Data.HashTable.hashInt seems somewhat sub-optimal

[Simon.Frankau at barclayscapital.com]

I tried submitting this bug through the tracker, but it seemed to give
me permissions errors - probably a firewall issue here. :( Apologies.
Anyway...

Prelude> Data.HashTable.hashInt 0
0
Prelude> Data.HashTable.hashInt 1
-1
Prelude> Data.HashTable.hashInt 2
-1
Prelude> Data.HashTable.hashInt 3
-2
Prelude> Data.HashTable.hashInt 4
-2
Prelude> Data.HashTable.hashInt 5
-2
Prelude> Data.HashTable.hashInt 6
-3
Prelude> Data.HashTable.hashInt 7
-3
Prelude> Data.HashTable.hashInt 8
-4
Prelude> Data.HashTable.hashInt 9
-4
Prelude> Data.HashTable.hashInt 10
-4
Prelude> Data.HashTable.hashInt 200
-77
Prelude> Data.HashTable.hashInt 201
-77
Prelude> Data.HashTable.hashInt 202
-78

I prefer to use hashing to decrease the likelihood of collisions, not
increase them. ;)

I think the original algorithm was quite possibly supposed to use the
bottom 32 bits of the result, although this means that changes in higher
bits of the input do not affect lower bits of the hash. It should
probably also use unsigned multiplication. Perhaps a better solution
would be to xor or add together the high and low 32 bits? (I'm not sure
as I haven't read the literature).

Simon Frankau (sgf at arbitrary.name)
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