Converter.cs source code in C# .NET

                        Code:
                         / 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / DataEntity / System / Data / Common / Utils / Boolean / Converter.cs / 1305376 / Converter.cs
                        
                        
                            //---------------------------------------------------------------------- 
// 
//      Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// 
// @owner [....]
// @backupOwner [....] 
//--------------------------------------------------------------------- 

using System; 
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;
using System.Collections.ObjectModel; 
using System.Globalization;
using System.Linq; 
 
namespace System.Data.Common.Utils.Boolean
{ 

    /// 
    /// Handles conversion of expressions to different forms (decision diagram, etc)
    ///  
    internal sealed class Converter
    { 
        private readonly Vertex _vertex; 
        private readonly ConversionContext _context;
        private DnfSentence _dnf; 
        private CnfSentence _cnf;

        internal Converter(BoolExpr expr, ConversionContext context)
        { 
            _context = context ?? IdentifierService.Instance.CreateConversionContext();
            _vertex = ToDecisionDiagramConverter.TranslateToRobdd(expr, _context); 
        } 

        internal Vertex Vertex 
        {
            get { return _vertex; }
        }
 
        internal DnfSentence Dnf
        { 
            get 
            {
                InitializeNormalForms(); 
                return _dnf;
            }
        }
 
        internal CnfSentence Cnf
        { 
            get 
            {
                InitializeNormalForms(); 
                return _cnf;
            }
        }
 
        /// 
        /// Converts the decision diagram (Vertex) wrapped by this converter and translates it into DNF 
        /// and CNF forms. I'll first explain the strategy with respect to DNF, and then explain how CNF 
        /// is achieved in parallel. A DNF sentence representing the expression is simply a disjunction
        /// of every rooted path through the decision diagram ending in one. For instance, given the 
        /// following decision diagram:
        ///
        ///                         A
        ///                       0/ \1 
        ///                      B     C
        ///                    0/ \1 0/ \1 
        ///                 One   Zero   One 
        ///
        /// the following paths evaluate to 'One' 
        ///
        ///                 !A, !B
        ///                 A, C
        /// 
        /// and the corresponding DNF is (!A.!B) + (A.C)
        /// 
        /// It is easy to compute CNF from the DNF of the negation, e.g.: 
        ///
        ///     !((A.B) + (C.D)) iff. (!A+!B) . (!C+!D) 
        ///
        /// To compute the CNF form in parallel, we negate the expression (by swapping One and Zero sinks)
        /// and collect negation of the literals along the path. In the above example, the following paths
        /// evaluate to 'Zero': 
        ///
        ///                 !A, B 
        ///                 A, !C 
        ///
        /// and the CNF (which takes the negation of all literals in the path) is (!A+B) . (A+!C) 
        /// 
        private void InitializeNormalForms()
        {
            if (null == _cnf) 
            {
                // short-circuit if the root is true/false 
                if (_vertex.IsOne()) 
                {
                    // And() -> True 
                    _cnf = new CnfSentence(Set>.Empty);
                    // Or(And()) -> True
                    var emptyClause = new DnfClause(Set>.Empty);
                    var emptyClauseSet = new Set>(); 
                    emptyClauseSet.Add(emptyClause);
                    _dnf = new DnfSentence(emptyClauseSet.MakeReadOnly()); 
                } 
                else if (_vertex.IsZero())
                { 
                    // And(Or()) -> False
                    var emptyClause = new CnfClause(Set>.Empty);
                    var emptyClauseSet = new Set>();
                    emptyClauseSet.Add(emptyClause); 
                    _cnf = new CnfSentence(emptyClauseSet.MakeReadOnly());
                    // Or() -> False 
                    _dnf = new DnfSentence(Set>.Empty); 
                }
                else 
                {
                    // construct clauses by walking the tree and constructing a clause for each sink
                    Set> dnfClauses = new Set>();
                    Set> cnfClauses = new Set>(); 
                    Set> path = new Set>();
 
                    FindAllPaths(_vertex, cnfClauses, dnfClauses, path); 

                    _cnf = new CnfSentence(cnfClauses.MakeReadOnly()); 
                    _dnf = new DnfSentence(dnfClauses.MakeReadOnly());
                }
            }
        } 

        private void FindAllPaths(Vertex vertex, Set> cnfClauses, Set> dnfClauses, 
            Set> path) 
        {
            if (vertex.IsOne()) 
            {
                // create DNF clause
                var clause = new DnfClause(path);
                dnfClauses.Add(clause); 
            }
            else if (vertex.IsZero()) 
            { 
                // create CNF clause
                var clause = new CnfClause(new Set>(path.Select(l => l.MakeNegated()))); 
                cnfClauses.Add(clause);
            }
            else
            { 
                // keep on walking...
                foreach (var successor in _context.GetSuccessors(vertex)) 
                { 
                    path.Add(successor.Literal);
                    FindAllPaths(successor.Vertex, cnfClauses, dnfClauses, path); 
                    path.Remove(successor.Literal);
                }
            }
        } 
    }
} 

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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