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Asked: 2026-05-12T23:00:16+00:00

Let’s say I want to build a perfect hash table for looking up an

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Let’s say I want to build a perfect hash table for looking up an array where the predefined keys are 12 Months, thus I would want

hash("January")==0
hash("December")==11

I run my Month names through gperf and got a nice hash function, but it appears to give out 16 buckets(or rather the range is 16)!

#define MIN_HASH_VALUE 3
#define MAX_HASH_VALUE 18
/* maximum key range = 16, duplicates = 0 */

Looking at the generated gperf code, its hash function code does a simple return of len plus char value lookup from a 256 size table. Somehow, in my head I imagined a fancy looking function… 🙂

What if I want exactly 12 buckets(that is I do not want to skip over unused buckets)? For small sets as this, it really doesn’t matter, but when I have 1000 predefined keys and want exactly 1000 buckets in a row?

Can one find a deterministic way to do this?

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1 Answer

  1. Editorial Team

    The only alternative to gperf I know is cmph : http://cmph.sourceforge.net/ but, as Jerome said in the comment, having 16 buckets provides you some speed benefit.

    When I first looked at minimal perfect hasihing I found very interesting readings on CiteseerX but I resisted the temptation to try coding one of those solutions myself. I know I would end up with an inferior solution respect to gperf or cmph or, even assuming the solution was comparable, I would have to spend a lot of time on it.

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