CanonSharp/CanonSharp.Common/LexicalAnalyzer/RegularExpression.cs

namespace CanonSharp.Common.LexicalAnalyzer;

public abstract class RegularExpression
{
    public abstract NondeterministicFiniteAutomation Convert2Nfa();

    public static RegularExpression Empty => new EmptyExpression();

    public static RegularExpression Single(char c) => new SymbolExpression(c);

    public static RegularExpression Alternate(RegularExpression left, RegularExpression right) =>
        new AlternationExpression(left, right);

    public static RegularExpression Concatenate(RegularExpression first, RegularExpression second) =>
        new ConcatenationExpression(first, second);

    public static RegularExpression Kleene(RegularExpression inner) => new KleeneExpression(inner);
}

public class EmptyExpression : RegularExpression
{
    public override NondeterministicFiniteAutomation Convert2Nfa()
    {
        NondeterministicState final = new();
        NondeterministicState start = new();
        start.AddTransaction(EmptyChar.Empty, final);

        return new NondeterministicFiniteAutomation(start, [final]);
    }
}

public class SymbolExpression(char symbol) : RegularExpression
{
    public char Symbol { get; } = symbol;

    public override NondeterministicFiniteAutomation Convert2Nfa()
    {
        NondeterministicState final = new();
        NondeterministicState start = new();
        start.AddTransaction(new EmptyChar(Symbol), final);

        return new NondeterministicFiniteAutomation(start, [final]);
    }
}

public class AlternationExpression(RegularExpression left, RegularExpression right) : RegularExpression
{
    public RegularExpression Left { get; } = left;

    public RegularExpression Right { get; } = right;

    public override NondeterministicFiniteAutomation Convert2Nfa()
    {
        NondeterministicFiniteAutomation left = Left.Convert2Nfa();
        NondeterministicFiniteAutomation right = Right.Convert2Nfa();

        NondeterministicState final = new();
        foreach (NondeterministicState state in left.FinalStates.Concat(right.FinalStates))
        {
            state.AddTransaction(EmptyChar.Empty, final);
        }

        NondeterministicState start = new();
        start.AddTransaction(EmptyChar.Empty, left.Start);
        start.AddTransaction(EmptyChar.Empty, right.Start);

        return new NondeterministicFiniteAutomation(start, [final]);
    }
}

public class ConcatenationExpression(RegularExpression first, RegularExpression second) : RegularExpression
{
    public RegularExpression First { get; } = first;

    public RegularExpression Second { get; } = second;

    public override NondeterministicFiniteAutomation Convert2Nfa()
    {
        NondeterministicFiniteAutomation first = First.Convert2Nfa();
        NondeterministicFiniteAutomation second = Second.Convert2Nfa();

        foreach (NondeterministicState state in first.FinalStates)
        {
            state.AddTransaction(EmptyChar.Empty, second.Start);
        }

        return new NondeterministicFiniteAutomation(first.Start, second.FinalStates);
    }
}

public class KleeneExpression(RegularExpression inner) : RegularExpression
{
    public RegularExpression Inner { get; } = inner;

    public override NondeterministicFiniteAutomation Convert2Nfa()
    {
        NondeterministicFiniteAutomation inner = Inner.Convert2Nfa();

        NondeterministicState final = new();
        final.AddTransaction(EmptyChar.Empty, inner.Start);

        foreach (NondeterministicState state in inner.FinalStates)
        {
            state.AddTransaction(EmptyChar.Empty, final);
        }

        return new NondeterministicFiniteAutomation(final, [final]);
    }
}