using System;

namespace PKHeX.Core
{
    /// <summary>
    /// Checksum algorithms used by the games.
    /// </summary>
    public static class Checksums
    {
        /// <summary>Calculates the CRC16-CCITT checksum over an input byte array.</summary>
        /// <param name="data">Input byte array</param>
        /// <returns>Checksum</returns>
        public static ushort CRC16_CCITT(ReadOnlySpan<byte> data)
        {
            byte top = 0xFF;
            byte bot = 0xFF;
            foreach (var b in data)
            {
                var x = b ^ top;
                x ^= (x >> 4);
                top = (byte)(bot ^ (x >> 3) ^ (x << 4));
                bot = (byte)(x ^ (x << 5));
            }
            return (ushort)(top << 8 | bot);
        }

        private static readonly ushort[] crc16 =
        {
            0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
            0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
            0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
            0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
            0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
            0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
            0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
            0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
            0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
            0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
            0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
            0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
            0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
            0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
            0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
            0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
            0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
            0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
            0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
            0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
            0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
            0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
            0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
            0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
            0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
            0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
            0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
            0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
            0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
            0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
            0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
            0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
        };

        /// <summary>Calculates the 16bit checksum over an input byte array.</summary>
        /// <param name="data">Input byte array</param>
        /// <param name="initial">Initial value for checksum</param>
        /// <returns>Checksum</returns>
        private static ushort CRC16(ReadOnlySpan<byte> data, ushort initial)
        {
            ushort chk = initial;
            foreach (var b in data)
                chk = (ushort)(crc16[(b ^ chk) & 0xFF] ^ chk >> 8);
            return chk;
        }

        /// <summary>Calculates the 16bit checksum over an input byte array.</summary>
        /// <param name="data">Input byte array</param>
        public static ushort CRC16Invert(ReadOnlySpan<byte> data) => (ushort)~CRC16(data, unchecked((ushort)~0));

        /// <summary>Calculates the 16bit checksum over an input byte array.</summary>
        /// <param name="data">Input byte array</param>
        public static ushort CRC16NoInvert(ReadOnlySpan<byte> data) => CRC16(data, 0);

        /// <summary>Calculates the 32bit checksum over an input byte array. Used in GBA save files.</summary>
        /// <param name="data">Input byte array</param>
        /// <param name="start">Offset to start checksum at</param>
        /// <param name="length">Length of array to checksum</param>
        /// <param name="initial">Initial value for checksum</param>
        /// <returns>Checksum</returns>
        public static ushort CheckSum32(byte[] data, int start, int length, uint initial = 0)
        {
            uint val = initial;
            for (int i = start; i < start + length; i += 4)
                val += BitConverter.ToUInt32(data, i);
            return (ushort)(val + (val >> 16));
        }

        /// <summary>Calculates the 16bit checksum over an input byte array. Used in N64 Stadium save files.</summary>
        /// <param name="data">Input byte array</param>
        /// <param name="initial">Initial value for checksum</param>
        /// <returns>Checksum</returns>
        public static ushort CheckSum16(ReadOnlySpan<byte> data, ushort initial = 0)
        {
            ushort acc = initial;
            foreach (byte b in data)
                acc += b;
            return acc;
        }

        /// <summary>Calculates the 32bit checksum over an input byte array. Used in GBA save files.</summary>
        /// <param name="data">Input byte array</param>
        /// <param name="initial">Initial value for checksum</param>
        /// <returns>Checksum</returns>
        public static ushort CheckSum32(byte[] data, uint initial = 0) => CheckSum32(data, 0, data.Length, initial);

        /// <summary>Calculates the 32bit checksum over an input byte array. Used in GC R/S BOX.</summary>
        /// <param name="data">Input byte array</param>
        /// <returns>Checksum</returns>
        public static uint CheckSum16BigInvert(ReadOnlySpan<byte> data)
        {
            ushort chk = 0; // initial value
            while ((data.Length & ~1) != 0)
            {
                chk += BigEndian.ToUInt16(data);
                data = data[2..];
            }
            return (uint)(chk << 16 | (ushort)(0xF004u - chk));
        }
    }
}