On Tue, Sep 28, 2010 at 05:01:13PM +0200, Lars Ellenberg wrote:
> # HG changeset patch
> # User Lars Ellenberg <l...@linbit.com>
> # Date 1285682508 -7200
> # Node ID 94f4ee6025502c7ef710775d458ef766c51ad6db
> # Parent 066f484922ec549f84ea59c138d4e6cf16b2e14b
> Low: add cl_rand_from_intervall(a, b) helper
>
> There are a few places throughout the (heartbeat) code which want to generate
> random ints from a certain interval.
> Don't have them all fix the same normalization problems over and over again.
>
> diff -r 066f484922ec -r 94f4ee602550 include/clplumbing/cl_random.h
> --- a/include/clplumbing/cl_random.h Tue Sep 28 13:12:33 2010 +0200
> +++ b/include/clplumbing/cl_random.h Tue Sep 28 16:01:48 2010 +0200
> @@ -18,6 +18,40 @@
> * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
> */
>
> +#include <stdlib.h>
> +unsigned int cl_randseed(void);
> +/* Is currently rand() based.
> + * Does srand(cl_randseed()) once internally.
> + * Assumes mainloop is setup. */
> +int get_next_random(void);
>
> -unsigned int cl_randseed(void);
> -int get_next_random(void); /* Assumes mainloop setup */
> +/* generate some random number in the range [a;b];
> + * typically used to randomly delay messages. */
> +#define HAVE_CL_RAND_FROM_INTERVAL 1
> +static inline
> +int cl_rand_from_interval(const int a, const int b)
> +{
> + /*
> + * Be careful here, you don't know RAND_MAX at coding time,
> + * only at compile time. If you think
> + * (int)(a + (rand()*(b-a)+(RAND_MAX/2))/RAND_MAX);
> + * was correct, think again with RAND_MAX = INT_MAX,
> + * which is the case for many rand() implementations nowadays.
> + *
> + * Don't do modulo, either, as that will skew the distribution, and
> + * still has possible wraparounds, or an insufficient input set for too
> + * small RAND_MAX.
> + *
> + * Rather do the whole calculation in 64 bit, which should be correct
> + * as long as r, a, b, and RAND_MAX are all int.
> + * Of course, if you prefer, you can do it with floating point as well.
> + */
> +#if 1 /* use long long */
> + long long r = get_next_random();
> + r = a + (r * (b-a) + RAND_MAX/2)/RAND_MAX;
> +#else /* use floating point */
> + int r = get_next_random();
> + r = a + (int)(1.0 / RAND_MAX * r * (b-a) + 0.5);
> +#endif
> + return r;
> +}
Looks good to me.
Dejan
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