Messages in this thread | | | Date | 22 Feb 2001 20:38:00 +0200 | From | (Kai Henningsen) | Subject | Re: [rfc] Near-constant time directory index for Ext2 |
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phillips@innominate.de (Daniel Phillips) wrote on 20.02.01 in <01022020011905.18944@gimli>:
> But the current hash function is just a place holder, waiting for > an better version based on some solid theory. I currently favor the > idea of using crc32 as the default hash function, but I welcome > suggestions.
I once liked those things, too - but I've learned better since.
Quoting _Handbook_of_Algorithms_and_Data_Structures_ (Gonnet/Baeza-Yates, ISBM 0-201-41607-7, Addison-Wesley):
--- snip --- 3.3.1 Practical hashing functions
[...]
A universal class of hashing functions is a class with the property that given any input, the average performance of all the functions is good. [...] For example, h(k) = (a * k + b) mod m with integers a != 0 and b is a universal class of hash functions. [...] Keys which are strings or sequences of words (including those which are of variable length) are best treated by considering them as a number base b. Let the string s be composed of k characters s1s2...sk. Then
h(s) = ( sum(i=0..k-1) B^i*s(k-i) ) mod m
To obtain a more efficient version of this function we can compute
h(s) = ( ( sum(i=0..k-1) B^i*s(k-i) ) mod 2^w ) mod m
where w is the number of bits in a computer word, and the mod 2^w operation is done by the hardware. For this function the value B = 131 is recommended, as B^i has a maximum cycle mod 2^k for 8<=k<=64.
Hashing function for strings
int hashfunction(s) char *s;
{ int i; for(i=0; *s; s++) i = 131*i + *s; return(i % m); } --- snip --- I've actually used that function for a hash containing something like a million phone numbers as keys, and there were *very* few collisions. Similarly for another hash containgng megabytes of RFC 822 message-ids.
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