FNV與FNV-1a Hash演算法說明【轉】

weixin_34391854發表於2017-02-28

轉自:http://blog.csdn.net/jiayanhui2877/article/details/12090575

The core of the FNV hash

The core of the FNV-1 hash algorithm is as follows:
hash = offset_basis
for each octet_of_data to be hashed
 hash = hash * FNV_prime
 hash = hash xor octet_of_data
return hash

The offset_basis andFNV_prime can be found in theparameters of the FNV-1/FNV-1a hash section below.

FNV-1a alternate algorithm

There is a minor variation of the FNV hash algorithm known asFNV-1a:
hash = offset_basis
for each octet_of_data to be hashed
 hash = hash xor octet_of_data
 hash = hash * FNV_prime
return hash

The only difference between the FNV-1a hash and the FNV-1 hashis the order of the xor and multiply.The FNV-1a hashuses the same FNV_prime and offset_basisas the FNV-1 hash of the same n-bit size.


Parameters of the FNV-1/FNV-1a hash

The FNV-1 hash parameters are as follows:
  • hash is an n bit unsigned integer,where n is the bit length of hash.
  • The multiplication is performed modulo 2nwhere n is the bit length of hash.
  • The xor is performed on the low orderoctet (8 bits) of hash.
  • The FNV_prime is dependent on n, the size of the hash:
    32 bit FNV_prime =224 + 28 + 0x93 = 16777619

    64 bit FNV_prime = 240 + 28 + 0xb3 = 1099511628211

    128 bit FNV_prime = 288 + 28 + 0x3b = 309485009821345068724781371

    256 bit FNV_prime = 2168 + 28 + 0x63 = 374144419156711147060143317175368453031918731002211

    512 bit FNV_prime = 2344 + 28 + 0x57 =
    35835915874844867368919076489095108449946327955754392558399825615420669938882575
    126094039892345713852759


    1024 bit FNV_prime = 2680 + 28 + 0x8d =
    50164565101131186554345988110352789550307653454047907443030175238311120551081474
    51509157692220295382716162651878526895249385292291816524375083746691371804094271
    873160484737966720260389217684476157468082573

    Part of the magic of FNV is the selection of the FNV_primefor a given sized unsigned integer.Some primes do hash better than other primes for a given integer size.

  • The offset_basis for FNV-1 is dependent on n, the size of the hash:
    32 bit offset_basis = 2166136261

    64 bit offset_basis = 14695981039346656037

    128 bit offset_basis = 144066263297769815596495629667062367629

    256 bit offset_basis =
    100029257958052580907070968620625704837092796014241193945225284501741471925557


    512 bit offset_basis =
    96593031294966694980094354007163104660904187456726378961083743294344626579945829
    32197716438449813051892206539805784495328239340083876191928701583869517785


    1024 bit offset_basis =
    14197795064947621068722070641403218320880622795441933960878474914617582723252296
    73230371772215086409652120235554936562817466910857181476047101507614802975596980
    40773201576924585630032153049571501574036444603635505054127112859663616102678680
    82893823963790439336411086884584107735010676915

    NOTE: Older versions of this web page incorretly indicated that the 128 bitFNV_prime was 2168 + 28 + 0x59.This was not correct.While that satisfied all of the significant FNV_prime properties,it was not the smallest 128 bit FNV_prime.The 128 bit offset_basischanged from275519064689413815358837431229664493455to144066263297769815596495629667062367629was changed as a result of the 128 bit FNV_prime correction.(Sorry about that!)

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