CanonSharp/CanonSharp.Common/LexicalAnalyzer/DeterministicFiniteAutomation.cs

namespace CanonSharp.Common.LexicalAnalyzer;

public class DeterministicState : IEquatable<DeterministicState>
{
    public Guid Id { get; } = Guid.NewGuid();

    public Dictionary<char, DeterministicState> Transaction { get; } = [];

    public bool Equals(DeterministicState? other) => other is not null && Id.Equals(other.Id);

    public override bool Equals(object? obj) => obj is DeterministicState other && Equals(other);

    public override int GetHashCode() => Id.GetHashCode();
}

public class DeterministicFiniteAutomation
{
    public DeterministicState Start { get; }

    public HashSet<DeterministicState> FinalStates { get; }

    private DeterministicFiniteAutomation(DeterministicState start, HashSet<DeterministicState> finalStates)
    {
        Start = start;
        FinalStates = finalStates;
    }

    private record Pair(HashSet<NondeterministicState> States, DeterministicState State);

    public static DeterministicFiniteAutomation Create(NondeterministicFiniteAutomation nfa)
    {
        Dictionary<NondeterministicStateSet, DeterministicState> map = [];
        HashSet<DeterministicState> visited = [];
        Queue<Pair> queue = [];
        HashSet<DeterministicState> finalStates = [];

        HashSet<NondeterministicState> startClosure = nfa.Start.CalculateEmptyClosure();
        DeterministicState start = new();
        map.Add(new NondeterministicStateSet(startClosure), start);
        queue.Enqueue(new Pair(startClosure, start));

        while (queue.TryDequeue(out Pair? pair))
        {
            if (pair.States.Any(s => nfa.FinalStates.Contains(s)))
            {
                finalStates.Add(pair.State);
            }

            Dictionary<char, HashSet<NondeterministicState>> next = [];

            foreach (NondeterministicState state in pair.States)
            {
                foreach (KeyValuePair<EmptyChar,HashSet<NondeterministicState>> transaction in
                         state.Transactions.Where(p => !p.Key.IsEmpty))
                {
                    HashSet<NondeterministicState> closure = [];

                    foreach (NondeterministicState s in transaction.Value)
                    {
                        closure.UnionWith(s.CalculateEmptyClosure());
                    }

                    if (next.TryGetValue(transaction.Key.Char, out HashSet<NondeterministicState>? n))
                    {
                        n.UnionWith(closure);
                    }
                    next.Add(transaction.Key.Char, closure);
                }
            }

            foreach (KeyValuePair<char,HashSet<NondeterministicState>> transaction in next)
            {
                NondeterministicStateSet set = new(transaction.Value);
                if (!map.TryGetValue(set, out DeterministicState? nextState))
                {
                    nextState = new DeterministicState();
                    map.Add(set, nextState);
                }

                pair.State.Transaction.Add(transaction.Key, nextState);

                if (visited.Add(nextState))
                {
                    queue.Enqueue(new Pair(transaction.Value, nextState));
                }
            }
        }

        return new DeterministicFiniteAutomation(start, finalStates);
    }
}