glib/gprimes.c

   1 /* GLIB - Library of useful routines for C programming
   2  * Copyright (C) 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the License, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with this library; if not, write to the
  16  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17  * Boston, MA 02111-1307, USA.
  18  */
  19
  20 /*
  21  * Modified by the GLib Team and others 1997-2000.  See the AUTHORS
  22  * file for a list of people on the GLib Team.  See the ChangeLog
  23  * files for a list of changes.  These files are distributed with
  24  * GLib at ftp://ftp.gtk.org/pub/gtk/.
  25  */
  26
  27 /*
  28  * MT safe
  29  */
  30
  31 #include "config.h"
  32
  33 #include "gprimes.h"
  34
  35
  36 static const guint g_primes[] =
  37 {
  38   11,
  39   19,
  40   37,
  41   73,
  42   109,
  43   163,
  44   251,
  45   367,
  46   557,
  47   823,
  48   1237,
  49   1861,
  50   2777,
  51   4177,
  52   6247,
  53   9371,
  54   14057,
  55   21089,
  56   31627,
  57   47431,
  58   71143,
  59   106721,
  60   160073,
  61   240101,
  62   360163,
  63   540217,
  64   810343,
  65   1215497,
  66   1823231,
  67   2734867,
  68   4102283,
  69   6153409,
  70   9230113,
  71   13845163,
  72 };
  73
  74 /**
  75  * g_spaced_primes_closest:
  76  * @num: a #guint
  77  *
  78  * Gets the smallest prime number from a built-in array of primes which
  79  * is larger than @num. This is used within GLib to calculate the optimum
  80  * size of a #GHashTable.
  81  *
  82  * The built-in array of primes ranges from 11 to 13845163 such that
  83  * each prime is approximately 1.5-2 times the previous prime.
  84  *
  85  * Returns: the smallest prime number from a built-in array of primes
  86  *     which is larger than @num
  87  */
  88 guint
  89 g_spaced_primes_closest (guint num)
  90 {
  91   gint i;
  92
  93   for (i = 0; i < G_N_ELEMENTS (g_primes); i++)
  94     if (g_primes[i] > num)
  95       return g_primes[i];
  96
  97   return g_primes[G_N_ELEMENTS (g_primes) - 1];
  98 }