//---------------------------------------------------------------------
// <copyright file="KnowledgeBase.cs" company="Microsoft">
//      Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
//
// @owner Microsoft
// @backupOwner Microsoft
//---------------------------------------------------------------------
 
using System;
using System.Collections.Generic;
using System.Text;
using System.Globalization;
using System.Collections.ObjectModel;
using System.Diagnostics;
 
namespace System.Data.Common.Utils.Boolean
{
    /// <summary>
    /// Data structure supporting storage of facts and proof (resolution) of queries given
    /// those facts.
    /// 
    /// For instance, we may know the following facts:
    /// 
    ///     A --> B
    ///     A
    /// 
    /// Given these facts, the knowledge base can prove the query:
    /// 
    ///     B
    /// 
    /// through resolution.
    /// </summary>
    /// <typeparam name="T_Identifier">Type of leaf term identifiers in fact expressions.</typeparam>
    internal class KnowledgeBase<T_Identifier>
    {
        private readonly List<BoolExpr<T_Identifier>> _facts;
        private Vertex _knowledge;
        private readonly ConversionContext<T_Identifier> _context;
 
        /// <summary>
        /// Initialize a new knowledge base.
        /// </summary>
        internal KnowledgeBase()
        {
            _facts = new List<BoolExpr<T_Identifier>>();
            _knowledge = Vertex.One; // we know '1', but nothing else at present
            _context = IdentifierService<T_Identifier>.Instance.CreateConversionContext();
        }
 
        /// <summary>
        /// Adds all facts from another knowledge base
        /// </summary>
        /// <param name="kb">The other knowledge base</param>
        internal void AddKnowledgeBase(KnowledgeBase<T_Identifier> kb)
        {
            foreach (BoolExpr<T_Identifier> fact in kb._facts)
            {
                AddFact(fact);
            }
        }
 
        /// <summary>
        /// Adds the given fact to this KB.
        /// </summary>
        /// <param name="fact">Simple fact.</param>
        internal virtual void AddFact(BoolExpr<T_Identifier> fact)
        {
            _facts.Add(fact);
            Converter<T_Identifier> converter = new Converter<T_Identifier>(fact, _context);
            Vertex factVertex = converter.Vertex;
            _knowledge = _context.Solver.And(_knowledge, factVertex);
        }
 
        /// <summary>
        /// Adds the given implication to this KB, where implication is of the form:
        /// 
        ///     condition --> implies
        /// </summary>
        /// <param name="condition">Condition</param>
        /// <param name="implies">Entailed expression</param>
        internal void AddImplication(BoolExpr<T_Identifier> condition, BoolExpr<T_Identifier> implies)
        {
            AddFact(new Implication(condition, implies));
        }
 
        /// <summary>
        /// Adds an equivalence to this KB, of the form:
        /// 
        ///     left iff. right
        /// </summary>
        /// <param name="left">Left operand</param>
        /// <param name="right">Right operand</param>
        internal void AddEquivalence(BoolExpr<T_Identifier> left, BoolExpr<T_Identifier> right)
        {
            AddFact(new Equivalence(left, right));
        }
 
        public override string ToString()
        {
            StringBuilder builder = new StringBuilder();
            builder.AppendLine("Facts:");
            foreach (BoolExpr<T_Identifier> fact in _facts)
            {
                builder.Append("\t").AppendLine(fact.ToString());
            }
            return builder.ToString();
        }
 
        // Private class improving debugging output for implication facts 
        // (fact appears as A --> B rather than !A + B)
        private class Implication : OrExpr<T_Identifier>
        {
            BoolExpr<T_Identifier> _condition;
            BoolExpr<T_Identifier> _implies;
 
            // (condition --> implies) iff. (!condition OR implies) 
            internal Implication(BoolExpr<T_Identifier> condition, BoolExpr<T_Identifier> implies)
                : base(condition.MakeNegated(), implies)
            {
                _condition = condition;
                _implies = implies;
            }
 
            public override string ToString()
            {
                return StringUtil.FormatInvariant("{0} --> {1}", _condition, _implies);
            }
        }
 
        // Private class improving debugging output for equivalence facts 
        // (fact appears as A <--> B rather than (!A + B) . (A + !B))
        private class Equivalence : AndExpr<T_Identifier>
        {
            BoolExpr<T_Identifier> _left;
            BoolExpr<T_Identifier> _right;
 
            // (left iff. right) iff. (left --> right AND right --> left)
            internal Equivalence(BoolExpr<T_Identifier> left, BoolExpr<T_Identifier> right)
                : base(new Implication(left, right), new Implication(right, left))
            {
                _left = left;
                _right = right;
            }
 
            public override string ToString()
            {
                return StringUtil.FormatInvariant("{0} <--> {1}", _left, _right);
            }
        }
    }
}