linux-kernel - Re: [RFC PATCH] random: add get_random

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Message-ID: <CAHmME9p3TR6ESekKRQxFsfQe3rZp7T4-4WLOn-SL98zUnAo2DQ@mail.gmail.com>
Date:   Sun, 24 Mar 2019 21:47:50 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     George Spelvin <lkml@....org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC PATCH] random: add get_random_max() function

I generally use a slightly simpler algorithm in various different projects:

//[0, bound)
static unsigned long random_bounded(unsigned long bound)
{
       unsigned long ret;
       const unsigned long max_mod_bound = (1 + ~bound) % bound;

       if (bound < 2)
               return 0;
       do
               ret = random_integer();
       while (ret < max_mod_bound);
       return ret % bound;
}

//[min, max_plus_one)
static unsigned long random_range(unsigned long min, unsigned long max_plus_one)
{
       return random_bounded(max_plus_one - min) + min;
}

Is the motivation behind using Lemire that you avoid the division (via
the modulo) in favor of a multiplication?