")

	case *ast.Property:
		if this.js.Len() == 0 {
			this.js.WriteString("doc.")
		}
		this.js.WriteString(expr.Path)

	case *ast.LiteralNumber:
		this.js.WriteString(fmt.Sprintf("%v", expr.Val))

	case *ast.LiteralString:
		this.js.WriteString(expr.Val)

	default:
		return e, errors.New("Expression is not supported by indexing currently: " + e.String())

	}
	return e, nil
}

func (this *BaseOperator) Evaluate(e ast.Expression, item *dparval.Value) (*dparval.Value, error) {
	// first ensure the query is avaliable
	item.SetAttachment("query", this.query)
	return e.Evaluate(item)
}

func (this *ExpressionSargable) evaluateConstant(e ast.Expression) (*dparval.Value, error) {
	return e.Evaluate(dparval.NewValue(map[string]interface{}{}))
}