162 lines
3.1 KiB
C
162 lines
3.1 KiB
C
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/* drand.c
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*
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* Pseudorandom number generator
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*
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*
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*
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* SYNOPSIS:
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*
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* double y, drand();
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*
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* drand( &y );
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*
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*
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*
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* DESCRIPTION:
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*
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* Yields a random number 1.0 <= y < 2.0.
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*
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* The three-generator congruential algorithm by Brian
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* Wichmann and David Hill (BYTE magazine, March, 1987,
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* pp 127-8) is used. The period, given by them, is
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* 6953607871644.
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*
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* Versions invoked by the different arithmetic compile
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* time options DEC, IBMPC, and MIEEE, produce
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* approximately the same sequences, differing only in the
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* least significant bits of the numbers. The UNK option
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* implements the algorithm as recommended in the BYTE
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* article. It may be used on all computers. However,
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* the low order bits of a double precision number may
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* not be adequately random, and may vary due to arithmetic
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* implementation details on different computers.
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*
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* The other compile options generate an additional random
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* integer that overwrites the low order bits of the double
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* precision number. This reduces the period by a factor of
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* two but tends to overcome the problems mentioned.
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*
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*/
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/* Three-generator random number algorithm
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* of Brian Wichmann and David Hill
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* BYTE magazine, March, 1987 pp 127-8
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*
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* The period, given by them, is (p-1)(q-1)(r-1)/4 = 6.95e12.
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*/
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#include "mconf.h"
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#ifdef ANSIPROT
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static int ranwh ( void );
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#else
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static int ranwh();
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#endif
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static int sx = 1;
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static int sy = 10000;
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static int sz = 3000;
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static union {
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double d;
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unsigned short s[4];
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} unkans;
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/* This function implements the three
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* congruential generators.
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*/
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static int ranwh()
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{
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int r, s;
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/* sx = sx * 171 mod 30269 */
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r = sx/177;
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s = sx - 177 * r;
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sx = 171 * s - 2 * r;
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if( sx < 0 )
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sx += 30269;
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/* sy = sy * 172 mod 30307 */
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r = sy/176;
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s = sy - 176 * r;
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sy = 172 * s - 35 * r;
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if( sy < 0 )
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sy += 30307;
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/* sz = 170 * sz mod 30323 */
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r = sz/178;
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s = sz - 178 * r;
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sz = 170 * s - 63 * r;
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if( sz < 0 )
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sz += 30323;
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/* The results are in static sx, sy, sz. */
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return 0;
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}
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/* drand.c
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*
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* Random double precision floating point number between 1 and 2.
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*
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* C callable:
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* drand( &x );
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*/
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int drand( a )
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double *a;
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{
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unsigned short r;
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#ifdef DEC
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unsigned short s, t;
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#endif
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/* This algorithm of Wichmann and Hill computes a floating point
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* result:
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*/
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ranwh();
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unkans.d = sx/30269.0 + sy/30307.0 + sz/30323.0;
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r = unkans.d;
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unkans.d -= r;
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unkans.d += 1.0;
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/* if UNK option, do nothing further.
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* Otherwise, make a random 16 bit integer
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* to overwrite the least significant word
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* of unkans.
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*/
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#ifdef UNK
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/* do nothing */
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#else
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ranwh();
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r = sx * sy + sz;
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#endif
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#ifdef DEC
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/* To make the numbers as similar as possible
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* in all arithmetics, the random integer has
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* to be inserted 3 bits higher up in a DEC number.
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* An alternative would be put it 3 bits lower down
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* in all the other number types.
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*/
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s = unkans.s[2];
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t = s & 07; /* save these bits to put in at the bottom */
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s &= 0177770;
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s |= (r >> 13) & 07;
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unkans.s[2] = s;
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t |= r << 3;
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unkans.s[3] = t;
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#endif
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#ifdef IBMPC
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unkans.s[0] = r;
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#endif
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#ifdef MIEEE
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unkans.s[3] = r;
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#endif
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*a = unkans.d;
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return 0;
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}
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