Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert"misuses" of reciprocal_divide

From: Daniel Borkmann
Date: Thu Jan 16 2014 - 04:22:25 EST

On 01/16/2014 01:29 AM, Joe Perches wrote:
On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:
Many functions have open coded a function that return a random
number in range [0,N-1]. Also, only because we have a function
that is named reciprocal_divide(), it has not much to do with
the pupose where it is being used when a previous reciprocal_value()
has not been obtained.


I do not like the camelcase name and thought the
prandom_u32_max was better.

Hm, you wanted to have the name intuitive, right ... so
u32 prandom_u32_lt_N(u32 N) suggests "less then N". If you
are saying "_max" here, then you should keep in mind that
the maximum result you get from here is "max-1", not "max".
But whatever, it's just a name.

How about using

u32 prandom_u32_max(u32 max)
return (u32)(((u64)prandom_u32() * max) >> 32);

u32 prandom_u32_range(u32 a, u32 b)
if (b < a)
swap(a, b);

return a + (u32)(((u64)prandom_u32() * (b - a)) >> 32);

I didn't introduce the last one as it wasn't used in the patch.
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