using System.Buffers.Binary;
using System.Text;

namespace HSchool.Protocol;

/// <summary>Little-endian cursor over a received frame. Mirror of <see cref="PacketWriter"/>.</summary>
public ref struct PacketReader(ReadOnlySpan<byte> buffer)
{
    private readonly ReadOnlySpan<byte> _buffer = buffer;
    private int _position = 0;

    public readonly int Position => _position;

    public readonly int Remaining => _buffer.Length - _position;

    public byte ReadByte()
    {
        EnsureAvailable(sizeof(byte));
        var value = _buffer[_position];
        _position += sizeof(byte);
        return value;
    }

    public MessageType ReadMessageType() => (MessageType)ReadByte();

    public ushort ReadUInt16()
    {
        EnsureAvailable(sizeof(ushort));
        var value = BinaryPrimitives.ReadUInt16LittleEndian(_buffer[_position..]);
        _position += sizeof(ushort);
        return value;
    }

    public uint ReadUInt32()
    {
        EnsureAvailable(sizeof(uint));
        var value = BinaryPrimitives.ReadUInt32LittleEndian(_buffer[_position..]);
        _position += sizeof(uint);
        return value;
    }

    /// <summary><c>u16</c> byte length, then UTF-8. Empty string is a zero length.</summary>
    public string ReadString()
    {
        var byteCount = ReadUInt16();
        EnsureAvailable(byteCount);
        var value = Encoding.UTF8.GetString(_buffer.Slice(_position, byteCount));
        _position += byteCount;
        return value;
    }

    public short ReadInt16()
    {
        EnsureAvailable(sizeof(short));
        var value = BinaryPrimitives.ReadInt16LittleEndian(_buffer[_position..]);
        _position += sizeof(short);
        return value;
    }

    public int ReadInt32()
    {
        EnsureAvailable(sizeof(int));
        var value = BinaryPrimitives.ReadInt32LittleEndian(_buffer[_position..]);
        _position += sizeof(int);
        return value;
    }

    public long ReadInt64()
    {
        EnsureAvailable(sizeof(long));
        var value = BinaryPrimitives.ReadInt64LittleEndian(_buffer[_position..]);
        _position += sizeof(long);
        return value;
    }

    private readonly void EnsureAvailable(int bytes)
    {
        if (_position + bytes > _buffer.Length)
        {
            throw new ProtocolException(
                $"Truncated frame: need {bytes} bytes at offset {_position}, only {Remaining} available.");
        }
    }
}