using CanonSharp.Common.Abstractions;

namespace CanonSharp.Common.Reader;

public class SourceReader : ISourceReader
{
    private readonly StreamReader _reader;
    private char? _lookAhead;

    public SourceReader(string filename)
    {
        FileInfo source = new(filename);

        if (!source.Exists)
        {
            throw new InvalidOperationException();
        }

        _reader = new StreamReader(filename);
    }

    public char Read()
    {
        if (_lookAhead.HasValue)
        {
            char result = _lookAhead.Value;
            _lookAhead = null;
            return result;
        }

        if (!TryFetchChar(out char c))
        {
            throw new InvalidOperationException();
        }

        return c;
    }

    public bool TryPeek(out char c)
    {
        if (_lookAhead.HasValue)
        {
            c = _lookAhead.Value;
            return true;
        }

        if (!TryFetchChar(out c))
        {
            return false;
        }

        _lookAhead = c;
        return true;
    }

    private readonly char[] _buffer = new char[1024];
    private int _length;
    private int _count;

    private bool TryFetchChar(out char c)
    {
        if (_length == _count)
        {
            _length = _reader.Read(_buffer);
            _count = 0;
        }

        if (_length == 0)
        {
            c = char.MinValue;
            return false;
        }

        c = _buffer[_count];
        _count += 1;
        return true;
    }
}