Positive integer from Python hash() function

Question

I want to use the Python hash() function to get integer hashes from objects. But built-in hash() can give negative values, and I want only positive. And I want it to work sensibly on both 32-bit and 64-bit platforms.

I.e. on 32-bit Python, hash() can return an integer in the range -2**31 to 2**31 - 1. On 64-bit systems, hash() can return an integer in the range -2**63 to 2**63 - 1.

But I want a hash in the range 0 to 2**32-1 on 32-bit systems, and 0 to 2**64-1 on 64-bit systems.

What is the best way to convert the hash value to its equivalent positive value within the range of the 32- or 64-bit target platform?

(Context: I'm trying to make a new random.Random style class. According to the random.Random.seed() docs, the seed "optional argument x can be any hashable object." So I'd like to duplicate that functionality, except that my seed algorithm can't handle negative integer values, only positive.)

Solution 1

Using sys.maxsize:

>>> import sys
>>> sys.maxsize
9223372036854775807L
>>> hash('asdf')
-618826466
>>> hash('asdf') % ((sys.maxsize + 1) * 2)
18446744073090725150L

Alternative using ctypes.c_size_t:

>>> import ctypes
>>> ctypes.c_size_t(hash('asdf')).value
18446744073090725150L

Solution 2

Just using sys.maxsize is wrong for obvious reasons (it being `2*n-1 and not 2*n), but the fix is easy enough:

h = hash(obj)
h += sys.maxsize + 1

for performance reasons you may want to split the sys.maxsize + 1 into two separate assignments to avoid creating a long integer temporarily for most negative numbers. Although I doubt this is going to matter much

Solution 3

(Edit: at first I thought you always wanted a 32-bit value)

Simply AND it with a mask of the desired size. Generally sys.maxsize will already be such a mask, since it's a power of 2 minus 1.

import sys
assert (sys.maxsize & (sys.maxsize+1)) == 0 # checks that maxsize+1 is a power of 2 

new_hash = hash & sys.maxsize

Solution 4

How about:

h = hash(o)
if h < 0:
  h += sys.maxsize

This uses sys.maxsize to be portable between 32- and 64-bit systems.

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Comments

[Voo]

There's no reason whatsoever to use a modulus here. I mean sure it works but it's less efficient and harder to read.

[falsetru]

You code could produce duplicated value. Try -0x7fffffffffffffff + sys.maxsize + 1 in 64bit system.

[Voo]

Ah yes true enough, shouldn't be conditional. Where's my head today?

[falsetru]

Why /2 instead of *2?

[falsetru]

@CraigMcQueen, I added an alternative way. Check it out.

[Voo]

Actually it's neither / nor * 2. We have a range of [-2**31, 2**31-1], but we want [-2**31+2**31, 2**31-1+2**31] (example is for 32bit system). So it's just an addition by the lower boundary (2**31).. really confused today I am.

[Exectron]

That's a good idea, although I want a 64-bit (or 32-bit) value, not a 63-bit (or 31-bit) value.

[Mark Ransom]

@CraigMcQueen, sorry I thought that maxsize was already the right size.