Message.cs

using System;
using System.Buffers.Binary;
using System.Collections.Generic;
using System.Linq;
 
namespace OSDP.Net.Messages
{
    public abstract class Message
    {
        public const byte AddressMask = 0x7F;

        public const byte StartOfMessage = 0x53;

        public const ushort MsgTypeIndex = 5;

        public const ushort MacSize = 4;

        public const byte FirstPaddingByte = 0x80;
 
        private static readonly ushort[] CrcTable =
        [
            0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7, 0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C,
            0xD1AD, 0xE1CE, 0xF1EF, 0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6, 0x9339, 0x8318,
            0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE, 0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4,
            0x5485, 0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D, 0x3653, 0x2672, 0x1611, 0x0630,
            0x76D7, 0x66F6, 0x5695, 0x46B4, 0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC, 0x48C4,
            0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823, 0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969,
            0xA90A, 0xB92B, 0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12, 0xDBFD, 0xCBDC, 0xFBBF,
            0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A, 0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,
            0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49, 0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13,
            0x2E32, 0x1E51, 0x0E70, 0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78, 0x9188, 0x81A9,
            0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F, 0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046,
            0x6067, 0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E, 0x02B1, 0x1290, 0x22F3, 0x32D2,
            0x4235, 0x5214, 0x6277, 0x7256, 0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D, 0x34E2,
            0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405, 0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E,
            0xC71D, 0xD73C, 0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634, 0xD94C, 0xC96D, 0xF90E,
            0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB, 0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
            0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A, 0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1,
            0x1AD0, 0x2AB3, 0x3A92, 0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9, 0x7C26, 0x6C07,
            0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1, 0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9,
            0x9FF8, 0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
        ];

        public byte Address { get; protected set; }
        
        public MessageType MessageType { get; protected set; }

        protected abstract ReadOnlySpan<byte> Data();
 
        internal static byte[] ConvertShortToBytes(ushort value)
        {
            var byteArray = BitConverter.GetBytes(value);
            if (!BitConverter.IsLittleEndian)
            {
                Array.Reverse(byteArray);
            }
 
            return byteArray;
        }
 
        internal static ushort ConvertBytesToUnsignedShort(ReadOnlySpan<byte> data, bool useLittleEndian = false)
        {
            return BitConverter.IsLittleEndian || useLittleEndian
                ? BinaryPrimitives.ReadUInt16LittleEndian(data)
                : BinaryPrimitives.ReadUInt16BigEndian(data);
        }
 
        internal static short ConvertBytesToShort(ReadOnlySpan<byte> data, bool useLittleEndian = false)
        {
            return BitConverter.IsLittleEndian || useLittleEndian
                ? BinaryPrimitives.ReadInt16LittleEndian(data)
                : BinaryPrimitives.ReadInt16BigEndian(data);
        }

        protected static ushort CalculateCrc(ReadOnlySpan<byte> packet)
        {
            ushort crc = 0x1D0F;
            foreach (var t in packet)
            {
                crc = (ushort)((crc << 8) ^ CrcTable[((crc >> 8) ^ t) & 0xFF]);
            }
 
            return crc;
        }

        protected static byte CalculateChecksum(byte[] packet)
        {
            return (byte) (0x100 - packet.Aggregate(0, (source, element) => source + element) & 0xff);
        }

        protected static void AddPacketLength(Span<byte> packet, ushort additionalLength = 0)
        {
            var packetLength = ConvertShortToBytes((ushort)(packet.Length + additionalLength)).ToArray();
            packet[2] = packetLength[0];
            packet[3] = packetLength[1];
        }

        protected internal static void AddCrc(Span<byte> packet)
        {
            ushort crc = CalculateCrc(packet.Slice(0, packet.Length - 2));
            var crcBytes = ConvertShortToBytes(crc).ToArray();
            packet[packet.Length - 2] = crcBytes[0];
            packet[packet.Length - 1] = crcBytes[1];
        }

        protected static void AddChecksum(IList<byte> packet)
        {
            packet[packet.Count - 1] = CalculateChecksum(packet.Take(packet.Count - 1).ToArray());
        }
 
        internal static int ConvertBytesToInt(byte[] bytes)
        {
            if (!BitConverter.IsLittleEndian)
            {
                Array.Reverse(bytes);
            }
 
            return BitConverter.ToInt32(bytes, 0);
        }
 
        internal static IEnumerable<byte> ConvertIntToBytes(int value)
        {
            var byteArray = BitConverter.GetBytes(value);
            if (!BitConverter.IsLittleEndian)
            {
                Array.Reverse(byteArray);
            }
 
            return byteArray;
        }
 
        internal static bool BuildMultiPartMessageData(ushort wholeMessageSize, ushort offset, ushort lengthOfFragment,
            ReadOnlySpan<byte> fragment, Span<byte> data)
        {
            fragment.CopyTo(data.Slice(offset, lengthOfFragment));
 
            return wholeMessageSize == offset + lengthOfFragment;
        }

        public static ushort CalculateMaximumMessageSize(ushort dataSize, bool isEncrypted = false, ushort cryptoLength = 16)
        {
            const ushort clearTextDifference = 8;
            const ushort encryptedDifference = 16;
 
            return (ushort)(dataSize - (isEncrypted ? encryptedDifference + (dataSize % cryptoLength) : clearTextDifference));
        }
    }
}