Evaluator.cs

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// 
//  Copyright 2010-2015 Deveel
// 
//    Licensed under the Apache License, Version 2.0 (the "License");
//    you may not use this file except in compliance with the License.
//    You may obtain a copy of the License at
// 
//        http://www.apache.org/licenses/LICENSE-2.0
// 
//    Unless required by applicable law or agreed to in writing, software
//    distributed under the License is distributed on an "AS IS" BASIS,
//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//    See the License for the specific language governing permissions and
//    limitations under the License.
//

using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace Deveel.Data.Linq {
    static class Evaluator {
        public static Expression PartialEval(Expression expression, Func<Expression, bool> fnCanBeEvaluated) {
            return new SubtreeEvaluator(new Nominator(fnCanBeEvaluated).Nominate(expression)).Eval(expression);
        }

        public static Expression PartialEval(Expression expression) {
            return PartialEval(expression, CanBeEvaluatedLocally);
        }

        private static bool CanBeEvaluatedLocally(Expression expression) {
            return expression.NodeType != ExpressionType.Parameter;
        }

        class SubtreeEvaluator : ExpressionVisitor {
            HashSet<Expression> candidates;

            internal SubtreeEvaluator(HashSet<Expression> candidates) {
                this.candidates = candidates;
            }

            internal Expression Eval(Expression exp) {
                return this.Visit(exp);
            }

            public override Expression Visit(Expression exp) {
                if (exp == null) {
                    return null;
                }
                if (candidates.Contains(exp)) {
                    return Evaluate(exp);
                }
                return base.Visit(exp);
            }

            private Expression Evaluate(Expression e) {
                if (e.NodeType == ExpressionType.Constant) {
                    return e;
                }
                LambdaExpression lambda = Expression.Lambda(e);
                Delegate fn = lambda.Compile();
                return Expression.Constant(fn.DynamicInvoke(null), e.Type);
            }
        }

        class Nominator : ExpressionVisitor {
            Func<Expression, bool> fnCanBeEvaluated;
            HashSet<Expression> candidates;
            bool cannotBeEvaluated;

            internal Nominator(Func<Expression, bool> fnCanBeEvaluated) {
                this.fnCanBeEvaluated = fnCanBeEvaluated;
            }

            internal HashSet<Expression> Nominate(Expression expression) {
                this.candidates = new HashSet<Expression>();
                this.Visit(expression);
                return this.candidates;
            }

            public override Expression Visit(Expression expression) {
                if (expression != null) {
                    bool saveCannotBeEvaluated = this.cannotBeEvaluated;
                    this.cannotBeEvaluated = false;
                    base.Visit(expression);
                    if (!this.cannotBeEvaluated) {
                        if (this.fnCanBeEvaluated(expression)) {
                            this.candidates.Add(expression);
                        } else {
                            this.cannotBeEvaluated = true;
                        }
                    }
                    this.cannotBeEvaluated |= saveCannotBeEvaluated;
                }
                return expression;
            }
        }
    }
}