lib/buffer-lcm.c

   1 /* buffer-lcm.c - compute a good buffer size for dealing with two files
   2
   3    Copyright (C) 2002, 2005, 2009-2010 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation, either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 #include "buffer-lcm.h"
  21
  22 /* Return a buffer size suitable for doing I/O with files whose block
  23    sizes are A and B.  However, never return a value greater than
  24    LCM_MAX.  */
  25
  26 size_t
  27 buffer_lcm (size_t a, size_t b, size_t lcm_max)
  28 {
  29   size_t size;
  30
  31   /* Use reasonable values if buffer sizes are zero.  */
  32   if (!a)
  33     size = b ? b : 8 * 1024;
  34   else
  35     {
  36       if (b)
  37         {
  38           /* Return lcm (A, B) if it is in range; otherwise, fall back
  39              on A.  */
  40
  41           size_t lcm, m, n, q, r;
  42
  43           /* N = gcd (A, B).  */
  44           for (m = a, n = b;  (r = m % n) != 0;  m = n, n = r)
  45             continue;
  46
  47           /* LCM = lcm (A, B), if in range.  */
  48           q = a / n;
  49           lcm = q * b;
  50           if (lcm <= lcm_max && lcm / b == q)
  51             return lcm;
  52         }
  53
  54       size = a;
  55     }
  56
  57   return size <= lcm_max ? size : lcm_max;
  58 }