Lair Of The Multimedia Guru

February 6, 2006

CRC32 vs. Adler32

You need to choose a checksum and dunno which, well the 2 most popular ones are the CRC and Adler32 checksum

Speed / innermost loop

CRC32:

while(buffer<end -3){
    crc ^= le2me_32(*(uint32_t*)buffer); buffer+=4;
    crc =  tab[3][ crc     &0xFF]
          ^tab[2][(crc>>8 )&0xFF]
          ^tab[1][(crc>>16)&0xFF]
          ^tab[0][(crc>>24)     ];
}

Adler32:

while(buffer<end -3){
    s1 += *buffer++; s2+=s1;
    s1 += *buffer++; s2+=s1;
    s1 += *buffer++; s2+=s1;
    s1 += *buffer++; s2+=s1;
}

as we can see adler needs 12 adds and 4 1byte reads and crc needs 1 4byte read, 4 xor, 4 table lookups, 1 add, 3 and, 3 shift, without benchmarking i would say adler should be faster

Burst error detecting capabilities

CRC-32 can detect every burst error of 32 or less bits
Adler-32 will fail with at least one 17bit burst error (0 00000010 00000000 vs. 1 00000000 00000001)

Bit error detecting capabilities

CRC-32 differs depening upon generator polynom see misusing-crcs-for-error-correction
Adler-32 can detect every 2bit error within 65521 byte or so but will fail with some 3 bit errors

Random error detecting capabilities

CRC-32 will produce a evenly distributed checksum for messages >4byte
Adler-32 will produce a quite unevenly distributed checksum for small mesages so that the effective number of bits in the checksum is significantly reduced for short messags

adler32
len=     1, collisions=488296949(0.390637%), effective bits=7.999957
len=     2, collisions=  1903788(0.001523%), effective bits=16.002699
len=     4, collisions=   372789(0.000298%), effective bits=18.355140
len=     8, collisions=    94799(0.000076%), effective bits=20.330556
len=    16, collisions=    23902(0.000019%), effective bits=22.318296
len=    32, collisions=     6068(0.000005%), effective bits=24.296135
len=    64, collisions=     1573(0.000001%), effective bits=26.243837
len=   128, collisions=      669(0.000001%), effective bits=27.477278
len=   256, collisions=      437(0.000000%), effective bits=28.091651
len=   512, collisions=      331(0.000000%), effective bits=28.492453
len=  1024, collisions=      219(0.000000%), effective bits=29.088353
----
crc32
len=     1, collisions=488296915(0.390637%), effective bits=7.999957
len=     2, collisions=  1903782(0.001523%), effective bits=16.002703
len=     4, collisions=       22(0.000000%), effective bits=32.403708
len=     8, collisions=       29(0.000000%), effective bits=32.005159
len=    16, collisions=       30(0.000000%), effective bits=31.956249
len=    32, collisions=       28(0.000000%), effective bits=32.055785
len=    64, collisions=       39(0.000000%), effective bits=31.577738
len=   128, collisions=       39(0.000000%), effective bits=31.577738
len=   256, collisions=       32(0.000000%), effective bits=31.863140
len=   512, collisions=       32(0.000000%), effective bits=31.863140
len=  1024, collisions=       42(0.000000%), effective bits=31.470823

Note, generated with: check_collision.c

Filed under: Error Correcting Codes — Michael @ 19:27

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