/// <summary>
///     CRC-16 calculator using the ANSI C12.18 reversed CCITT polynomial.
/// </summary>
public static class Crc16
{
#region Lookup Table
    private static ReadOnlySpan<ushort> Table => [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];

#endregion
#region Private Methods
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static ushort SwapBytes(ushort value) => (ushort)((value >> 8) | (value << 8));
#endregion
#region Public Methods
    /// <summary>
    ///     Computes the CRC-16 over the specified span of bytes using the ANSI C12.18 algorithm.
    /// </summary>
    /// <param name = "data">The data to compute the CRC over.</param>
    /// <returns>The computed CRC-16 value.</returns>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ushort Compute(ReadOnlySpan<byte> data)
    {
        ushort crc = 0xFFFF;
        foreach (var b in data)
            crc = (ushort)((crc >> 8) ^ Table[(crc ^ b) & 0xFF]);
        return (ushort)~crc;
    }

    /// <summary>
    ///     Computes the CRC-16 for a PSEM frame (excludes STP byte, includes data through payload).
    ///     The CRC covers bytes from index 1 (after STP) through the end of the data payload.
    /// </summary>
    /// <param name = "frame">The complete frame data including STP.</param>
    /// <param name = "headerAndDataLength">Number of bytes after STP to include in CRC (header + data).</param>
    /// <returns>The computed CRC-16 value with bytes swapped per ANSI C12.18.</returns>
    public static ushort ComputeFrame(ReadOnlySpan<byte> frame, int headerAndDataLength)
    {
        var crc = Compute(frame.Slice(1, headerAndDataLength));
        return SwapBytes(crc);
    }

    /// <summary>
    ///     Verifies that the CRC at the end of a frame matches the computed CRC.
    /// </summary>
    /// <param name = "frame">The complete frame including STP and trailing CRC bytes.</param>
    /// <returns><c>true</c> if the CRC is valid; otherwise, <c>false</c>.</returns>
    public static bool Verify(ReadOnlySpan<byte> frame)
    {
        if (frame.Length < 8)
            return false;
        var dataLength = (frame[5] << 8) | frame[6];
        var crcOffset = 1 + 5 + dataLength; // after STP + 5 header bytes + data
        if (frame.Length < crcOffset + 2)
            return false;
        var computed = Compute(frame[1..crcOffset]);
        var swapped = SwapBytes(computed);
        var stored = (ushort)((frame[crcOffset] << 8) | frame[crcOffset + 1]);
        return swapped == stored;
    }
#endregion
}