1/* Iterate over previous character in UTF-8 string.
2   Copyright (C) 2002, 2006-2007, 2009-2010 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 <http://www.gnu.org/licenses/>.  */
17
18#include <config.h>
19
20/* Specification.  */
21#include "unistr.h"
22
23const uint8_t *
24u8_prev (ucs4_t *puc, const uint8_t *s, const uint8_t *start)
25{
26  /* Keep in sync with unistr.h and utf8-ucs4.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 CONFIG_UNICODE_SAFETY
37      if ((c_1 ^ 0x80) < 0x40)
38#endif
39        if (s - 1 != start)
40          {
41            uint8_t c_2 = s[-2];
42
43            if (c_2 >= 0xc2 && c_2 < 0xe0)
44              {
45                *puc = ((unsigned int) (c_2 & 0x1f) << 6)
46                       | (unsigned int) (c_1 ^ 0x80);
47                return s - 2;
48              }
49#if CONFIG_UNICODE_SAFETY
50            if ((c_2 ^ 0x80) < 0x40)
51#endif
52              if (s - 2 != start)
53                {
54                  uint8_t c_3 = s[-3];
55
56                  if (c_3 >= 0xe0 && c_3 < 0xf0
57#if CONFIG_UNICODE_SAFETY
58                      && (c_3 >= 0xe1 || c_2 >= 0xa0)
59                      && (c_3 != 0xed || c_2 < 0xa0)
60#endif
61                     )
62                    {
63                      *puc = ((unsigned int) (c_3 & 0x0f) << 12)
64                             | ((unsigned int) (c_2 ^ 0x80) << 6)
65                             | (unsigned int) (c_1 ^ 0x80);
66                      return s - 3;
67                    }
68#if CONFIG_UNICODE_SAFETY
69                  if ((c_3 ^ 0x80) < 0x40)
70#endif
71                    if (s - 3 != start)
72                      {
73                        uint8_t c_4 = s[-4];
74
75                        if (c_4 >= 0xf0 && c_4 < 0xf8
76#if CONFIG_UNICODE_SAFETY
77                            && (c_4 >= 0xf1 || c_3 >= 0x90)
78                            && (c_4 < 0xf4 || (c_4 == 0xf4 && c_3 < 0x90))
79#endif
80                           )
81                          {
82                            *puc = ((unsigned int) (c_4 & 0x07) << 18)
83                                   | ((unsigned int) (c_3 ^ 0x80) << 12)
84                                   | ((unsigned int) (c_2 ^ 0x80) << 6)
85                                   | (unsigned int) (c_1 ^ 0x80);
86                            return s - 4;
87                          }
88                      }
89                }
90          }
91    }
92  return NULL;
93}
94