using System;
using System.Runtime.InteropServices;
using static System.Buffers.Binary.BinaryPrimitives;

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 ReadOnlySpan<ushort> crc16 => new ushort[]
    {
        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,
    };

    private static ReadOnlySpan<uint> crc32 => new uint[]
    {
        0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
        0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
        0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
        0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
        0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
        0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
        0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
        0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
        0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
        0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
        0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
        0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
        0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
        0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
        0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
        0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
        0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
        0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
        0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
        0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
        0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
        0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
        0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
        0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
        0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
        0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
        0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
        0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
        0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
        0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
        0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
        0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
    };

    /// <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[(byte)(b ^ chk)] ^ (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="initial">Initial value for checksum</param>
    /// <returns>Checksum</returns>
    public static ushort CheckSum32(ReadOnlySpan<byte> data, uint initial = 0)
    {
        uint chk = initial;
        foreach (var u32 in MemoryMarshal.Cast<byte, uint>(data))
        {
            if (BitConverter.IsLittleEndian)
                chk += u32;
            else
                chk += ReverseEndianness(u32);
        }
        return (ushort)(chk + (chk >> 16));
    }

    /// <summary>Calculates the 32bit 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 uint CRC32(ReadOnlySpan<byte> data, uint initial)
    {
        uint chk = initial;
        foreach (var b in data)
            chk = crc32[(byte)(b ^ chk)] ^ (chk >> 8);
        return chk;
    }

    /// <summary>Calculates the 16bit checksum over an input byte array.</summary>
    /// <param name="data">Input byte array</param>
    public static uint CRC32Invert(ReadOnlySpan<byte> data) => ~CRC32(data, unchecked((uint)~0));

    /// <summary>Calculates the 16bit checksum over an input byte array.</summary>
    /// <param name="data">Input byte array</param>
    public static uint CRC32NoInvert(ReadOnlySpan<byte> data) => CRC32(data, 0);

    /// <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 GC R/S BOX.</summary>
    /// <param name="data">Input byte array</param>
    /// <returns>Checksum</returns>
    public static uint CheckSum16BigInvert(ReadOnlySpan<byte> data)
    {
        var chk = Add16BigEndian(data);
        return (uint)((chk << 16) | (ushort)(0xF004u - chk));
    }

    /// <summary>
    /// Gets the 16-bit checksum of a byte array reading as Little Endian.
    /// </summary>
    /// <param name="data">Input byte array</param>
    /// <returns>Checksum</returns>
    public static ushort Add16(ReadOnlySpan<byte> data)
    {
        ushort chk = 0;
        foreach (var u16 in MemoryMarshal.Cast<byte, ushort>(data))
        {
            if (BitConverter.IsLittleEndian)
                chk += u16;
            else
                chk += ReverseEndianness(u16);
        }
        return chk;
    }

    /// <summary>
    /// Gets the 16-bit checksum of a byte array reading as Big Endian.
    /// </summary>
    /// <param name="data">Input byte array</param>
    /// <returns>Checksum</returns>
    public static ushort Add16BigEndian(ReadOnlySpan<byte> data)
    {
        ushort chk = 0;
        foreach (var u16 in MemoryMarshal.Cast<byte, ushort>(data))
        {
            if (!BitConverter.IsLittleEndian)
                chk += u16;
            else
                chk += ReverseEndianness(u16);
        }
        return chk;
    }
}