/* Check UTF-8 string.

   Copyright (C) 2002, 2006-2007, 2009-2023 Free Software Foundation, Inc.

   Written by Bruno Haible <bruno@clisp.org>, 2002.

   This file is free software: you can redistribute it and/or modify

   it under the terms of the GNU Lesser General Public License as

   published by the Free Software Foundation; either version 2.1 of the

   License, or (at your option) any later version.

   This file 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_check (const uint8_t *s, size_t n)

{

  const uint8_t *s_end = s + n;

  while (s < s_end)

    {

      /* Keep in sync with unistr.h and u8-mbtouc-aux.c.  */

      uint8_t c = *s;

      if (c < 0x80)

        {

          s++;

          continue;

        }

      if (c >= 0xc2)

        {

          if (c < 0xe0)

            {

              if (s + 2 <= s_end

                  && (s[1] ^ 0x80) < 0x40)

                {

                  s += 2;

                  continue;

                }

            }

          else if (c < 0xf0)

            {

              if (s + 3 <= s_end

                  && (s[1] ^ 0x80) < 0x40 && (s[2] ^ 0x80) < 0x40

                  && (c >= 0xe1 || s[1] >= 0xa0)

                  && (c != 0xed || s[1] < 0xa0))

                {

                  s += 3;

                  continue;

                }

            }

          else if (c < 0xf8)

            {

              if (s + 4 <= s_end

                  && (s[1] ^ 0x80) < 0x40 && (s[2] ^ 0x80) < 0x40

                  && (s[3] ^ 0x80) < 0x40

                  && (c >= 0xf1 || s[1] >= 0x90)

                  && (c < 0xf4 || (c == 0xf4 && s[1] < 0x90)))

                {

                  s += 4;

                  continue;

                }

            }

        }

      /* invalid or incomplete multibyte character */

      return s;

    }

  return NULL;

}