wmMatrix/yarandom.c

   1 #define HAVE_UNISTD_H 1
   2 #define GETTIMEOFDAY_TWO_ARGS 1
   3 /* yarandom.c -- Yet Another Random Number Generator.
   4
   5    The unportable mess that is rand(), random(), drand48() and friends led me
   6    to ask Phil Karlton <karlton@netscape.com> what the Right Thing to Do was.
   7    He responded with this.  It is non-cryptographically secure, reasonably
   8    random (more so than anything that is in any C library), and very fast.
   9
  10    I don't understand how it works at all, but he says "look at Knuth,
  11    Vol. 2 (original edition), page 26, Algorithm A.  In this case n=55,
  12    k=20 and m=2^32."
  13
  14    So there you have it.
  15
  16    ---------------------------
  17    Note: xlockmore 4.03a10 uses this very simple RNG:
  18
  19         if ((seed = seed % 44488 * 48271 - seed / 44488 * 3399) < 0)
  20           seed += 2147483647;
  21         return seed-1;
  22
  23    of which it says
  24
  25         ``Dr. Park's algorithm published in the Oct. '88 ACM  "Random Number
  26           Generators: Good Ones Are Hard To Find" His version available at
  27           ftp://cs.wm.edu/pub/rngs.tar Present form by many authors.''
  28
  29    Karlton says: ``the usual problem with that kind of RNG turns out to
  30    be unexepected short cycles for some word lengths.''
  31
  32    Karlton's RNG is faster, since it does three adds and two stores, while the
  33    xlockmore RNG does two multiplies, two divides, three adds, and one store.
  34
  35    Compiler optimizations make a big difference here:
  36        gcc -O:     difference is 1.2x.
  37        gcc -O2:    difference is 1.4x.
  38        gcc -O3:    difference is 1.5x.
  39        SGI cc -O:  difference is 2.4x.
  40        SGI cc -O2: difference is 2.4x.
  41        SGI cc -O3: difference is 5.1x.
  42    Irix 6.2; Indy r5k; SGI cc version 6; gcc version 2.7.2.1.
  43  */
  44
  45
  46 #ifdef HAVE_UNISTD_H
  47 # include <unistd.h>  /* for getpid() */
  48 #endif
  49 #include <sys/time.h> /* for gettimeofday() */
  50
  51 #include "yarandom.h"
  52
  53
  54 /* The following 'random' numbers are taken from CRC, 18th Edition, page 622.
  55    Each array element was taken from the corresponding line in the table,
  56    except that a[0] was from line 100. 8s and 9s in the table were simply
  57    skipped. The high order digit was taken mod 4.
  58  */
  59 #define VectorSize 55
  60 static unsigned int a[VectorSize] = {
  61  035340171546, 010401501101, 022364657325, 024130436022, 002167303062, /*  5 */
  62  037570375137, 037210607110, 016272055420, 023011770546, 017143426366, /* 10 */
  63  014753657433, 021657231332, 023553406142, 004236526362, 010365611275, /* 14 */
  64  007117336710, 011051276551, 002362132524, 001011540233, 012162531646, /* 20 */
  65  007056762337, 006631245521, 014164542224, 032633236305, 023342700176, /* 25 */
  66  002433062234, 015257225043, 026762051606, 000742573230, 005366042132, /* 30 */
  67  012126416411, 000520471171, 000725646277, 020116577576, 025765742604, /* 35 */
  68  007633473735, 015674255275, 017555634041, 006503154145, 021576344247, /* 40 */
  69  014577627653, 002707523333, 034146376720, 030060227734, 013765414060, /* 45 */
  70  036072251540, 007255221037, 024364674123, 006200353166, 010126373326, /* 50 */
  71  015664104320, 016401041535, 016215305520, 033115351014, 017411670323  /* 55 */
  72 };
  73
  74 static int i1, i2;
  75
  76 unsigned int
  77 ya_random (void)
  78 {
  79   register int ret = a[i1] + a[i2];
  80   a[i1] = ret;
  81   if (++i1 >= VectorSize) i1 = 0;
  82   if (++i2 >= VectorSize) i2 = 0;
  83   return ret;
  84 }
  85
  86 void
  87 ya_rand_init(unsigned int seed)
  88 {
  89   int i;
  90   if (seed == 0)
  91     {
  92       struct timeval tp;
  93 #ifdef GETTIMEOFDAY_TWO_ARGS
  94       struct timezone tzp;
  95       gettimeofday(&tp, &tzp);
  96 #else
  97       gettimeofday(&tp);
  98 #endif
  99       /* ignore overflow */
 100       seed = (999*tp.tv_sec) + (1001*tp.tv_usec) + (1003 * getpid());
 101     }
 102
 103   a[0] += seed;
 104   for (i = 1; i < VectorSize; i++)
 105     {
 106       seed = a[i-1]*1001 + seed*999;
 107       a[i] += seed;
 108     }
 109
 110   i1 = a[0] % VectorSize;
 111   i2 = (i1 + 024) % VectorSize;
 112 }