1  /* Iterate over previous character in UTF-8 string.
       2     Copyright (C) 2002, 2006-2007, 2009-2023 Free Software Foundation, Inc.
       3     Written by Bruno Haible <bruno@clisp.org>, 2002.
       4  
       5     This file is free software: you can redistribute it and/or modify
       6     it under the terms of the GNU Lesser General Public License as
       7     published by the Free Software Foundation; either version 2.1 of the
       8     License, or (at your option) any later version.
       9  
      10     This file 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 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 uint8_t *
      24  u8_prev (ucs4_t *puc, const uint8_t *s, const uint8_t *start)
      25  {
      26    /* Keep in sync with unistr.h and u8-mbtouc-aux.c.  */
      27    if (s != start)
      28      {
      29        uint8_t c_1 = s[-1];
      30  
      31        if (c_1 < 0x80)
      32          {
      33            *puc = c_1;
      34            return s - 1;
      35          }
      36        if ((c_1 ^ 0x80) < 0x40)
      37          if (s - 1 != start)
      38            {
      39              uint8_t c_2 = s[-2];
      40  
      41              if (c_2 >= 0xc2 && c_2 < 0xe0)
      42                {
      43                  *puc = ((unsigned int) (c_2 & 0x1f) << 6)
      44                         | (unsigned int) (c_1 ^ 0x80);
      45                  return s - 2;
      46                }
      47              if ((c_2 ^ 0x80) < 0x40)
      48                if (s - 2 != start)
      49                  {
      50                    uint8_t c_3 = s[-3];
      51  
      52                    if (c_3 >= 0xe0 && c_3 < 0xf0
      53                        && (c_3 >= 0xe1 || c_2 >= 0xa0)
      54                        && (c_3 != 0xed || c_2 < 0xa0))
      55                      {
      56                        *puc = ((unsigned int) (c_3 & 0x0f) << 12)
      57                               | ((unsigned int) (c_2 ^ 0x80) << 6)
      58                               | (unsigned int) (c_1 ^ 0x80);
      59                        return s - 3;
      60                      }
      61                    if ((c_3 ^ 0x80) < 0x40)
      62                      if (s - 3 != start)
      63                        {
      64                          uint8_t c_4 = s[-4];
      65  
      66                          if (c_4 >= 0xf0 && c_4 <= 0xf4
      67                              && (c_4 >= 0xf1 || c_3 >= 0x90)
      68                              && (c_4 < 0xf4 || (/* c_4 == 0xf4 && */ c_3 < 0x90)))
      69                            {
      70                              *puc = ((unsigned int) (c_4 & 0x07) << 18)
      71                                     | ((unsigned int) (c_3 ^ 0x80) << 12)
      72                                     | ((unsigned int) (c_2 ^ 0x80) << 6)
      73                                     | (unsigned int) (c_1 ^ 0x80);
      74                              return s - 4;
      75                            }
      76                        }
      77                  }
      78            }
      79      }
      80    return NULL;
      81  }