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r250.c
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/******************************************************************************
* Module: r250.c
* Description: implements R250 random number generator, from S.
* Kirkpatrick and E. Stoll, Journal of Computational Physics, 40, p.
* 517 (1981).
* Written by: W. L. Maier
*
* METHOD...
* 16 parallel copies of a linear shift register with
* period 2^250 - 1. FAR longer period than the usual
* linear congruent generator, and commonly faster as
* well. (For details see the above paper, and the
* article in DDJ referenced below.)
*
* HISTORY...
* Sep 92: Number returned by dr250() is in range [0,1) instead
* of [0,1], so for example a random angle in the
* interval [0, 2*PI) can be calculated
* conveniently. (J. R. Van Zandt <[email protected]>)
* Aug 92: Initialization is optional. Default condition is
* equivalent to initializing with the seed 12345,
* so that the sequence of random numbers begins:
* 1173, 53403, 52352, 35341... (9996 numbers
* skipped) ...57769, 14511, 46930, 11942, 7978,
* 56163, 46506, 45768, 21162, 43113... Using ^=
* operator rather than two separate statements.
* Initializing with own linear congruent
* pseudorandom number generator for portability.
* Function prototypes moved to a header file.
* Implemented r250n() to generate numbers
* uniformly distributed in a specific range
* [0,n), because the common expedient rand()%n is
* incorrect. (J. R. Van Zandt <[email protected]>)
* May 91: Published by W. L. Maier, "A Fast Pseudo Random Number
* Generator", Dr. Dobb's Journal #176.
******************************************************************************/
#include <stdlib.h>
#include "r250.h"
/**** Static variables ****/
static int r250_index = 0;
static unsigned int r250_buffer[250] = {
15301,57764,10921,56345,19316,43154,54727,49252,32360,49582,
26124,25833,34404,11030,26232,13965,16051,63635,55860,5184,
15931,39782,16845,11371,38624,10328,9139,1684,48668,59388,
13297,1364,56028,15687,63279,27771,5277,44628,31973,46977,
16327,23408,36065,52272,33610,61549,58364,3472,21367,56357,
56345,54035,7712,55884,39774,10241,50164,47995,1718,46887,
47892,6010,29575,54972,30458,21966,54449,10387,4492,644,
57031,41607,61820,54588,40849,54052,59875,43128,50370,44691,
286,12071,3574,61384,15592,45677,9711,23022,35256,45493,
48913,146,9053,5881,36635,43280,53464,8529,34344,64955,
38266,12730,101,16208,12607,58921,22036,8221,31337,11984,
20290,26734,19552,48,31940,43448,34762,53344,60664,12809,
57318,17436,44730,19375,30,17425,14117,5416,23853,55783,
57995,32074,26526,2192,11447,11,53446,35152,64610,64883,
26899,25357,7667,3577,39414,51161,4,58427,57342,58557,
53233,1066,29237,36808,19370,17493,37568,3,61468,38876,
17586,64937,21716,56472,58160,44955,55221,63880,1,32200,
62066,22911,24090,10438,40783,36364,14999,2489,43284,9898,
39612,9245,593,34857,41054,30162,65497,53340,27209,45417,
37497,4612,58397,52910,56313,62716,22377,40310,15190,34471,
64005,18090,11326,50839,62901,59284,5580,15231,9467,13161,
58500,7259,317,50968,2962,23006,32280,6994,18751,5148,
52739,49370,51892,18552,52264,54031,2804,17360,1919,19639,
2323,9448,43821,11022,45500,31509,49180,35598,38883,19754,
987,11521,55494,38056,20664,2629,50986,31009,54043,59743
};
static unsigned myrand();
static void mysrand(unsigned newseed);
/**** Function: r250_init
Description: initializes r250 random number generator. ****/
void r250_init(int seed)
{
/*---------------------------------------------------------------------------*/
int j, k;
unsigned int mask;
unsigned int msb;
/*---------------------------------------------------------------------------*/
mysrand(seed);
r250_index = 0;
for (j = 0; j < 250; j++) /* Fill the r250 buffer with 15-bit values */
r250_buffer[j] = myrand();
for (j = 0; j < 250; j++) /* Set some of the MS bits to 1 */
if (myrand() > 16384)
r250_buffer[j] |= 0x8000;
msb = 0x8000; /* To turn on the diagonal bit */
mask = 0xffff; /* To turn off the leftmost bits */
for (j = 0; j < 16; j++)
{
k = 11 * j + 3; /* Select a word to operate on */
r250_buffer[k] &= mask; /* Turn off bits left of the diagonal */
r250_buffer[k] |= msb; /* Turn on the diagonal bit */
mask >>= 1;
msb >>= 1;
}
}
/**** Function: r250 Description: returns a random unsigned integer k
uniformly distributed in the interval 0 <= k < 65536. ****/
unsigned int r250()
{
/*---------------------------------------------------------------------------*/
register int j;
register unsigned int new_rand;
/*---------------------------------------------------------------------------*/
if (r250_index >= 147)
j = r250_index - 147; /* Wrap pointer around */
else
j = r250_index + 103;
new_rand = r250_buffer[r250_index] ^= r250_buffer[j];
if (r250_index >= 249) /* Increment pointer for next time */
r250_index = 0;
else
r250_index++;
return new_rand;
}
/**** Function: r250n Description: returns a random unsigned integer k
uniformly distributed in the interval 0 <= k < n ****/
unsigned int r250n(unsigned n)
{
/*---------------------------------------------------------------------------*/
register int j;
register unsigned int new_rand, limit;
/*---------------------------------------------------------------------------*/
limit = (65535U/n)*n;
do
{new_rand = r250();
if (r250_index >= 147)
j = r250_index - 147; /* Wrap pointer around */
else
j = r250_index + 103;
new_rand = r250_buffer[r250_index] ^= r250_buffer[j];
if (r250_index >= 249) /* Increment pointer for next time */
r250_index = 0;
else
r250_index++;
} while(new_rand >= limit);
return new_rand%n;
}
/**** Function: dr250
Description: returns a random double z in range 0 <= z < 1. ****/
double dr250()
{
/*---------------------------------------------------------------------------*/
register int j;
register unsigned int new_rand;
/*---------------------------------------------------------------------------*/
if (r250_index >= 147)
j = r250_index - 147; /* Wrap pointer around */
else
j = r250_index + 103;
new_rand = r250_buffer[r250_index] ^= r250_buffer[j];
if (r250_index >= 249) /* Increment pointer for next time */
r250_index = 0;
else
r250_index++;
return new_rand / 65536.; /* Return a number in [0.0 to 1.0) */
}
/*** linear congruent pseudorandom number generator for initialization ***/
static unsigned long seed=1;
/* Return a pseudorandom number in the interval 0 <= n < 32768.
This produces the following sequence of pseudorandom
numbers:
346, 130, 10982, 1090... (9996 numbers skipped) ...23369,
2020, 5703, 12762, 10828, 16252, 28648, 27041, 23444, 6604...
*/
static unsigned myrand()
{
seed = seed*0x015a4e35L + 1;
return (seed>>16)&0x7fff;
}
/* Initialize above linear congruent pseudorandom number generator */
static void mysrand(unsigned newseed)
{ seed = newseed;
}