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 2.1 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, see <http://www.gnu.org/licenses/>.
  16  */
  17
  18 /*
  19  * Modified by the GLib Team and others 1997-2000.  See the AUTHORS
  20  * file for a list of people on the GLib Team.  See the ChangeLog
  21  * files for a list of changes.  These files are distributed with
  22  * GLib at ftp://ftp.gtk.org/pub/gtk/.
  23  */
  24
  25 /*
  26  * MT safe
  27  */
  28
  29 #include "config.h"
  30
  31 #include "gprimes.h"
  32
  33
  34 static const guint g_primes[] =
  35 {
  36   11,
  37   19,
  38   37,
  39   73,
  40   109,
  41   163,
  42   251,
  43   367,
  44   557,
  45   823,
  46   1237,
  47   1861,
  48   2777,
  49   4177,
  50   6247,
  51   9371,
  52   14057,
  53   21089,
  54   31627,
  55   47431,
  56   71143,
  57   106721,
  58   160073,
  59   240101,
  60   360163,
  61   540217,
  62   810343,
  63   1215497,
  64   1823231,
  65   2734867,
  66   4102283,
  67   6153409,
  68   9230113,
  69   13845163,
  70 };
  71
  72 /**
  73  * g_spaced_primes_closest:
  74  * @num: a #guint
  75  *
  76  * Gets the smallest prime number from a built-in array of primes which
  77  * is larger than @num. This is used within GLib to calculate the optimum
  78  * size of a #GHashTable.
  79  *
  80  * The built-in array of primes ranges from 11 to 13845163 such that
  81  * each prime is approximately 1.5-2 times the previous prime.
  82  *
  83  * Returns: the smallest prime number from a built-in array of primes
  84  *     which is larger than @num
  85  */
  86 guint
  87 g_spaced_primes_closest (guint num)
  88 {
  89   gint i;
  90
  91   for (i = 0; i < G_N_ELEMENTS (g_primes); i++)
  92     if (g_primes[i] > num)
  93       return g_primes[i];
  94
  95   return g_primes[G_N_ELEMENTS (g_primes) - 1];
  96 }