gl/lib/buffer-lcm.c

   1 /* buffer-lcm.c - compute a good buffer size for dealing with two files
   2
   3    Copyright (C) 2002-2023 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 <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 #include <config.h>
  21 #include "buffer-lcm.h"
  22
  23 /* Return a buffer size suitable for doing I/O with files whose block
  24    sizes are A and B.  However, never return a value greater than
  25    LCM_MAX.  */
  26
  27 size_t
  28 buffer_lcm (size_t a, size_t b, size_t lcm_max)
  29 {
  30   size_t size;
  31
  32   /* Use reasonable values if buffer sizes are zero.  */
  33   if (!a)
  34     size = b ? b : 8 * 1024;
  35   else
  36     {
  37       if (b)
  38         {
  39           /* Return lcm (A, B) if it is in range; otherwise, fall back
  40              on A.  */
  41
  42           size_t lcm, m, n, q, r;
  43
  44           /* N = gcd (A, B).  */
  45           for (m = a, n = b;  (r = m % n) != 0;  m = n, n = r)
  46             continue;
  47
  48           /* LCM = lcm (A, B), if in range.  */
  49           q = a / n;
  50           lcm = q * b;
  51           if (lcm <= lcm_max && lcm / b == q)
  52             return lcm;
  53         }
  54
  55       size = a;
  56     }
  57
  58   return size <= lcm_max ? size : lcm_max;
  59 }