lib/unistr/u16-prev.c

   1 /* Iterate over previous character in UTF-16 string.
   2    Copyright (C) 2002, 2006-2007, 2009-2020 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2002.
   4
   5    This program is free software: you can redistribute it and/or modify it
   6    under the terms of the GNU Lesser General Public License as published
   7    by 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #include <config.h>
  19
  20 /* Specification.  */
  21 #include "unistr.h"
  22
  23 const uint16_t *
  24 u16_prev (ucs4_t *puc, const uint16_t *s, const uint16_t *start)
  25 {
  26   /* Keep in sync with unistr.h and utf16-ucs4.c.  */
  27   if (s != start)
  28     {
  29       uint16_t c_1 = s[-1];
  30
  31       if (c_1 < 0xd800 || c_1 >= 0xe000)
  32         {
  33           *puc = c_1;
  34           return s - 1;
  35         }
  36       if (c_1 >= 0xdc00)
  37         if (s - 1 != start)
  38           {
  39             uint16_t c_2 = s[-2];
  40
  41             if (c_2 >= 0xd800 && c_2 < 0xdc00)
  42               {
  43                 *puc = 0x10000 + ((c_2 - 0xd800) << 10) + (c_1 - 0xdc00);
  44                 return s - 2;
  45               }
  46           }
  47     }
  48   return NULL;
  49 }