Question

I have a problem with this algorithm:

function crc16 = crc16eval(D)
% CRC16EVAL CRC-CCITT check with the polynomial: x^16+x^12+x^5+1
D = uint16(D);
crchi = 255;
crclo = 255;
t = '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';
crc16htab = hex2dec(reshape(t,2,length(t)/2)');
t = '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';
crc16ltab = hex2dec(reshape(t,2,length(t)/2)');
for k = 1:length(D)
    ix = double(bitxor(crchi,D(k)))+1;
    crchi = bitxor(crclo,crc16htab(ix));
    crclo = crc16ltab(ix);
end
crc16 = crchi*256+crclo;
end

I need translate that code to Python, and I have done the next:

def crc16eval(D):
    crchi = 255
    crclo = 255
    t = '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'
    # crc16htab = hex2dec(reshape(t,2,length(t)/2)');

    tarray = [int(n, 16) for n in t] # Recorro el string t, y por cada caracter creo un nuevo entero en el array

    crc16htab = reshape(tarray, (2, (len(t)/2) )).transpose()
    #print crc16htab

    t = '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'
    # crc16ltab = hex2dec(reshape(t,2,length(t)/2)');

    tarray = [int(n, 16) for n in t] # Recorro el string t, y por cada caracter creo un nuevo entero en el array

    crc16ltab = reshape(tarray, (2, (len(t)/2))).transpose()
    #print crc16ltab

    for k in range(len(D)):
        ix = crchi ^ D[k]
        crchi = crclo ^ crc16htab[ix]
        crclo = crc16ltab[ix]

    return crchi*256+crclo

My problem is the next:

When I execute de Python code, It’s take a loooong time to calculate de xor, I think the the problem is that

crclo = crc16ltab[ix]

is a matrix and that’s take a long time to calculate. Which is the problem?

The pseudo-code of this algorithm is the next:

The algorithm for the CRC-CCITT is below described. Note that all operations are on bytes.
A = new byte
B = temp byte
CRCHI = High byte (most significant) of the 16-bit CRC
CRCLO = Low byte (least significant) of the 16-bit CRC
START:
    FOR A = FIRST_BYTE TO LAST_BYTE IN BLOCK DO:
        A = A XOR CRCHI
        CRCHI = A
        SHIFT A RIGHT FOUR TIMES (ZERO FILL)
        A = A XOR CRCHI                  {IJKLMNOP}
        CRCHI = CRCLO                    { swap CRCHI, CRCLO }
        CRCLO = A
        ROTATE A LEFT 4 TIMES            {MNOPIJKL}
        B=A                              { temp save }
        ROTATE A LEFT ONCE               {NOPIJKLM}
        A = A AND $1F                    {000IJLLM}
        CRCHI = A XOR CRCHI
        A = B AND $F0                    {MNOP0000}
        CRCHI = A XOR CRCHI              { CRCHI complete }
        ROTATE B LEFT ONCE               {NOP0000M}
        B = B AND $ E0                   {NOP00000}
        CRCLO = B XOR CRCLO              { CRCLO complete }
    DOEND;
FINISH.

My question is: Why my python code take long time to execute? What is wrong? The problem I think is in

for k in range(len(D)):
    ix = crchi ^ D[k]
    crchi = crclo ^ crc16htab[ix]
    crclo = crc16ltab[ix]

Thanks a lot!

Answer 1

This was the final solution, too late to post here maybe… But I think that maybe It’s useful for someone.

# CRC16EVAL CRC-CCITT check with the polynomial: x^16+x^12+x^5+1
def crc16eval(D):
    crchi = 255
    crclo = 255

    crc16htab = [0, 16, 32, 48, 64, 80, 96, 112, 129, 145, 161, 177, 193, 209, 225, 241, 18, 2, 50, 34, 82, 66, 114, 98, 147, 131, 179, 163, 211, 195, 243, 227, 36, 52, 4, 20, 100, 116, 68, 84, 165, 181, 133, 149, 229, 245, 197, 213, 54, 38, 22, 6, 118, 102, 86, 70, 183, 167, 151, 135, 247, 231, 215, 199, 72, 88, 104, 120, 8, 24, 40, 56, 201, 217, 233, 249, 137, 153, 169, 185, 90, 74, 122, 106, 26, 10, 58, 42, 219, 203, 251, 235, 155, 139, 187, 171, 108, 124, 76, 92, 44, 60, 12, 28, 237, 253, 205, 221, 173, 189, 141, 157, 126, 110, 94, 78, 62, 46, 30, 14, 255, 239, 223, 207, 191, 175, 159, 143, 145, 129, 177, 161, 209, 193, 241, 225, 16, 0, 48, 32, 80, 64, 112, 96, 131, 147, 163, 179, 195, 211, 227, 243, 2, 18, 34, 50, 66, 82, 98, 114, 181, 165, 149, 133, 245, 229, 213, 197, 52, 36, 20, 4, 116, 100, 84, 68, 167, 183, 135, 151, 231, 247, 199, 215, 38, 54, 6, 22, 102, 118, 70, 86, 217, 201, 249, 233, 153, 137, 185, 169, 88, 72, 120, 104, 24, 8, 56, 40, 203, 219, 235, 251, 139, 155, 171, 187, 74, 90, 106, 122, 10, 26, 42, 58, 253, 237, 221, 205, 189, 173, 157, 141, 124, 108, 92, 76, 60, 44, 28, 12, 239, 255, 207, 223, 175, 191, 143, 159, 110, 126, 78, 94, 46, 62, 14, 30]

    crc16ltab = [0, 33, 66, 99, 132, 165, 198, 231, 8, 41, 74, 107, 140, 173, 206, 239, 49, 16, 115, 82, 181, 148, 247, 214, 57, 24, 123, 90, 189, 156, 255, 222, 98, 67, 32, 1, 230, 199, 164, 133, 106, 75, 40, 9, 238, 207, 172, 141, 83, 114, 17, 48, 215, 246, 149, 180, 91, 122, 25, 56, 223, 254, 157, 188, 196, 229, 134, 167, 64, 97, 2, 35, 204, 237, 142, 175, 72, 105, 10, 43, 245, 212, 183, 150, 113, 80, 51, 18, 253, 220, 191, 158, 121, 88, 59, 26, 166, 135, 228, 197, 34, 3, 96, 65, 174, 143, 236, 205, 42, 11, 104, 73, 151, 182, 213, 244, 19, 50, 81, 112, 159, 190, 221, 252, 27, 58, 89, 120, 136, 169, 202, 235, 12, 45, 78, 111, 128, 161, 194, 227, 4, 37, 70, 103, 185, 152, 251, 218, 61, 28, 127, 94, 177, 144, 243, 210, 53, 20, 119, 86, 234, 203, 168, 137, 110, 79, 44, 13, 226, 195, 160, 129, 102, 71, 36, 5, 219, 250, 153, 184, 95, 126, 29, 60, 211, 242, 145, 176, 87, 118, 21, 52, 76, 109, 14, 47, 200, 233, 138, 171, 68, 101, 6, 39, 192, 225, 130, 163, 125, 92, 63, 30, 249, 216, 187, 154, 117, 84, 55, 22, 241, 208, 179, 146, 46, 15, 108, 77, 170, 139, 232, 201, 38, 7, 100, 69, 162, 131, 224, 193, 31, 62, 93, 124, 155, 186, 217, 248, 23, 54, 85, 116, 147, 178, 209, 240]

    for k in range(len(D)):
        ix = crchi ^ D[k]
        crchi = crclo ^ crc16htab[ix]
        crclo = crc16ltab[ix]

    return crchi*256+crclo

Antonio.