CanonSharp/CanonSharp.Pascal/Parser/LexicalTokenReadState.cs

using CanonSharp.Combinator.Abstractions;
using CanonSharp.Pascal.Scanner;

namespace CanonSharp.Pascal.Parser;

public sealed class LexicalTokenReadState : IReadState<LexicalToken, LexicalTokenReadState>
{
    private readonly List<LexicalToken> _tokens;
    private readonly int _pos;

    public LexicalToken Current => _tokens[_pos];

    public bool HasValue => _pos < _tokens.Count;

    public LexicalTokenReadState Next => new(_tokens, _pos + 1);

    private LexicalTokenReadState(List<LexicalToken> tokens, int pos)
    {
        _tokens = tokens;
        _pos = pos;
    }

    public LexicalTokenReadState(IEnumerable<LexicalToken> tokens)
    {
        _tokens = tokens.ToList();
        _pos = 0;
    }

    public bool Equals(LexicalTokenReadState? other)
    {
        if (other is null)
        {
            return false;
        }

        if (_tokens.Count != other._tokens.Count)
        {
            return false;
        }

        foreach ((LexicalToken first, LexicalToken second) in _tokens.Zip(other._tokens))
        {
            if (!first.Equals(second))
            {
                return false;
            }
        }

        return _pos == other._pos;
    }

    public override string ToString() => HasValue ? Current.ToString() : "End of input stream.";
}