[Haskell-cafe] Very imperfect hash function

Steve Schafer steve at fenestra.com
Thu Jan 28 14:07:27 EST 2010

I'm looking for some algorithmic suggestions:

I have a set of several hundred key/value pairs. The keys are 32-bit
integers, and are all distinct. The values are also integers, but the
number of values is small (only six in my current problem). So,
obviously, several keys map to the same value.

For some subsets of keys, examining only a small portion of the key's
bits is enough to determine the associated value. For example, there may
be 250 keys that all have the same most-significant byte, and all 250
map to the same value. There are also keys at the other extreme, where
two keys that differ in only one bit position map to different values.

The data are currently in a large lookup table. To save space, I'd like
to convert that into a sort of hash function:

 hash :: key -> value

My question is this: Is there any kind of generic approach that can make
use of the knowledge about the internal redundancy of the keys to come
up with an efficient function?

Steve Schafer
Fenestra Technologies Corp.

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