using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace IrcTokens
{
    public class StatefulDecoder
    {
        private byte[] _buffer;
        private Encoding _encoding;
        private Encoding _fallback;

        public Encoding Encoding
        {
            get => _encoding ?? Encoding.GetEncoding(Encoding.UTF8.CodePage, EncoderFallback.ExceptionFallback,
                       DecoderFallback.ExceptionFallback);
            set
            {
                if (value != null)
                {
                    _encoding = Encoding.GetEncoding(value.CodePage, EncoderFallback.ExceptionFallback,
                        DecoderFallback.ReplacementFallback);
                }
            }
        }

        public Encoding Fallback
        {
            get => _fallback ?? Encoding.GetEncoding(Encoding.GetEncoding("iso-8859-1").CodePage,
                EncoderFallback.ExceptionFallback, DecoderFallback.ExceptionFallback);
            set
            {
                if (value != null)
                {
                    _fallback = Encoding.GetEncoding(value.CodePage, EncoderFallback.ReplacementFallback,
                        DecoderFallback.ReplacementFallback);
                }
            }
        }

        public string Pending => Encoding.GetString(_buffer);

        public StatefulDecoder()
        {
            Clear();
        }

        public void Clear()
        {
            _buffer = Array.Empty<byte>();
        }

        public List<Line> Push(string data)
        {
            return Push(Encoding.GetBytes(data));
        }

        public List<Line> Push(byte[] data)
        {
            if (data == null || data.Length == 0)
            {
                return null;
            }

            _buffer = _buffer.Concat(data).ToArray();

            // simulate string.Split('\n') before decoding
            var newLineIndices = _buffer.Select((b, i) => b == '\n' ? i : -1).Where(i => i != -1).ToArray();
            var lines = new List<byte[]>();

            for (int i = 0, currentIndex = 0; i < newLineIndices.Length; ++i)
            {
                var n = new byte[newLineIndices[i] - currentIndex];
                Array.Copy(_buffer, currentIndex, n, 0, newLineIndices[i] - currentIndex);
                currentIndex = newLineIndices[i] + 1;
                lines.Add(n);
            }

            var listLines = lines.Select(l => l.ToList()).ToList();

            // simulate string.Trim('\r') before decoding
            foreach (var line in listLines)
            {
                var i = 0;
                while (line[i] == '\r')
                {
                    line.RemoveAt(i);
                    i++;
                }

                i = line.Count - 1;
                while (line[i] == '\r')
                {
                    line.RemoveAt(i);
                    i--;
                }
            }

            var decodeLines = new List<string>();
            foreach (var line in listLines.Select(l => l.ToArray()))
            {
                try
                {
                    decodeLines.Add(Encoding.GetString(line));
                }
                catch (DecoderFallbackException)
                {
                    decodeLines.Add(Fallback.GetString(line));
                }
            }

            return decodeLines
                .Select(l => new Line(l))
                .ToList();
        }
    }
}