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

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Date:	Thu, 16 Jan 2014 10:22:04 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Joe Perches <joe@...ches.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert
 "misuses" of reciprocal_divide

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.
>
> prandom_u32_lt_N?
>
> 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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