File: Evaluator.cs

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//Copyright 2010 Microsoft Corporation
//
//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.

namespace System.Data.Services.Client
{
    using System;
    using System.Collections.Generic;
    using System.Diagnostics;
    using System.Linq.Expressions;

#if ASTORIA_LIGHT    
    internal class HashSet<T> : Dictionary<T, T>, IEnumerable<T> where T : class
    {
        public HashSet() { }

        public HashSet(IEqualityComparer<T> comparer) : base(comparer) { }

        public HashSet(IEnumerable<T> collection, IEqualityComparer<T> comparer) : base(comparer)
        {
           this.UnionWith(collection);
        }

        public bool Add(T value) { if (!base.ContainsKey(value)) { base.Add(value, value); return true; } return false; }

        public bool Contains(T value) { return base.ContainsKey(value); }

        new public bool Remove(T value) { return base.Remove(value); }

        new public IEnumerator<T> GetEnumerator() { return base.Keys.GetEnumerator(); }

        public void UnionWith(IEnumerable<T> other)
        {
            if (other == null)
            {
                throw new ArgumentNullException("other");
            }
        
            foreach (T local in other)
            {
                this.Add(local);
            }
        }
    }
#endif
    
    internal static class Evaluator
    {
        internal static Expression PartialEval(Expression expression, Func<Expression, bool> canBeEvaluated)
        {
            Nominator nominator = new Nominator(canBeEvaluated);
            HashSet<Expression> candidates = nominator.Nominate(expression);
            return new SubtreeEvaluator(candidates).Eval(expression);
        }

        internal static Expression PartialEval(Expression expression)
        {
            return PartialEval(expression, Evaluator.CanBeEvaluatedLocally);
        }

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

        internal class SubtreeEvaluator : DataServiceExpressionVisitor
        {
            private HashSet<Expression> candidates;

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

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

            internal override Expression Visit(Expression exp)
            {
                if (exp == null)
                {
                    return null;
                }

                if (this.candidates.Contains(exp))
                {
                    return Evaluate(exp);
                }

                return base.Visit(exp);
            }

            private static Expression Evaluate(Expression e)
            {
                if (e.NodeType == ExpressionType.Constant)
                {
                    return e;
                }

#if ASTORIA_LIGHT
                LambdaExpression lambda = ExpressionHelpers.CreateLambda(e, new ParameterExpression[0]); 
#else
                LambdaExpression lambda = Expression.Lambda(e);
#endif
                Delegate fn = lambda.Compile();
                object constantValue = fn.DynamicInvoke(null);
                Debug.Assert(!(constantValue is Expression), "!(constantValue is Expression)");
                
                Type constantType = e.Type;
                if (constantValue != null && constantType.IsArray && constantType.GetElementType() == constantValue.GetType().GetElementType())
                {
                    constantType = constantValue.GetType();
                }

                return Expression.Constant(constantValue, constantType);
            }
        }

        internal class Nominator : DataServiceExpressionVisitor
        {
            private Func<Expression, bool> functionCanBeEvaluated;

            private HashSet<Expression> candidates;

            private bool cannotBeEvaluated;

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

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

            internal override Expression Visit(Expression expression)
            {
                if (expression != null)
                {
                    bool saveCannotBeEvaluated = this.cannotBeEvaluated;
                    this.cannotBeEvaluated = false;

                    base.Visit(expression);

                    if (!this.cannotBeEvaluated)
                    {
                        if (this.functionCanBeEvaluated(expression))
                        {
                            this.candidates.Add(expression);
                        }
                        else
                        {
                            this.cannotBeEvaluated = true;
                        }
                    }

                    this.cannotBeEvaluated |= saveCannotBeEvaluated;
                }

                return expression;
            }
        }
    } 
}