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/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / DataEntity / System / Data / Common / Utils / Boolean / Simplifier.cs / 1305376 / Simplifier.cs
//---------------------------------------------------------------------- //// Copyright (c) Microsoft Corporation. All rights reserved. // // // @owner [....] // @backupOwner [....] //--------------------------------------------------------------------- using System; using System.Collections.Generic; using System.Text; using System.Diagnostics; namespace System.Data.Common.Utils.Boolean { // Simplifier visitor for Boolean expressions. Performs the following // simplifications bottom-up: // - Eliminate True and False (A Or False iff. A, A And True iff. A) // - Resolve tautology (A Or !A iff. True, True Or A iff. True) and // contradiction (A And !A iff. False, False And A iff. False) // - Flatten nested negations (!!A iff. A) // - Evaluate bound literals (!True iff. False, etc.) // - Flatten unary/empty And/Or expressions internal class Simplifier: BasicVisitor { internal static readonly Simplifier Instance = new Simplifier (); protected Simplifier() { } internal override BoolExpr VisitNot(NotExpr expression) { BoolExpr child = expression.Child.Accept(this); switch (child.ExprType) { case ExprType.Not: return ((NotExpr )child).Child; case ExprType.True: return FalseExpr .Value; case ExprType.False: return TrueExpr .Value; default: return base.VisitNot(expression); } } internal override BoolExpr VisitAnd(AndExpr expression) { return SimplifyTree(expression); } internal override BoolExpr VisitOr(OrExpr expression) { return SimplifyTree(expression); } private BoolExpr SimplifyTree(TreeExpr tree) { bool isAnd = ExprType.And == tree.ExprType; Debug.Assert(isAnd || ExprType.Or == tree.ExprType); // Get list of simplified children, flattening nested And/Or expressions List > simplifiedChildren = new List >(tree.Children.Count); foreach (BoolExpr child in tree.Children) { BoolExpr simplifiedChild = child.Accept(this); // And(And(A, B), C) iff. And(A, B, C) // Or(Or(A, B), C) iff. Or(A, B, C) if (simplifiedChild.ExprType == tree.ExprType) { simplifiedChildren.AddRange(((TreeExpr )simplifiedChild).Children); } else { simplifiedChildren.Add(simplifiedChild); } } // Track negated children separately to identify tautologies and contradictions Dictionary , bool> negatedChildren = new Dictionary , bool>(tree.Children.Count); List > otherChildren = new List >(tree.Children.Count); foreach (BoolExpr simplifiedChild in simplifiedChildren) { switch (simplifiedChild.ExprType) { case ExprType.Not: negatedChildren[((NotExpr )simplifiedChild).Child] = true; break; case ExprType.False: // False And A --> False if (isAnd) { return FalseExpr .Value; } // False || A --> A (omit False from child collections) break; case ExprType.True: // True Or A --> True if (!isAnd) { return TrueExpr .Value; } // True And A --> A (omit True from child collections) break; default: otherChildren.Add(simplifiedChild); break; } } List > children = new List >(); foreach (BoolExpr child in otherChildren) { if (negatedChildren.ContainsKey(child)) { // A && !A --> False, A || !A --> True if (isAnd) { return FalseExpr .Value; } else { return TrueExpr .Value; } } children.Add(child); } foreach (BoolExpr child in negatedChildren.Keys) { children.Add(child.MakeNegated()); } if (0 == children.Count) { // And() iff. True if (isAnd) { return TrueExpr .Value; } // Or() iff. False else { return FalseExpr .Value; } } else if (1 == children.Count) { // Or(A) iff. A, And(A) iff. A return children[0]; } else { // Construct simplified And/Or expression TreeExpr result; if (isAnd) { result = new AndExpr (children); } else { result = new OrExpr (children); } return result; } } } } // File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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