using System;
using System.Buffers.Binary;

namespace ShrinkNetwork
{
    internal enum ShrinkKcpEnvelopeKind : byte
    {
        ConnectRequest = 1,
        ConnectAccept = 2,
        Disconnect = 3,
        Data = 4
    }

    internal static class ShrinkKcpTransportProtocol
    {
        private const int KindSize = 1;
        private const int ConversationSize = 4;
        private const int NonceSize = 8;

        public static byte[] CreateConnectRequest(long nonce, uint requestedConversationId)
        {
            var buffer = new byte[KindSize + NonceSize + ConversationSize];
            buffer[0] = (byte)ShrinkKcpEnvelopeKind.ConnectRequest;
            BinaryPrimitives.WriteInt64LittleEndian(buffer.AsSpan(1, NonceSize), nonce);
            BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(1 + NonceSize, ConversationSize), requestedConversationId);
            return buffer;
        }

        public static bool TryReadConnectRequest(byte[] datagram, out long nonce, out uint requestedConversationId)
        {
            nonce = default;
            requestedConversationId = default;
            if (datagram == null || datagram.Length != KindSize + NonceSize + ConversationSize)
                return false;
            if (datagram[0] != (byte)ShrinkKcpEnvelopeKind.ConnectRequest)
                return false;

            nonce = BinaryPrimitives.ReadInt64LittleEndian(datagram.AsSpan(1, NonceSize));
            requestedConversationId = BinaryPrimitives.ReadUInt32LittleEndian(datagram.AsSpan(1 + NonceSize, ConversationSize));
            return true;
        }

        public static byte[] CreateConnectAccept(long nonce, uint conversationId)
        {
            var buffer = new byte[KindSize + NonceSize + ConversationSize];
            buffer[0] = (byte)ShrinkKcpEnvelopeKind.ConnectAccept;
            BinaryPrimitives.WriteInt64LittleEndian(buffer.AsSpan(1, NonceSize), nonce);
            BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(1 + NonceSize, ConversationSize), conversationId);
            return buffer;
        }

        public static bool TryReadConnectAccept(byte[] datagram, out long nonce, out uint conversationId)
        {
            nonce = default;
            conversationId = default;
            if (datagram == null || datagram.Length != KindSize + NonceSize + ConversationSize)
                return false;
            if (datagram[0] != (byte)ShrinkKcpEnvelopeKind.ConnectAccept)
                return false;

            nonce = BinaryPrimitives.ReadInt64LittleEndian(datagram.AsSpan(1, NonceSize));
            conversationId = BinaryPrimitives.ReadUInt32LittleEndian(datagram.AsSpan(1 + NonceSize, ConversationSize));
            return true;
        }

        public static byte[] CreateDisconnect(uint conversationId)
        {
            var buffer = new byte[KindSize + ConversationSize];
            buffer[0] = (byte)ShrinkKcpEnvelopeKind.Disconnect;
            BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(1, ConversationSize), conversationId);
            return buffer;
        }

        public static bool TryReadDisconnect(byte[] datagram, out uint conversationId)
        {
            conversationId = default;
            if (datagram == null || datagram.Length != KindSize + ConversationSize)
                return false;
            if (datagram[0] != (byte)ShrinkKcpEnvelopeKind.Disconnect)
                return false;

            conversationId = BinaryPrimitives.ReadUInt32LittleEndian(datagram.AsSpan(1, ConversationSize));
            return true;
        }

        public static byte[] CreateDataPacket(uint conversationId, byte[] payload)
        {
            payload ??= Array.Empty<byte>();

            var buffer = new byte[KindSize + ConversationSize + payload.Length];
            buffer[0] = (byte)ShrinkKcpEnvelopeKind.Data;
            BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(1, ConversationSize), conversationId);
            if (payload.Length > 0)
                Buffer.BlockCopy(payload, 0, buffer, KindSize + ConversationSize, payload.Length);
            return buffer;
        }

        public static bool TryReadDataPacket(byte[] datagram, out uint conversationId, out int payloadOffset, out int payloadLength)
        {
            conversationId = default;
            payloadOffset = default;
            payloadLength = default;
            if (datagram == null || datagram.Length < KindSize + ConversationSize)
                return false;
            if (datagram[0] != (byte)ShrinkKcpEnvelopeKind.Data)
                return false;

            conversationId = BinaryPrimitives.ReadUInt32LittleEndian(datagram.AsSpan(1, ConversationSize));
            payloadOffset = KindSize + ConversationSize;
            payloadLength = datagram.Length - payloadOffset;
            return payloadLength >= 0;
        }
    }
}