newlib/libc/stdlib/rand48.c

   1 /*
   2  * Copyright (c) 1993 Martin Birgmeier
   3  * All rights reserved.
   4  *
   5  * You may redistribute unmodified or modified versions of this source
   6  * code provided that the above copyright notice and this and the
   7  * following conditions are retained.
   8  *
   9  * This software is provided ``as is'', and comes with no warranties
  10  * of any kind. I shall in no event be liable for anything that happens
  11  * to anyone/anything when using this software.
  12  */
  13
  14 /*
  15 FUNCTION
  16    <<rand48>>, <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>, <<mrand48>>, <<jrand48>>, <<srand48>>, <<seed48>>, <<lcong48>>---pseudo-random number generators and initialization routines
  17
  18 INDEX
  19        rand48
  20 INDEX
  21        drand48
  22 INDEX
  23        erand48
  24 INDEX
  25        lrand48
  26 INDEX
  27        nrand48
  28 INDEX
  29        mrand48
  30 INDEX
  31        jrand48
  32 INDEX
  33        srand48
  34 INDEX
  35        seed48
  36 INDEX
  37        lcong48
  38
  39 SYNOPSIS
  40        #include <stdlib.h>
  41        double drand48(void);
  42        double erand48(unsigned short <[xseed]>[3]);
  43        long lrand48(void);
  44        long nrand48(unsigned short <[xseed]>[3]);
  45        long mrand48(void);
  46        long jrand48(unsigned short <[xseed]>[3]);
  47        void srand48(long <[seed]>);
  48        unsigned short *seed48(unsigned short <[xseed]>[3]);
  49        void lcong48(unsigned short <[p]>[7]);
  50
  51 DESCRIPTION
  52 The <<rand48>> family of functions generates pseudo-random numbers
  53 using a linear congruential algorithm working on integers 48 bits in size.
  54 The particular formula employed is
  55 r(n+1) = (a * r(n) + c) mod m
  56 where the default values are
  57 for the multiplicand a = 0xfdeece66d = 25214903917 and
  58 the addend c = 0xb = 11. The modulo is always fixed at m = 2 ** 48.
  59 r(n) is called the seed of the random number generator.
  60
  61 For all the six generator routines described next, the first
  62 computational step is to perform a single iteration of the algorithm.
  63
  64 <<drand48>> and <<erand48>>
  65 return values of type double. The full 48 bits of r(n+1) are
  66 loaded into the mantissa of the returned value, with the exponent set
  67 such that the values produced lie in the interval [0.0, 1.0].
  68
  69 <<lrand48>> and <<nrand48>>
  70 return values of type long in the range
  71 [0, 2**31-1]. The high-order (31) bits of
  72 r(n+1) are loaded into the lower bits of the returned value, with
  73 the topmost (sign) bit set to zero.
  74
  75 <<mrand48>> and <<jrand48>>
  76 return values of type long in the range
  77 [-2**31, 2**31-1]. The high-order (32) bits of
  78 r(n+1) are loaded into the returned value.
  79
  80 <<drand48>>, <<lrand48>>, and <<mrand48>>
  81 use an internal buffer to store r(n). For these functions
  82 the initial value of r(0) = 0x1234abcd330e = 20017429951246.
  83
  84 On the other hand, <<erand48>>, <<nrand48>>, and <<jrand48>>
  85 use a user-supplied buffer to store the seed r(n),
  86 which consists of an array of 3 shorts, where the zeroth member
  87 holds the least significant bits.
  88
  89 All functions share the same multiplicand and addend.
  90
  91 <<srand48>> is used to initialize the internal buffer r(n) of
  92 <<drand48>>, <<lrand48>>, and <<mrand48>>
  93 such that the 32 bits of the seed value are copied into the upper 32 bits
  94 of r(n), with the lower 16 bits of r(n) arbitrarily being set to 0x330e.
  95 Additionally, the constant multiplicand and addend of the algorithm are
  96 reset to the default values given above.
  97
  98 <<seed48>> also initializes the internal buffer r(n) of
  99 <<drand48>>, <<lrand48>>, and <<mrand48>>,
 100 but here all 48 bits of the seed can be specified in an array of 3 shorts,
 101 where the zeroth member specifies the lowest bits. Again,
 102 the constant multiplicand and addend of the algorithm are
 103 reset to the default values given above.
 104 <<seed48>> returns a pointer to an array of 3 shorts which contains
 105 the old seed.
 106 This array is statically allocated, thus its contents are lost after
 107 each new call to <<seed48>>.
 108
 109 Finally, <<lcong48>> allows full control over the multiplicand and
 110 addend used in <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>,
 111 <<mrand48>>, and <<jrand48>>,
 112 and the seed used in <<drand48>>, <<lrand48>>, and <<mrand48>>.
 113 An array of 7 shorts is passed as parameter; the first three shorts are
 114 used to initialize the seed; the second three are used to initialize the
 115 multiplicand; and the last short is used to initialize the addend.
 116 It is thus not possible to use values greater than 0xffff as the addend.
 117
 118 Note that all three methods of seeding the random number generator
 119 always also set the multiplicand and addend for any of the six
 120 generator calls.
 121
 122 For a more powerful random number generator, see <<random>>.
 123
 124 PORTABILITY
 125 SUS requires these functions.
 126
 127 No supporting OS subroutines are required.
 128 */
 129
 130 #include "rand48.h"
 131
 132 void
 133 __dorand48 (struct _reent *r,
 134        unsigned short xseed[3])
 135 {
 136   unsigned long accu;
 137   unsigned short temp[2];
 138
 139   _REENT_CHECK_RAND48(r);
 140   accu = (unsigned long) __rand48_mult[0] * (unsigned long) xseed[0] +
 141     (unsigned long) __rand48_add;
 142   temp[0] = (unsigned short) accu;     /* lower 16 bits */
 143   accu >>= sizeof(unsigned short) * 8;
 144   accu += (unsigned long) __rand48_mult[0] * (unsigned long) xseed[1] +
 145     (unsigned long) __rand48_mult[1] * (unsigned long) xseed[0];
 146   temp[1] = (unsigned short) accu;     /* middle 16 bits */
 147   accu >>= sizeof(unsigned short) * 8;
 148   accu += __rand48_mult[0] * xseed[2] + __rand48_mult[1] * xseed[1] + __rand48_mult[2] * xseed[0];
 149   xseed[0] = temp[0];
 150   xseed[1] = temp[1];
 151   xseed[2] = (unsigned short) accu;
 152 }