/* Iterate over previous character in UTF-8 string.
   Copyright (C) 2002, 2006-2007, 2009-2018 Free Software Foundation, Inc.
   Written by Bruno Haible <bruno@clisp.org>, 2002.

   This program is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * the GNU General Public License as published by the Free
       Software Foundation; either version 2 of the License, or (at your
       option) any later version.

   or both in parallel, as here.
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include "unistr.h"

const uint8_t *
u8_prev (ucs4_t *puc, const uint8_t *s, const uint8_t *start)
{
  /* Keep in sync with unistr.h and u8-mbtouc-aux.c.  */
  if (s != start)
    {
      uint8_t c_1 = s[-1];

      if (c_1 < 0x80)
        {
          *puc = c_1;
          return s - 1;
        }
      if ((c_1 ^ 0x80) < 0x40)
        if (s - 1 != start)
          {
            uint8_t c_2 = s[-2];

            if (c_2 >= 0xc2 && c_2 < 0xe0)
              {
                *puc = ((unsigned int) (c_2 & 0x1f) << 6)
                       | (unsigned int) (c_1 ^ 0x80);
                return s - 2;
              }
            if ((c_2 ^ 0x80) < 0x40)
              if (s - 2 != start)
                {
                  uint8_t c_3 = s[-3];

                  if (c_3 >= 0xe0 && c_3 < 0xf0
                      && (c_3 >= 0xe1 || c_2 >= 0xa0)
                      && (c_3 != 0xed || c_2 < 0xa0))
                    {
                      *puc = ((unsigned int) (c_3 & 0x0f) << 12)
                             | ((unsigned int) (c_2 ^ 0x80) << 6)
                             | (unsigned int) (c_1 ^ 0x80);
                      return s - 3;
                    }
                  if ((c_3 ^ 0x80) < 0x40)
                    if (s - 3 != start)
                      {
                        uint8_t c_4 = s[-4];

                        if (c_4 >= 0xf0 && c_4 < 0xf8
                            && (c_4 >= 0xf1 || c_3 >= 0x90)
                            && (c_4 < 0xf4 || (c_4 == 0xf4 && c_3 < 0x90)))
                          {
                            *puc = ((unsigned int) (c_4 & 0x07) << 18)
                                   | ((unsigned int) (c_3 ^ 0x80) << 12)
                                   | ((unsigned int) (c_2 ^ 0x80) << 6)
                                   | (unsigned int) (c_1 ^ 0x80);
                            return s - 4;
                          }
                      }
                }
          }
    }
  return NULL;
}
