CRC-16/CCITT-FALSE

crc16.c

uint16_t crc16(char* pData, int length)
{
    uint8_t i;
    uint16_t wCrc = 0xffff;
    while (length--) {
        wCrc ^= *(unsigned char *)pData++ << 8;
        for (i=0; i < 8; i++)
            wCrc = wCrc & 0x8000 ? (wCrc << 1) ^ 0x1021 : wCrc << 1;
    }
    return wCrc & 0xffff;
}

C# version with a precalc lookup table.

public static class Crc16Calculator {
    private const ushort InitialValue = 0xFFFF;

    private static readonly ushort[] ccittFalseCrc16Table = {
        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 static ushort CalcCcittFalse(ReadOnlySpan<byte> bytes, int length) {
        var crc16 = InitialValue;

        var byteIndex = 0;
        while (length-- > 0) {
            var tableIndex = crc16 >> 8 ^ bytes[byteIndex++];
            crc16 = (ushort)(crc16 << 8 ^ ccittFalseCrc16Table[tableIndex]);
        }

        return crc16;
    }
}

Javascript version:

function crc16(data, offset, length) {
    if (data == null || offset < 0 || offset > data.length - 1 || offset + length > data.length) {
        return 0;
    }

    crc = 0xFFFF;
    for (i = 0; i < length; ++i) {
        crc ^= data[offset + i] << 8;
        for (j = 0; j < 8; ++j) {
            crc = (crc & 0x8000) > 0 ? (crc << 1) ^ 0x1021 : crc << 1;
        }
    }
    return crc & 0xFFFF;
}

/**
 * data: string
 */
function crc16(data, offset, length) {
    if (data == null || offset < 0 || offset > data.length - 1 || offset + length > data.length) {
        return 0;
    }

    crc = 0xFFFF;
    for (i = 0; i < length; ++i) {
        crc ^= data.charCodeAt(offset + i) << 8;
        for (j = 0; j < 8; ++j) {
            crc = (crc & 0x8000) > 0 ? (crc << 1) ^ 0x1021 : crc << 1;
        }
    }
    return crc & 0xFFFF;
}

https://github.com/IgorAlov/crc16

My variant of the function [ MySQL / MariaDB Implementation ]

DELIMITER $$

USE `database`$$

DROP FUNCTION IF EXISTS `CRC16`$$

CREATE FUNCTION `CRC16`(_STRING VARCHAR(255)) RETURNS INT(11)
    DETERMINISTIC
BEGIN
    DECLARE _CRC INT;
    DECLARE _ord INT;
    DECLARE _n INT;
    DECLARE _x INT;
    DECLARE _strlend INT;
    SET _CRC := 0xffff;

    IF (_STRING IS NULL) THEN
       RETURN NULL;
    END IF;

    SET _n  := 1;  
    SET _strlend := LENGTH(_STRING) ;
          
    loop_crc:  LOOP

       IF  _n > _strlend THEN 
         LEAVE  loop_crc;
       END  IF;

       SET _ord := ORD(SUBSTRING(_STRING, _n, 1) );
       SET _x := ((_CRC>>8) ^ _ord) & 0xff;
       SET _x := _x ^ (_x>>4);       
       SET _CRC := ((_CRC<<8) ^ (_x<<12) ^ (_x<<5) ^ _x) & 0xffff;
       SET  _n := _n + 1;
     END LOOP;
   RETURN _CRC; 

 END$$

DELIMITER ;

example query:
SELECT HEX(CRC16('bashtest'));

result:

hex(CRC16('bashtest'))  
33D0