I tested some different algorithms, measuring speed and number of collisions.
I used three different key sets:
For each corpus, the number of collisions and the average time spent hashing was recorded.
Each result contains the average hash time, and the number of collisions
Hash Lowercase Random UUID Numbers
============= ============= =========== ==============
Murmur 145 ns 259 ns 92 ns
6 collis 5 collis 0 collis
FNV-1a 152 ns 504 ns 86 ns
4 collis 4 collis 0 collis
FNV-1 184 ns 730 ns 92 ns
1 collis 5 collis 0 collis*
DBJ2a 158 ns 443 ns 91 ns
5 collis 6 collis 0 collis***
DJB2 156 ns 437 ns 93 ns
7 collis 6 collis 0 collis***
SDBM 148 ns 484 ns 90 ns
4 collis 6 collis 0 collis**
SuperFastHash 164 ns 344 ns 118 ns
85 collis 4 collis 18742 collis
CRC32 250 ns 946 ns 130 ns
2 collis 0 collis 0 collis
LoseLose 338 ns - -
Collisions actually happen?
Yes. I started writing my test program to see if hash collisions actually happen - and are not just a theoretical construct. They do indeed happen:
creamwove collides with
costarring collides with
declinate collides with
altarage collides with
altarages collides with
cataract collides with
roquette collides with
shawl collides with
dowlases collides with
cricketings collides with
longans collides with
hetairas collides with
heliotropes collides with
depravement collides with
stylist collides with
joyful collides with
redescribed collides with
dram collides with
haggadot collides with
adorablenesses collides with
playwright collides with
playwrighting collides with
treponematoses collides with
codding collides with
exhibiters collides with
dahabiah collides with
encharm collides with
grahams collides with
- ...snip 79 collisions...
night collides with
nights collides with
finks collides with
The other subjective measure is how randomly distributed the hashes are. Mapping the resulting HashTables shows how evenly the data is distributed. All the hash functions show good distribution when mapping the table linearly:
Or as a Hilbert Map (XKCD is always relevant):
Except when hashing number strings (
"216553") (e.g. zip codes), where patterns begin to emerge in most of the hashing algorithms:
All except FNV-1a, which still look plenty random to me:
In fact, Murmur2 seems to have even better randomness with
When i look at the
FNV-1a "number" map, i think i see subtle vertical patterns. With Murmur i see no patterns at all. What do you think?
* in the above table denotes how bad the randomness is. With
FNV-1a being the best, and
DJB2x being the worst:
I originally wrote this program to decide if I even had to worry about collisions: I do.
And then it turned into making sure that the hash functions were sufficiently random.
The FNV1 hash comes in variants that return 32, 64, 128, 256, 512 and 1024 bit hashes.
The FNV-1a algorithm is:
hash = FNV_offset_basis
for each octetOfData to be hashed
hash = hash xor octetOfData
hash = hash * FNV_prime
Where the constants
FNV_prime depend on the return hash size you want:
Hash Size Prime Offset
=========== =========================== =================================
32-bit 16777619 2166136261
64-bit 1099511628211 14695981039346656037
128-bit 309485009821345068724781371 144066263297769815596495629667062367629
prime: 2^88 + 2^8 + 0x3b = 309485009821345068724781371
prime: 2^344 + 2^8 + 0x57 = 35835915874844867368919076489095108449946327955754392558399825615420669938882575126094039892345713852759
prime: 2^680 + 2^8 + 0x8d = 5016456510113118655434598811035278955030765345404790744303017523831112055108147451509157692220295382716162651878526895249385292291816524375083746691371804094271873160484737966720260389217684476157468082573
See the main FNV page for details.
As a practical matter:
Guid can be useful
All my results are with the 32-bit variant.
FNV-1 better than FNV-1a?
No. FNV-1a is all around better. There was more collision with FNV-1a when using the english word corpus:
Hash Word Collisions
Now compare lowercase and uppercase:
Hash lowercase word Collisions UPPERCASE word collisions
====== ========================= =========================
FNV-1 1 9
FNV-1a 4 11
In this case FNV-1a isn't "400%" worse than FN-1, only 20% worse.
I think the more important takeaway is that there's two classes of algorithms when it comes to collisions:
- collisions rare: FNV-1, FNV-1a, DJB2, DJB2a, SDBM
- collisions common: SuperFastHash, Loselose
And then there's the how evenly distributed the hashes are:
- outstanding distribution: Murmur2, FNV-1a, SuperFastHas
- excellent distribution: FNV-1
- good distribution: SDBM, DJB2, DJB2a
- horrible distribution: Loselose
Murmur? Sure, why not
@whatshisname wondered how a CRC32 would perform, added numbers to the table.
CRC32 is pretty good. Few collisions, but slower, and the overhead of a 1k lookup table.
snip all erroneous stuff about CRC distribution - my bad
Up until today I was going to use FNV-1a as my de facto hash-table hashing algorithm. But now I'm switching to Murmur2:
- Better randomnessification of all classes of input
And I really, really hope there's something wrong with the
SuperFastHash algorithm i found; it's too bad to be as popular as it is.
Update: From the MurmurHash3 homepage on Google:
(1) - SuperFastHash has very poor collision properties, which have been documented elsewhere.
So I guess it's not just me.
Update: I realized why
Murmur is faster than the others. MurmurHash2 operates on 4 bytes at a time. Most algorithms are byte by byte:
for each octet in Key
This means that as keys get longer Murmur gets its chance to shine.
A timely post by Raymond Chen reiterates the fact that "random" guids are not meant to be used for their randomness. They, or a subset of them, are unsuitable as a hash key:
Even the Version 4 GUID algorithm is not guaranteed to be unpredictable, because the algorithm does not specify the quality of the random number generator. The Wikipedia article for GUID contains primary research which suggests that future and previous GUIDs can be predicted based on knowledge of the random number generator state, since the generator is not cryptographically strong.
Randomess is not the same as collision avoidance; which is why it would be a mistake to try to invent your own "hashing" algorithm by taking some subset of a "random" guid:
int HashKeyFromGuid(Guid type4uuid)
//A "4" is put somewhere in the guid.
//i can't remember exactly where, but it doesn't matter for
//the illustrative purposes of this pseudocode
int guidVersion = ((type4uuid.D3 & 0x0f00) >> 8);
Assert(guidVersion == 4);
Note: Again, i put "random guid" in quotes because it's the "random" variant of guids. A more accurate description would be
Type 4 uuid. But nobody knows what type 4, or types 1, 3 and 5 are. So it's just easier to call them "random" guids.