Edge.cs

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using Mercury.Exceptions;
using Mercury.Syntax;
using System;
using System.Linq;
using System.Text;

namespace Mercury.Syntax
{
//=============================================================================
//  Edge
//=============================================================================

    public class Edge : IEquatable<Edge>, IComparable<Edge>
    {
        //--[  Constructors  ]-------------------------------------------------

        public Edge(Rule rule, int dot, int left, int right)
        {
            if (rule == null) throw new ArgumentNullException("rule");

            Rule  = rule;       Dot   = dot;
            Left  = left;       Right = right;

            if (!IsValid()) throw new InvalidEdgeException(this);

            IsActive  = !Rule.IsEpsilon && (Dot < Rule.RHS.Count);
            IsPassive = !IsActive;

            NextSymbol = IsActive ? Rule.RHS[Dot] : null;
        }

        //--[  Properties  ]---------------------------------------------------

        public Rule Rule  { get; private set; }

        public int  Dot   { get; private set; }

        public int  Left  { get; private set; }

        public int  Right { get; private set; }

        public bool IsActive { get; private set; }

        public bool IsPassive { get; private set; }

        public Symbol NextSymbol { get; private set; }

        //--[  Methods  ]------------------------------------------------------

        public bool IsValid()
        { return (0 <= Dot) && (Dot <= Rule.RHS.Count) && (Left <= Right); }

        //--[  Overriden methods  ]--------------------------------------------

        #region [  Object       ]

        public override bool Equals(object obj)
        {
            Edge edge = obj as Edge;
            return edge == null ? false : Equals(edge);
        }

        public override int GetHashCode()
        { return 23 * Rule.GetHashCode() + 17 * Dot + 11 * Left + 7 * Right; }

        public override string ToString()
        {
            StringBuilder sb = new StringBuilder(string.Format("({0}, {1}) ", Left, Right))
                .Append(Rule.LHS).Append(" ->");

            for (int ix = 0; ix < Rule.RHS.Count; ++ix)
            {
                sb.Append(ix == Dot ? " . " : " ");
                if (!Rule.RHS[ix].IsEpsilon) sb.Append(Rule.RHS[ix].ToString());
            }

            if (Dot == Rule.RHS.Count) sb.Append(" .");

            return sb.ToString();
        }

        #endregion

        //--[  Interface implementation  ]-------------------------------------

        #region [  IEquatable   ]

        public bool Equals(Edge other)
        {
            return (other != null)             && (Dot   == other.Dot)
                && (Left  == other.Left)       && (Right == other.Right)
                && (Rule.Equals(other.Rule));
        }

        #endregion

        #region [  IComparable  ]

        public int CompareTo(Edge other)
        {
            int temp = Left.CompareTo(other.Left);

            if (temp == 0) temp = Right.CompareTo(other.Right);
            if (temp == 0) temp = Rule.CompareTo(other.Rule);
            if (temp == 0) temp = Dot.CompareTo(other.Dot);

            return temp;
        }

        #endregion

    }
}