compat/nrand48.c

   1 /*****************************************************************************
   2  * nrand48.c: POSIX erand48(), jrand48() and nrand48() replacements
   3  *****************************************************************************
   4  * Copyright © 2010 Rémi Denis-Courmont
   5  *
   6  * This program is free software; you can redistribute it and/or modify it
   7  * under the terms of the GNU Lesser General Public License as published by
   8  * the Free Software Foundation; either version 2.1 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public License
  17  * along with this program; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  19  *****************************************************************************/
  20
  21 #ifdef HAVE_CONFIG_H
  22 # include <config.h>
  23 #endif
  24
  25 #include <inttypes.h>
  26
  27 static uint64_t iterate48 (unsigned short subi[3])
  28 {
  29     const uint64_t a = UINT64_C(0x5DEECE66D);
  30     const unsigned c = 13;
  31     const uint64_t mask = UINT64_C(0xFFFFFFFFFFFF); // 48 bits
  32
  33     uint64_t x = ((uint64_t)subi[0] << 32)
  34                | ((uint32_t)subi[1] << 16)
  35                | subi[2];
  36
  37     x *= a;
  38     x += c;
  39     x &= mask;
  40
  41     subi[0] = (x >> 32) & 0xFFFF;
  42     subi[1] = (x >> 16) & 0xFFFF;
  43     subi[2] = (x >>  0) & 0XFFFF;
  44
  45     return x;
  46 }
  47
  48 double erand48 (unsigned short subi[3])
  49 {
  50     uint64_t r = iterate48 (subi);
  51     return ((double)r) / 281474976710655.;
  52 }
  53
  54 long jrand48 (unsigned short subi[3])
  55 {
  56     return ((int64_t)iterate48 (subi)) >> 16;
  57 }
  58
  59 long nrand48 (unsigned short subi[3])
  60 {
  61     return iterate48 (subi) >> 17;
  62 }