/* gutf8.c - Operations on UTF-8 strings.
 *
 * Copyright (C) 1999 Tom Tromey
 * Copyright (C) 2000, 2015-2022 Red Hat, Inc.
 * Copyright (C) 2022-2023 David Rheinsberg
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 *
 * This library 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 library 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 library; if not, see <http://www.gnu.org/licenses/>.
 */

#include "config.h"

#include <stdlib.h>
#ifdef HAVE_CODESET
#include <langinfo.h>
#endif
#include <string.h>
#include <stdbool.h>

#ifdef G_PLATFORM_WIN32
#include <stdio.h>
#include <windows.h>
#endif

#include "gconvert.h"
#include "ghash.h"
#include "gstrfuncs.h"
#include "gtestutils.h"
#include "gtypes.h"
#include "gthread.h"
#include "glibintl.h"
#include "gvalgrind.h"
#include "gunicodeprivate.h"

#define UTF8_COMPUTE(Char, Mask, Len)					      \
  if (Char < 128)							      \
    {									      \
      Len = 1;								      \
      Mask = 0x7f;							      \
    }									      \
  else if ((Char & 0xe0) == 0xc0)					      \
    {									      \
      Len = 2;								      \
      Mask = 0x1f;							      \
    }									      \
  else if ((Char & 0xf0) == 0xe0)					      \
    {									      \
      Len = 3;								      \
      Mask = 0x0f;							      \
    }									      \
  else if ((Char & 0xf8) == 0xf0)					      \
    {									      \
      Len = 4;								      \
      Mask = 0x07;							      \
    }									      \
  else if ((Char & 0xfc) == 0xf8)					      \
    {									      \
      Len = 5;								      \
      Mask = 0x03;							      \
    }									      \
  else if ((Char & 0xfe) == 0xfc)					      \
    {									      \
      Len = 6;								      \
      Mask = 0x01;							      \
    }									      \
  else									      \
    Len = -1;

#define UTF8_LENGTH(Char)              \
  ((Char) < 0x80 ? 1 :                 \
   ((Char) < 0x800 ? 2 :               \
    ((Char) < 0x10000 ? 3 :            \
     ((Char) < 0x200000 ? 4 :          \
      ((Char) < 0x4000000 ? 5 : 6)))))
   

#define UTF8_GET(Result, Chars, Count, Mask, Len)			      \
  (Result) = (Chars)[0] & (Mask);					      \
  for ((Count) = 1; (Count) < (Len); ++(Count))				      \
    {									      \
      if (((Chars)[(Count)] & 0xc0) != 0x80)				      \
	{								      \
	  (Result) = -1;						      \
	  break;							      \
	}								      \
      (Result) <<= 6;							      \
      (Result) |= ((Chars)[(Count)] & 0x3f);				      \
    }
    
/*
 * Check whether a Unicode (5.2) char is in a valid range.
 *
 * The first check comes from the Unicode guarantee to never encode
 * a point above 0x0010ffff, since UTF-16 couldn't represent it.
 * 
 * The second check covers surrogate pairs (category Cs).
 *
 * @param Char the character
 */
#define UNICODE_VALID(Char)                   \
    ((Char) < 0x110000 &&                     \
     (((Char) & 0xFFFFF800) != 0xD800))

    
static const gchar utf8_skip_data[256] = {
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,
  3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,5,5,5,5,6,6,1,1
};

const gchar * const g_utf8_skip = utf8_skip_data;

/**
 * g_utf8_find_prev_char:
 * @str: pointer to the beginning of a UTF-8 encoded string
 * @p: pointer to some position within @str
 * 
 * Given a position @p with a UTF-8 encoded string @str, find the start
 * of the previous UTF-8 character starting before @p. Returns `NULL` if no
 * UTF-8 characters are present in @str before @p.
 *
 * @p does not have to be at the beginning of a UTF-8 character. No check
 * is made to see if the character found is actually valid other than
 * it starts with an appropriate byte.
 *
 * Returns: (transfer none) (nullable): a pointer to the found character
 */
gchar *
g_utf8_find_prev_char (const gchar *str,
		       const gchar *p)
{
  while (p > str)
    {
      --p;
      if ((*p & 0xc0) != 0x80)
	return (gchar *)p;
    }
  return NULL;
}

/**
 * g_utf8_find_next_char:
 * @p: a pointer to a position within a UTF-8 encoded string
 * @end: (nullable): a pointer to the byte following the end of the string,
 *     or `NULL` to indicate that the string is nul-terminated
 *
 * Finds the start of the next UTF-8 character in the string after @p.
 *
 * @p does not have to be at the beginning of a UTF-8 character. No check
 * is made to see if the character found is actually valid other than
 * it starts with an appropriate byte.
 * 
 * If @end is `NULL`, the return value will never be `NULL`: if the end of the
 * string is reached, a pointer to the terminating nul byte is returned. If
 * @end is non-`NULL`, the return value will be `NULL` if the end of the string
 * is reached.
 *
 * Returns: (transfer none) (nullable): a pointer to the found character or `NULL` if @end is
 *    set and is reached
 */
gchar *
g_utf8_find_next_char (const gchar *p,
		       const gchar *end)
{
  if (end)
    {
      for (++p; p < end && (*p & 0xc0) == 0x80; ++p)
        ;
      return (p >= end) ? NULL : (gchar *)p;
    }
  else
    {
      for (++p; (*p & 0xc0) == 0x80; ++p)
        ;
      return (gchar *)p;
    }
}

/**
 * g_utf8_prev_char:
 * @p: a pointer to a position within a UTF-8 encoded string
 *
 * Finds the previous UTF-8 character in the string before @p.
 *
 * @p does not have to be at the beginning of a UTF-8 character. No check
 * is made to see if the character found is actually valid other than
 * it starts with an appropriate byte. If @p might be the first
 * character of the string, you must use [func@GLib.utf8_find_prev_char]
 * instead.
 * 
 * Returns: (transfer none) (not nullable): a pointer to the found character
 */
gchar *
g_utf8_prev_char (const gchar *p)
{
  while (TRUE)
    {
      p--;
      if ((*p & 0xc0) != 0x80)
	return (gchar *)p;
    }
}
 
/**
 * g_utf8_strlen:
 * @p: pointer to the start of a UTF-8 encoded string
 * @max: the maximum number of bytes to examine. If @max
 *   is less than 0, then the string is assumed to be
 *   nul-terminated. If @max is 0, @p will not be examined and
 *   may be `NULL`. If @max is greater than 0, up to @max
 *   bytes are examined
 *
 * Computes the length of the string in characters, not including
 * the terminating nul character. If the @max’th byte falls in the
 * middle of a character, the last (partial) character is not counted.
 *
 * Returns: the length of the string in characters
 */
glong
g_utf8_strlen (const gchar *p,
               gssize       max)
{
  glong len = 0;
  const gchar *start = p;
  g_return_val_if_fail (p != NULL || max == 0, 0);

  if (max < 0)
    {
      while (*p)
        {
          p = g_utf8_next_char (p);
          ++len;
        }
    }
  else
    {
      if (max == 0 || !*p)
        return 0;

      p = g_utf8_next_char (p);

      while (p - start < max && *p)
        {
          ++len;
          p = g_utf8_next_char (p);
        }

      /* only do the last len increment if we got a complete
       * char (don't count partial chars)
       */
      if (p - start <= max)
        ++len;
    }

  return len;
}

/**
 * g_utf8_substring:
 * @str: a UTF-8 encoded string
 * @start_pos: a character offset within @str
 * @end_pos: another character offset within @str,
 *   or `-1` to indicate the end of the string
 *
 * Copies a substring out of a UTF-8 encoded string.
 * The substring will contain @end_pos - @start_pos characters.
 *
 * Since GLib 2.72, `-1` can be passed to @end_pos to indicate the
 * end of the string.
 *
 * Returns: (transfer full): a newly allocated copy of the requested
 *   substring. Free with [func@GLib.free] when no longer needed.
 *
 * Since: 2.30
 */
gchar *
g_utf8_substring (const gchar *str,
                  glong        start_pos,
                  glong        end_pos)
{
  gchar *start, *end, *out;

  g_return_val_if_fail (end_pos >= start_pos || end_pos == -1, NULL);

  start = g_utf8_offset_to_pointer (str, start_pos);

  if (end_pos == -1)
    {
      glong length = g_utf8_strlen (start, -1);
      end = g_utf8_offset_to_pointer (start, length);
    }
  else
    {
      end = g_utf8_offset_to_pointer (start, end_pos - start_pos);
    }

  out = g_malloc (end - start + 1);
  memcpy (out, start, end - start);
  out[end - start] = 0;

  return out;
}

/**
 * g_utf8_get_char:
 * @p: a pointer to Unicode character encoded as UTF-8
 * 
 * Converts a sequence of bytes encoded as UTF-8 to a Unicode character.
 *
 * If @p does not point to a valid UTF-8 encoded character, results
 * are undefined. If you are not sure that the bytes are complete
 * valid Unicode characters, you should use [func@GLib.utf8_get_char_validated]
 * instead.
 * 
 * Returns: the resulting character
 */
gunichar
g_utf8_get_char (const gchar *p)
{
  int i, mask = 0, len;
  gunichar result;
  unsigned char c = (unsigned char) *p;

  UTF8_COMPUTE (c, mask, len);
  if (len == -1)
    return (gunichar)-1;
  UTF8_GET (result, p, i, mask, len);

  return result;
}

/**
 * g_utf8_offset_to_pointer:
 * @str: a UTF-8 encoded string
 * @offset: a character offset within @str
 *
 * Converts from an integer character offset to a pointer to a position
 * within the string.
 *
 * Since 2.10, this function allows to pass a negative @offset to
 * step backwards. It is usually worth stepping backwards from the end
 * instead of forwards if @offset is in the last fourth of the string,
 * since moving forward is about 3 times faster than moving backward.
 *
 * Note that this function doesn’t abort when reaching the end of @str.
 * Therefore you should be sure that @offset is within string boundaries
 * before calling that function. Call [func@GLib.utf8_strlen] when unsure.
 * This limitation exists as this function is called frequently during
 * text rendering and therefore has to be as fast as possible.
 *
 * Returns: (transfer none): the resulting pointer
 */
gchar *
g_utf8_offset_to_pointer  (const gchar *str,
			   glong        offset)
{
  const gchar *s = str;

  if (offset > 0) 
    while (offset--)
      s = g_utf8_next_char (s);
  else
    {
      const char *s1;

      /* This nice technique for fast backwards stepping 
       * through a UTF-8 string was dubbed "stutter stepping" 
       * by its inventor, Larry Ewing.
       */
      while (offset)
	{
	  s1 = s;
	  s += offset;
	  while ((*s & 0xc0) == 0x80)
	    s--;

	  offset += g_utf8_pointer_to_offset (s, s1);
	}
    }

  return (gchar *)s;
}

/**
 * g_utf8_pointer_to_offset:
 * @str: a UTF-8 encoded string
 * @pos: a pointer to a position within @str
 * 
 * Converts from a pointer to position within a string to an integer
 * character offset.
 *
 * Since 2.10, this function allows @pos to be before @str, and returns
 * a negative offset in this case.
 * 
 * Returns: the resulting character offset
 */
glong    
g_utf8_pointer_to_offset (const gchar *str,
			  const gchar *pos)
{
  const gchar *s = str;
  glong offset = 0;    

  if (pos < str) 
    offset = - g_utf8_pointer_to_offset (pos, str);
  else
    while (s < pos)
      {
	s = g_utf8_next_char (s);
	offset++;
      }
  
  return offset;
}


/**
 * g_utf8_strncpy:
 * @dest: (transfer none): buffer to fill with characters from @src
 * @src: UTF-8 encoded string
 * @n: character count
 * 
 * Like the standard C [`strncpy()`](man:strncpy) function, but copies a given
 * number of characters instead of a given number of bytes.
 *
 * The @src string must be valid UTF-8 encoded text. (Use
 * [func@GLib.utf8_validate] on all text before trying to use UTF-8 utility
 * functions with it.)
 * 
 * Note you must ensure @dest is at least 4 * @n + 1 to fit the
 * largest possible UTF-8 characters
 *
 * Returns: (transfer none): @dest
 */
gchar *
g_utf8_strncpy (gchar       *dest,
		const gchar *src,
		gsize        n)
{
  const gchar *s = src;
  while (n && *s)
    {
      s = g_utf8_next_char(s);
      n--;
    }
  strncpy(dest, src, s - src);
  dest[s - src] = 0;
  return dest;
}

/**
 * g_utf8_truncate_middle:
 * @string: (transfer none): a nul-terminated UTF-8 encoded string
 * @truncate_length: the new size of @string, in characters, including the ellipsis character
 *
 * Cuts off the middle of the string, preserving half of @truncate_length
 * characters at the beginning and half at the end.
 * 
 * If @string is already short enough, this returns a copy of @string.
 * If @truncate_length is `0`, an empty string is returned.
 *
 * Returns: (transfer full): a newly-allocated copy of @string ellipsized in the middle
 *
 * Since: 2.78
 */
gchar *
g_utf8_truncate_middle (const gchar *string,
                        gsize        truncate_length)
{
  const gchar *ellipsis = "…";
  const gsize ellipsis_bytes = strlen (ellipsis);

  gsize length;
  gsize left_substring_length;
  gchar *left_substring_end;
  gchar *right_substring_begin;
  gchar *right_substring_end;
  gsize left_bytes;
  gsize right_bytes;
  gchar *result;

  g_return_val_if_fail (string != NULL, NULL);

  length = g_utf8_strlen (string, -1);
  /* Current string already smaller than requested length */
  if (length <= truncate_length)
    return g_strdup (string);
  if (truncate_length == 0)
    return g_strdup ("");

  /* Find substrings to keep, ignore ellipsis character for that */
  truncate_length -= 1;

  left_substring_length = truncate_length / 2;

  left_substring_end = g_utf8_offset_to_pointer (string, left_substring_length);
  right_substring_begin = g_utf8_offset_to_pointer (left_substring_end,
                                                    length - truncate_length);
  right_substring_end = g_utf8_offset_to_pointer (right_substring_begin,
                                                  truncate_length - left_substring_length);

  g_assert (*right_substring_end == '\0');

  left_bytes = left_substring_end - string;
  right_bytes = right_substring_end - right_substring_begin;

  result = g_malloc (left_bytes + ellipsis_bytes + right_bytes + 1);

  strncpy (result, string, left_bytes);
  memcpy (result + left_bytes, ellipsis, ellipsis_bytes);
  strncpy (result + left_bytes + ellipsis_bytes, right_substring_begin, right_bytes);
  result[left_bytes + ellipsis_bytes + right_bytes] = '\0';

  return result;
}

/* unicode_strchr */

/**
 * g_unichar_to_utf8:
 * @c: a Unicode character code
 * @outbuf: (out caller-allocates) (optional): output buffer, must have at
 *   least 6 bytes of space. If `NULL`, the length will be computed and
 *   returned and nothing will be written to @outbuf.
 * 
 * Converts a single character to UTF-8.
 * 
 * Returns: number of bytes written
 */
int
g_unichar_to_utf8 (gunichar c,
		   gchar   *outbuf)
{
  /* If this gets modified, also update the copy in g_string_insert_unichar() */
  guint len = 0;    
  int first;
  int i;

  if (c < 0x80)
    {
      first = 0;
      len = 1;
    }
  else if (c < 0x800)
    {
      first = 0xc0;
      len = 2;
    }
  else if (c < 0x10000)
    {
      first = 0xe0;
      len = 3;
    }
   else if (c < 0x200000)
    {
      first = 0xf0;
      len = 4;
    }
  else if (c < 0x4000000)
    {
      first = 0xf8;
      len = 5;
    }
  else
    {
      first = 0xfc;
      len = 6;
    }

  if (outbuf)
    {
      for (i = len - 1; i > 0; --i)
	{
	  outbuf[i] = (c & 0x3f) | 0x80;
	  c >>= 6;
	}
      outbuf[0] = c | first;
    }

  return len;
}

/**
 * g_utf8_strchr:
 * @p: a nul-terminated UTF-8 encoded string
 * @len: the maximum length of @p
 * @c: a Unicode character
 * 
 * Finds the leftmost occurrence of the given Unicode character
 * in a UTF-8 encoded string, while limiting the search to @len bytes.
 * 
 * If @len is `-1`, allow unbounded search.
 *
 * Returns: (transfer none) (nullable): `NULL` if the string does not contain
 *   the character, otherwise, a pointer to the start of the leftmost occurrence
 *   of the character in the string.
 */
gchar *
g_utf8_strchr (const char *p,
	       gssize      len,
	       gunichar    c)
{
  gchar ch[10];

  gint charlen = g_unichar_to_utf8 (c, ch);
  ch[charlen] = '\0';
  
  return g_strstr_len (p, len, ch);
}


/**
 * g_utf8_strrchr:
 * @p: a nul-terminated UTF-8 encoded string
 * @len: the maximum length of @p
 * @c: a Unicode character
 * 
 * Find the rightmost occurrence of the given Unicode character
 * in a UTF-8 encoded string, while limiting the search to @len bytes.
 * 
 * If @len is `-1`, allow unbounded search.
 *
 * Returns: (transfer none) (nullable): `NULL` if the string does not contain
 *   the character, otherwise, a pointer to the start of the rightmost
 *   occurrence of the character in the string.
 */
gchar *
g_utf8_strrchr (const char *p,
		gssize      len,
		gunichar    c)
{
  gchar ch[10];

  gint charlen = g_unichar_to_utf8 (c, ch);
  ch[charlen] = '\0';
  
  return g_strrstr_len (p, len, ch);
}


/* Like g_utf8_get_char, but take a maximum length
 * and return (gunichar)-2 on incomplete trailing character;
 * also check for malformed or overlong sequences
 * and return (gunichar)-1 in this case.
 */
static inline gunichar
g_utf8_get_char_extended (const  gchar *p,
			  gssize max_len)
{
  gsize i, len;
  gunichar min_code;
  gunichar wc = (guchar) *p;
  const gunichar partial_sequence = (gunichar) -2;
  const gunichar malformed_sequence = (gunichar) -1;

  if (wc < 0x80)
    {
      return wc;
    }
  else if (G_UNLIKELY (wc < 0xc0))
    {
      return malformed_sequence;
    }
  else if (wc < 0xe0)
    {
      len = 2;
      wc &= 0x1f;
      min_code = 1 << 7;
    }
  else if (wc < 0xf0)
    {
      len = 3;
      wc &= 0x0f;
      min_code = 1 << 11;
    }
  else if (wc < 0xf8)
    {
      len = 4;
      wc &= 0x07;
      min_code = 1 << 16;
    }
  else if (wc < 0xfc)
    {
      len = 5;
      wc &= 0x03;
      min_code = 1 << 21;
    }
  else if (wc < 0xfe)
    {
      len = 6;
      wc &= 0x01;
      min_code = 1 << 26;
    }
  else
    {
      return malformed_sequence;
    }

  if (G_UNLIKELY (max_len >= 0 && len > (gsize) max_len))
    {
      for (i = 1; i < (gsize) max_len; i++)
	{
	  if ((((guchar *)p)[i] & 0xc0) != 0x80)
	    return malformed_sequence;
	}
      return partial_sequence;
    }

  for (i = 1; i < len; ++i)
    {
      gunichar ch = ((guchar *)p)[i];

      if (G_UNLIKELY ((ch & 0xc0) != 0x80))
	{
	  if (ch)
	    return malformed_sequence;
	  else
	    return partial_sequence;
	}

      wc <<= 6;
      wc |= (ch & 0x3f);
    }

  if (G_UNLIKELY (wc < min_code))
    return malformed_sequence;

  return wc;
}

/**
 * g_utf8_get_char_validated:
 * @p: a pointer to Unicode character encoded as UTF-8
 * @max_len: the maximum number of bytes to read, or `-1` if @p is nul-terminated
 *
 * Convert a sequence of bytes encoded as UTF-8 to a Unicode character.
 *
 * This function checks for incomplete characters, for invalid characters
 * such as characters that are out of the range of Unicode, and for
 * overlong encodings of valid characters.
 *
 * Note that [func@GLib.utf8_get_char_validated] returns `(gunichar)-2` if
 * @max_len is positive and any of the bytes in the first UTF-8 character
 * sequence are nul.
 * 
 * Returns: the resulting character. If @p points to a partial
 *   sequence at the end of a string that could begin a valid
 *   character (or if @max_len is zero), returns `(gunichar)-2`;
 *   otherwise, if @p does not point to a valid UTF-8 encoded
 *   Unicode character, returns `(gunichar)-1`.
 */
gunichar
g_utf8_get_char_validated (const gchar *p,
			   gssize       max_len)
{
  gunichar result;

  if (max_len == 0)
    return (gunichar)-2;

  result = g_utf8_get_char_extended (p, max_len);

  /* Disallow codepoint U+0000 as it’s a nul byte,
   * and all string handling in GLib is nul-terminated */
  if (result == 0 && max_len > 0)
    return (gunichar) -2;

  if (result & 0x80000000)
    return result;
  else if (!UNICODE_VALID (result))
    return (gunichar)-1;
  else
    return result;
}

#define CONT_BYTE_FAST(p) ((guchar)*p++ & 0x3f)

/**
 * g_utf8_to_ucs4_fast:
 * @str: a UTF-8 encoded string
 * @len: the maximum length of @str to use, in bytes. If @len is negative,
 *   then the string is nul-terminated.
 * @items_written: (out) (optional): location to store the
 *   number of characters in the result, or `NULL`.
 *
 * Convert a string from UTF-8 to a 32-bit fixed width
 * representation as UCS-4, assuming valid UTF-8 input.
 *
 * This function is roughly twice as fast as [func@GLib.utf8_to_ucs4]
 * but does no error checking on the input. A trailing nul character (U+0000)
 * will be added to the string after the converted text.
 * 
 * Returns: (transfer full): a pointer to a newly allocated UCS-4 string.
 *   This value must be freed with [func@GLib.free].
 */
gunichar *
g_utf8_to_ucs4_fast (const gchar *str,
		     glong        len,              
		     glong       *items_written)    
{
  gunichar *result;
  gint n_chars, i;
  const gchar *p;

  g_return_val_if_fail (str != NULL, NULL);

  p = str;
  n_chars = 0;
  if (len < 0)
    {
      while (*p)
	{
	  p = g_utf8_next_char (p);
	  ++n_chars;
	}
    }
  else
    {
      while (p < str + len && *p)
	{
	  p = g_utf8_next_char (p);
	  ++n_chars;
	}
    }
  
  result = g_new (gunichar, n_chars + 1);
  
  p = str;
  for (i=0; i < n_chars; i++)
    {
      guchar first = (guchar)*p++;
      gunichar wc;

      if (first < 0xc0)
	{
          /* We really hope first < 0x80, but we don't want to test an
           * extra branch for invalid input, which this function
           * does not care about. Handling unexpected continuation bytes
           * here will do the least damage. */
	  wc = first;
	}
      else
	{
          gunichar c1 = CONT_BYTE_FAST(p);
          if (first < 0xe0)
            {
              wc = ((first & 0x1f) << 6) | c1;
            }
          else
            {
              gunichar c2 = CONT_BYTE_FAST(p);
              if (first < 0xf0)
                {
                  wc = ((first & 0x0f) << 12) | (c1 << 6) | c2;
                }
              else
                {
                  gunichar c3 = CONT_BYTE_FAST(p);
                  wc = ((first & 0x07) << 18) | (c1 << 12) | (c2 << 6) | c3;
                  if (G_UNLIKELY (first >= 0xf8))
                    {
                      /* This can't be valid UTF-8, but g_utf8_next_char()
                       * and company allow out-of-range sequences */
                      gunichar mask = 1 << 20;
                      while ((wc & mask) != 0)
                        {
                          wc <<= 6;
                          wc |= CONT_BYTE_FAST(p);
                          mask <<= 5;
                        }
                      wc &= mask - 1;
                    }
                }
            }
	}
      result[i] = wc;
    }
  result[i] = 0;

  if (items_written)
    *items_written = i;

  return result;
}

static gpointer
try_malloc_n (gsize n_blocks, gsize n_block_bytes, GError **error)
{
    gpointer ptr = g_try_malloc_n (n_blocks, n_block_bytes);
    if (ptr == NULL)
      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_NO_MEMORY,
                           _("Failed to allocate memory"));
    return ptr;
}

/**
 * g_utf8_to_ucs4:
 * @str: a UTF-8 encoded string
 * @len: the maximum length of @str to use, in bytes. If @len is negative,
 *   then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of
  *  bytes read, or `NULL`.
 *   If `NULL`, then %G_CONVERT_ERROR_PARTIAL_INPUT will be
 *   returned in case @str contains a trailing partial
 *   character. If an error occurs then the index of the
 *   invalid input is stored here.
 * @items_written: (out) (optional): location to store number
 *   of characters written or `NULL`. The value here stored does not include
 *   the trailing nul character.
 * @error: location to store the error occurring, or `NULL` to ignore
 *   errors. Any of the errors in [error@GLib.ConvertError] other than
 *   [error@GLib.ConvertError.NO_CONVERSION] may occur.
 *
 * Convert a string from UTF-8 to a 32-bit fixed width representation as UCS-4.
 *
 * A trailing nul character (U+0000) will be added to the string after the
 * converted text.
 * 
 * Returns: (transfer full): a pointer to a newly allocated UCS-4 string.
 *   This value must be freed with [func@GLib.free].
 */
gunichar *
g_utf8_to_ucs4 (const gchar *str,
		glong        len,             
		glong       *items_read,      
		glong       *items_written,   
		GError     **error)
{
  gunichar *result = NULL;
  gint n_chars, i;
  const gchar *in;
  
  in = str;
  n_chars = 0;
  while ((len < 0 || str + len - in > 0) && *in)
    {
      gunichar wc = g_utf8_get_char_extended (in, len < 0 ? 6 : str + len - in);
      if (wc & 0x80000000)
	{
	  if (wc == (gunichar)-2)
	    {
	      if (items_read)
		break;
	      else
		g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_PARTIAL_INPUT,
                                     _("Partial character sequence at end of input"));
	    }
	  else
	    g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                 _("Invalid byte sequence in conversion input"));

	  goto err_out;
	}

      n_chars++;

      in = g_utf8_next_char (in);
    }

  result = try_malloc_n (n_chars + 1, sizeof (gunichar), error);
  if (result == NULL)
      goto err_out;

  in = str;
  for (i=0; i < n_chars; i++)
    {
      result[i] = g_utf8_get_char (in);
      in = g_utf8_next_char (in);
    }
  result[i] = 0;

  if (items_written)
    *items_written = n_chars;

 err_out:
  if (items_read)
    *items_read = in - str;

  return result;
}

/**
 * g_ucs4_to_utf8:
 * @str: (array length=len) (element-type gunichar): a UCS-4 encoded string
 * @len: the maximum length (number of characters) of @str to use. 
 *   If @len is negative, then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of
 *   characters read, or `NULL`.
 * @items_written: (out) (optional): location to store number
 *   of bytes written or `NULL`. The value here stored does not include the
 *   trailing nul byte.
 * @error: location to store the error occurring, or %NULL to ignore
 *   errors. Any of the errors in #GConvertError other than
 *   %G_CONVERT_ERROR_NO_CONVERSION may occur.
 *
 * Convert a string from a 32-bit fixed width representation as UCS-4.
 * to UTF-8.
 *
 * The result will be terminated with a nul byte.
 * 
 * Returns: (transfer full): a pointer to a newly allocated UTF-8 string.
 *   This value must be freed with [func@GLib.free]. If an error occurs,
 *   @items_read will be set to the position of the first invalid input
 *   character.
 */
gchar *
g_ucs4_to_utf8 (const gunichar *str,
		glong           len,              
		glong          *items_read,       
		glong          *items_written,    
		GError        **error)
{
  gint result_length;
  gchar *result = NULL;
  gchar *p;
  gint i;

  result_length = 0;
  for (i = 0; len < 0 || i < len ; i++)
    {
      if (!str[i])
	break;

      if (str[i] >= 0x80000000)
	{
	  g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                               _("Character out of range for UTF-8"));
	  goto err_out;
	}
      
      result_length += UTF8_LENGTH (str[i]);
    }

  result = try_malloc_n (result_length + 1, 1, error);
  if (result == NULL)
      goto err_out;

  p = result;

  i = 0;
  while (p < result + result_length)
    p += g_unichar_to_utf8 (str[i++], p);
  
  *p = '\0';

  if (items_written)
    *items_written = p - result;

 err_out:
  if (items_read)
    *items_read = i;

  return result;
}

#define SURROGATE_VALUE(h,l) (((h) - 0xd800) * 0x400 + (l) - 0xdc00 + 0x10000)

/**
 * g_utf16_to_utf8:
 * @str: (array length=len) (element-type guint16): a UTF-16 encoded string
 * @len: the maximum length (number of #gunichar2) of @str to use. 
 *   If @len is negative, then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of words read, or
 *   `NULL`. If `NULL`, then [error@GLib.ConvertError.PARTIAL_INPUT] will
 *   be returned in case @str contains a trailing partial character. If
 *   an error occurs then the index of the invalid input is stored here.
 *   It’s guaranteed to be non-negative.
 * @items_written: (out) (optional): location to store number
 *   of bytes written, or `NULL`. The value stored here does not include the
 *   trailing nul byte. It’s guaranteed to be non-negative.
 * @error: location to store the error occurring, or `NULL` to ignore
 *   errors. Any of the errors in [error@GLib.ConvertError] other than
 *   [error@GLib.ConvertError.NO_CONVERSION] may occur.
 *
 * Convert a string from UTF-16 to UTF-8.
 *
 * The result will be terminated with a nul byte.
 *
 * Note that the input is expected to be already in native endianness,
 * an initial byte-order-mark character is not handled specially.
 * [func@GLib.convert] can be used to convert a byte buffer of UTF-16 data of
 * ambiguous endianness.
 *
 * Further note that this function does not validate the result
 * string; it may (for example) include embedded nul characters. The only
 * validation done by this function is to ensure that the input can
 * be correctly interpreted as UTF-16, i.e. it doesn’t contain
 * unpaired surrogates or partial character sequences.
 *
 * Returns: (transfer full): a pointer to a newly allocated UTF-8 string.
 *   This value must be freed with [func@GLib.free].
 **/
gchar *
g_utf16_to_utf8 (const gunichar2  *str,
		 glong             len,
		 glong            *items_read,
		 glong            *items_written,
		 GError          **error)
{
  /* This function and g_utf16_to_ucs4 are almost exactly identical -
   * The lines that differ are marked.
   */
  const gunichar2 *in;
  gchar *out;
  gchar *result = NULL;
  gint n_bytes;
  gunichar high_surrogate;

  g_return_val_if_fail (str != NULL, NULL);

  n_bytes = 0;
  in = str;
  high_surrogate = 0;
  while ((len < 0 || in - str < len) && *in)
    {
      gunichar2 c = *in;
      gunichar wc;

      if (c >= 0xdc00 && c < 0xe000) /* low surrogate */
	{
	  if (high_surrogate)
	    {
	      wc = SURROGATE_VALUE (high_surrogate, c);
	      high_surrogate = 0;
	    }
	  else
	    {
	      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                   _("Invalid sequence in conversion input"));
	      goto err_out;
	    }
	}
      else
	{
	  if (high_surrogate)
	    {
	      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                   _("Invalid sequence in conversion input"));
	      goto err_out;
	    }

	  if (c >= 0xd800 && c < 0xdc00) /* high surrogate */
	    {
	      high_surrogate = c;
	      goto next1;
	    }
	  else
	    wc = c;
	}

      /********** DIFFERENT for UTF8/UCS4 **********/
      n_bytes += UTF8_LENGTH (wc);

    next1:
      in++;
    }

  if (high_surrogate && !items_read)
    {
      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_PARTIAL_INPUT,
                           _("Partial character sequence at end of input"));
      goto err_out;
    }
  
  /* At this point, everything is valid, and we just need to convert
   */
  /********** DIFFERENT for UTF8/UCS4 **********/
  result = try_malloc_n (n_bytes + 1, 1, error);
  if (result == NULL)
      goto err_out;

  high_surrogate = 0;
  out = result;
  in = str;
  while (out < result + n_bytes)
    {
      gunichar2 c = *in;
      gunichar wc;

      if (c >= 0xdc00 && c < 0xe000) /* low surrogate */
	{
	  wc = SURROGATE_VALUE (high_surrogate, c);
	  high_surrogate = 0;
	}
      else if (c >= 0xd800 && c < 0xdc00) /* high surrogate */
	{
	  high_surrogate = c;
	  goto next2;
	}
      else
	wc = c;

      /********** DIFFERENT for UTF8/UCS4 **********/
      out += g_unichar_to_utf8 (wc, out);

    next2:
      in++;
    }
  
  /********** DIFFERENT for UTF8/UCS4 **********/
  *out = '\0';

  if (items_written)
    /********** DIFFERENT for UTF8/UCS4 **********/
    *items_written = out - result;

 err_out:
  if (items_read)
    *items_read = in - str;

  return result;
}

/**
 * g_utf16_to_ucs4:
 * @str: (array length=len) (element-type guint16): a UTF-16 encoded string
 * @len: the maximum length (number of #gunichar2) of @str to use. 
 *   If @len is negative, then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of words read, or
 *   `NULL`. If `NULL`, then [error@GLib.ConvertError.PARTIAL_INPUT] will be
 *   returned in case @str contains a trailing partial character. If
 *   an error occurs then the index of the invalid input is stored here.
 * @items_written: (out) (optional): location to store number
 *   of characters written, or `NULL`. The value stored here does not include
 *   the trailing nul character.
 * @error: location to store the error occurring, or `NULL` to ignore
 *   errors. Any of the errors in [error@GLib.ConvertError] other than
 *   [error@GLib.ConvertError.NO_CONVERSION] may occur.
 *
 * Convert a string from UTF-16 to UCS-4.
 *
 * The result will be nul-terminated.
 * 
 * Returns: (transfer full): a pointer to a newly allocated UCS-4 string.
 *   This value must be freed with [func@GLib.free].
 */
gunichar *
g_utf16_to_ucs4 (const gunichar2  *str,
		 glong             len,              
		 glong            *items_read,       
		 glong            *items_written,    
		 GError          **error)
{
  const gunichar2 *in;
  gchar *out;
  gchar *result = NULL;
  size_t n_bytes;
  gunichar high_surrogate;

  g_return_val_if_fail (str != NULL, NULL);

  n_bytes = 0;
  in = str;
  high_surrogate = 0;
  while ((len < 0 || in - str < len) && *in)
    {
      gunichar2 c = *in;

      if (c >= 0xdc00 && c < 0xe000) /* low surrogate */
	{
	  if (high_surrogate)
	    {
	      high_surrogate = 0;
	    }
	  else
	    {
	      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                   _("Invalid sequence in conversion input"));
	      goto err_out;
	    }
	}
      else
	{
	  if (high_surrogate)
	    {
	      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                   _("Invalid sequence in conversion input"));
	      goto err_out;
	    }

	  if (c >= 0xd800 && c < 0xdc00) /* high surrogate */
	    {
	      high_surrogate = c;
	      goto next1;
	    }
	}

      /********** DIFFERENT for UTF8/UCS4 **********/
      n_bytes += sizeof (gunichar);

    next1:
      in++;
    }

  if (high_surrogate && !items_read)
    {
      g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_PARTIAL_INPUT,
                           _("Partial character sequence at end of input"));
      goto err_out;
    }
  
  /* At this point, everything is valid, and we just need to convert
   */
  /********** DIFFERENT for UTF8/UCS4 **********/
  result = try_malloc_n (n_bytes + 4, 1, error);
  if (result == NULL)
      goto err_out;

  high_surrogate = 0;
  out = result;
  in = str;
  while (out < result + n_bytes)
    {
      gunichar2 c = *in;
      gunichar wc;

      if (c >= 0xdc00 && c < 0xe000) /* low surrogate */
	{
	  wc = SURROGATE_VALUE (high_surrogate, c);
	  high_surrogate = 0;
	}
      else if (c >= 0xd800 && c < 0xdc00) /* high surrogate */
	{
	  high_surrogate = c;
	  goto next2;
	}
      else
	wc = c;

      /********** DIFFERENT for UTF8/UCS4 **********/
      *(gunichar *)out = wc;
      out += sizeof (gunichar);

    next2:
      in++;
    }

  /********** DIFFERENT for UTF8/UCS4 **********/
  *(gunichar *)out = 0;

  if (items_written)
    /********** DIFFERENT for UTF8/UCS4 **********/
    *items_written = (out - result) / sizeof (gunichar);

 err_out:
  if (items_read)
    *items_read = in - str;

  return (gunichar *)result;
}

/**
 * g_utf8_to_utf16:
 * @str: a UTF-8 encoded string
 * @len: the maximum length (number of bytes) of @str to use.
 *   If @len is negative, then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of bytes read, or
 *   `NULL`. If `NULL`, then [error@GLib.ConvertError.PARTIAL_INPUT] will
 *   be returned in case @str contains a trailing partial character. If
 *   an error occurs then the index of the invalid input is stored here.
 * @items_written: (out) (optional): location to store number
 *   of `gunichar2` written, or `NULL`. The value stored here does not include
 *   the trailing nul.
 * @error: location to store the error occurring, or `NULL` to ignore
 *   errors. Any of the errors in [error@GLib.ConvertError] other than
 *   [error@GLib.ConvertError.NO_CONVERSION] may occur.
 *
 * Convert a string from UTF-8 to UTF-16.
 *
 * A nul character (U+0000) will be added to the result after the converted text.
 *
 * Returns: (transfer full): a pointer to a newly allocated UTF-16 string.
 *   This value must be freed with [func@GLib.free].
 */
gunichar2 *
g_utf8_to_utf16 (const gchar *str,
		 glong        len,
		 glong       *items_read,
		 glong       *items_written,
		 GError     **error)
{
  gunichar2 *result = NULL;
  gint n16;
  const gchar *in;
  gint i;

  g_return_val_if_fail (str != NULL, NULL);

  in = str;
  n16 = 0;
  while ((len < 0 || str + len - in > 0) && *in)
    {
      gunichar wc = g_utf8_get_char_extended (in, len < 0 ? 6 : str + len - in);
      if (wc & 0x80000000)
	{
	  if (wc == (gunichar)-2)
	    {
	      if (items_read)
		break;
	      else
		g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_PARTIAL_INPUT,
                                     _("Partial character sequence at end of input"));
	    }
	  else
	    g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                                 _("Invalid byte sequence in conversion input"));

	  goto err_out;
	}

      if (wc < 0xd800)
	n16 += 1;
      else if (wc < 0xe000)
	{
	  g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                               _("Invalid sequence in conversion input"));

	  goto err_out;
	}
      else if (wc < 0x10000)
	n16 += 1;
      else if (wc < 0x110000)
	n16 += 2;
      else
	{
	  g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                               _("Character out of range for UTF-16"));

	  goto err_out;
	}
      
      in = g_utf8_next_char (in);
    }

  result = try_malloc_n (n16 + 1, sizeof (gunichar2), error);
  if (result == NULL)
      goto err_out;

  in = str;
  for (i = 0; i < n16;)
    {
      gunichar wc = g_utf8_get_char (in);

      if (wc < 0x10000)
	{
	  result[i++] = wc;
	}
      else
	{
	  result[i++] = (wc - 0x10000) / 0x400 + 0xd800;
	  result[i++] = (wc - 0x10000) % 0x400 + 0xdc00;
	}
      
      in = g_utf8_next_char (in);
    }

  result[i] = 0;

  if (items_written)
    *items_written = n16;

 err_out:
  if (items_read)
    *items_read = in - str;
  
  return result;
}

/**
 * g_ucs4_to_utf16:
 * @str: (array length=len) (element-type gunichar): a UCS-4 encoded string
 * @len: the maximum length (number of characters) of @str to use. 
 *   If @len is negative, then the string is nul-terminated.
 * @items_read: (out) (optional): location to store number of
 *   bytes read, or `NULL`. If an error occurs then the index of the invalid
 *   input is stored here.
 * @items_written: (out) (optional): location to store number
 *   of `gunichar2` written, or `NULL`. The value stored here does not include
 *   the trailing nul.
 * @error: location to store the error occurring, or `NULL` to ignore
 *   errors. Any of the errors in [error@GLib.ConvertError] other than
 *   [error@GLib.ConvertError.NO_CONVERSION] may occur.
 *
 * Convert a string from UCS-4 to UTF-16.
 *
 * A nul character (U+0000) will be added to the result after the converted text.
 * 
 * Returns: (transfer full): a pointer to a newly allocated UTF-16 string.
 *   This value must be freed with [func@GLib.free].
 */
gunichar2 *
g_ucs4_to_utf16 (const gunichar  *str,
		 glong            len,              
		 glong           *items_read,       
		 glong           *items_written,    
		 GError         **error)
{
  gunichar2 *result = NULL;
  gint n16;
  gint i, j;

  n16 = 0;
  i = 0;
  while ((len < 0 || i < len) && str[i])
    {
      gunichar wc = str[i];

      if (wc < 0xd800)
	n16 += 1;
      else if (wc < 0xe000)
	{
	  g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                               _("Invalid sequence in conversion input"));

	  goto err_out;
	}
      else if (wc < 0x10000)
	n16 += 1;
      else if (wc < 0x110000)
	n16 += 2;
      else
	{
	  g_set_error_literal (error, G_CONVERT_ERROR, G_CONVERT_ERROR_ILLEGAL_SEQUENCE,
                               _("Character out of range for UTF-16"));

	  goto err_out;
	}

      i++;
    }

  result = try_malloc_n (n16 + 1, sizeof (gunichar2), error);
  if (result == NULL)
      goto err_out;

  for (i = 0, j = 0; j < n16; i++)
    {
      gunichar wc = str[i];

      if (wc < 0x10000)
	{
	  result[j++] = wc;
	}
      else
	{
	  result[j++] = (wc - 0x10000) / 0x400 + 0xd800;
	  result[j++] = (wc - 0x10000) % 0x400 + 0xdc00;
	}
    }
  result[j] = 0;

  if (items_written)
    *items_written = n16;
  
 err_out:
  if (items_read)
    *items_read = i;
  
  return result;
}

/**< private >
 * find_invalid_or_incomplete_utf8_sequence:
 *
 * @string: the source string.
 *
 * Returns the first byte of a sequence that is either invalid
 * UTF-8 or incomplete UTF-8, or a pointer to the NULL terminator
 * if all of @string is valid UTF-8.
 */
static const char *
find_invalid_or_incomplete_utf8_sequence (const char *string)
{
  const char *end = string;

  g_utf8_validate (string, -1, &end);

  return end;
}

/**< private >
 * find_valid_and_complete_utf8_sequence:
 *
 * @string: a NULL-terminated source string.
 *
 * Returns the first byte of a sequence that is valid (and complete)
 * UTF-8, or a pointer to the NULL terminator if no such sequence
 * could be found.
 */
static const char *
find_valid_and_complete_utf8_sequence (const char *string)
{
  const unsigned char *iter = (const unsigned char *)string;

  for (;; iter++)
    {
      if (*iter < 128 ||
          ((*iter & 0xC0) == 0xC0 &&
           g_utf8_get_char_validated ((const char*)iter, -1) < (gunichar2)-2))
        {
          break;
        }
    }

  return (const char *) iter;
}


/**< private >
 * invalidly_encoded_string_to_utf16_get_output_length:
 *
 * @start: start of the source string.
 * @end: end of the source string (excluded).
 *
 * Returns the output length, as a count of gunichar2, that is necessary
 * for the generic translation of an invalidly-encoded string to UTF-16.
 */
static size_t
invalidly_encoded_string_to_utf16_get_output_length (const char *start,
                                                     const char *end)
{
  size_t count;

  g_assert ((uintptr_t)end >= (uintptr_t)start);

  /* We output one gunichar2 for each input byte */
  count = (uintptr_t)end - (uintptr_t)start;

  return count;
}

/**< private >
 * invalidly_encoded_string_to_utf16:
 *
 * @start: start of the string.
 * @end: end of the string (excluded).
 * @output: the output buffer. Must be long enough to hold
 *          the entire output.
 *
 * Performs a generic conversion of an invalidly-encoded string
 * to UTF-16. Note: the current implementation simply outputs
 * Unicode Replacement Characters "�" (U+FFFD) for each byte in
 * the source string.
 */
static size_t
invalidly_encoded_string_to_utf16 (const char *start,
                                   const char *end,
                                   gunichar2  *output)
{
  size_t count;

  g_assert ((uintptr_t)end >= (uintptr_t)start);
  count = (uintptr_t)end - (uintptr_t)start;

  for (size_t i = 0; i < count; i++)
    output[i] = 0xFFFD;

  return count;
}

/**< private >
 * invalidly_encoded_string_to_utf16_backtrack:
 *
 * @start: start of the source string.
 * @output_length: length within the output UTF-16 string
 *                 expressed as a count of gunichar2.
 *
 * Backtracks an output-length in count of gunichar2 to the
 * corresponding length, in bytes, of the source string.
 */
static size_t
invalidly_encoded_string_to_utf16_backtrack (const char *start,
                                             size_t      output_length)
{
  /* The conversion process outputs one gunichar2 (a complete
   * character) for each input byte, so the mapping is very
   * simple.
   */
  return output_length;
}


/**< private >
 * valid_utf8_to_utf16_get_output_length:
 *
 * @start: start of the source string. Must be valid UTF-8.
 * @end: end of the source string (excluded).
 *
 * Returns the output-length, in count of gunichar2, necessary for the
 * translation of a valid UTF-8 string to UTF-16.
 */
static size_t
valid_utf8_to_utf16_get_output_length (const char *start,
                                       const char *end)
{
  size_t count = 0;

  while (start < end)
    {
      gunichar codepoint = g_utf8_get_char (start);

      if (codepoint <= 0xFFFF)
        count += 1;
      else
        count += 2;

      start = g_utf8_next_char (start);
    }

  g_assert (start == end);

  return count;
}

/**< private >
 * valid_utf8_to_utf16:
 *
 * @start: start of the source string. Must be valid UTF-8
 * @end: end of the source string (excluded).
 * @output: the output buffer. Must be long enough to hold
 *          the entire output.
 *
 * Performs the conversion of a valid UTF-8 string to UTF-16.
 */
static size_t
valid_utf8_to_utf16 (const char *start,
                     const char *end,
                     gunichar2  *output)
{
  size_t count = 0;

  while (start < end)
    {
      gunichar codepoint = g_utf8_get_char (start);

      if (codepoint <= 0xFFFF)
        {
          output[count++] = (gunichar2) codepoint;
        }
      else
        {
          gunichar subtract = codepoint - 0x010000;
          output[count++] = 0xD800 + ((subtract >> 10) & 0x3FF);
          output[count++] = 0xDC00 + (subtract & 0x3FF);
        }

      start = g_utf8_next_char (start);
    }

  g_assert (start == end);

  return count;
}

/**< private >
 * valid_utf8_to_utf16_backtrack:
 *
 * @start: start of the source string. Must be valid UTF-8.
 * @output_length: length within the output UTF-16 string expressed
 *                 as a count of gunichar2.
 *
 * Backtracks an output-length in count of gunichar2 to the
 * corresponding length, in bytes, of the source string.
 */
static size_t
valid_utf8_to_utf16_backtrack (const char *start,
                               size_t      output_length)
{
  const char *iter = start;
  size_t count = 0;

  for (; *iter != '\0'; iter = g_utf8_next_char (iter))
    {
      if (output_length <= count)
        break;

      if (g_utf8_get_char (iter) <= 0xFFFF)
        count += 1;
      else
        count += 2;
    }

  return (uintptr_t)iter - (uintptr_t)start;
}


static size_t
utf8_to_utf16_make_valid_get_output_length (const char *string)
{
  const char *start = string;
  size_t count = 0;

  while (true)
    {
      const char *end = NULL;

      end = find_invalid_or_incomplete_utf8_sequence (start);
      count += valid_utf8_to_utf16_get_output_length (start, end);
      start = end;

      if (start[0] == '\0')
        break;

      end = find_valid_and_complete_utf8_sequence (start);
      g_assert ((uintptr_t)end > (uintptr_t)start);
      count += invalidly_encoded_string_to_utf16_get_output_length (start, end);
      start = end;

      if (start[0] == '\0')
        break;
    }

  return count;
}

static size_t
utf8_to_utf16_make_valid_backtrack (const char *string,
                                    size_t      output_length)
{
  const char *start = string;
  size_t count = 0;
  size_t l;

  while (true)
    {
      const char *end = NULL;

      end = find_invalid_or_incomplete_utf8_sequence (start);
      l = valid_utf8_to_utf16_get_output_length (start, end);
      if (output_length < count + l)
        return count + valid_utf8_to_utf16_backtrack (start, output_length);
      count += (uintptr_t)end - (uintptr_t)start;
      output_length -= l;
      start = end;

      if (start[0] == '\0')
        return (uintptr_t)start - (uintptr_t)string;

      end = find_valid_and_complete_utf8_sequence (start);
      g_assert ((uintptr_t)end > (uintptr_t)start);
      l = invalidly_encoded_string_to_utf16_get_output_length (start, end);
      if (output_length < l)
        return count + invalidly_encoded_string_to_utf16_backtrack (start, output_length);
      count += (uintptr_t)end - (uintptr_t)start;
      output_length -= l;
      start = end;

      if (start[0] == '\0')
        return (uintptr_t)start - (uintptr_t)string;
    }

  return count;
}


static size_t
utf8_to_utf16_make_valid (const char *string,
                          gunichar2  *output)
{
  const char *start = string;
  size_t count = 0;

  while (true)
    {
      const char *end = NULL;

      end = find_invalid_or_incomplete_utf8_sequence (start);
      count += valid_utf8_to_utf16 (start, end, &output[count]);
      start = end;

      if (start[0] == '\0')
        break;

      end = find_valid_and_complete_utf8_sequence (start);
      g_assert ((uintptr_t)end > (uintptr_t)start);
      count += invalidly_encoded_string_to_utf16 (start, end, &output[count]);
      start = end;

      if (start[0] == '\0')
        break;
    }

  return count;
}

/** < private >
 * g_utf8_to_utf16_make_valid:
 *
 * @utf8: source UTF-8 string. May contain invalid or incomplete sequences.
 * @buffer: optional auxiliary buffer where the output UTF-16 string will be
 *          stored if large enough to hold the output. Callers can pass NULL,
 *          in which case the output buffer is allocated on the heap.
 * @buffer_len: length, in count of gunichar2, of @buffer. This is used only
 *              if @buffer is not NULL.
 * @out_utf16: pointer that will be set the to output string. If @buffer is
 *             long enough to hold the data, *out_utf16 will equal @buffer
 *             upon return; otherwise *out_utf16 will point to heap-allocated
 *             data, which must be freed using `g_free`.
 * @out_utf16_len: pointer to size_t that will be set to the length of the
 *                 output UTF-16 string on return, in count of gunichar2.
 *                 Can be NULL.
 *
 * Performs conversion of an UTF-8 string that may contain invalid sequences
 * to UTF-16.
 *
 * On return, the caller should check if *out_utf16 equals @buffer and call
 * `g_free` accordingly.
 */
void
g_utf8_to_utf16_make_valid (const char  *utf8,
                            gunichar2   *buffer,
                            size_t       buffer_len,
                            gunichar2  **out_utf16,
                            size_t      *out_utf16_len)
{
  size_t output_length = utf8_to_utf16_make_valid_get_output_length (utf8);

  if (output_length < buffer_len)
    {
      *out_utf16 = buffer;
    }
  else
    {
      /* output_length cannot be greater than strlen (utf8), which
       * is less than SIZE_MAX since utf8 is null-terminated.
       * As such, (output_length + 1) cannot overflow.
       */
      *out_utf16 = g_new (gunichar2, output_length + 1);
    }

  utf8_to_utf16_make_valid (utf8, *out_utf16);

  /* Add the terminating NULL character */
  (*out_utf16)[output_length] = L'\0';

  if (out_utf16_len)
    *out_utf16_len = output_length;
}

/** < private >
 * g_utf8_to_utf16_make_valid_backtrack:
 *
 * @utf8: source UTF-8 string. May contain invalid or incomplete sequences.
 * @utf16_len: length within the output UTF-16 string expressed as a count
 *             of gunichar2.
 *
 * Backtracks an output-length in count of gunichar2 to the
 * corresponding length, in bytes, of the source string.
 */
size_t
g_utf8_to_utf16_make_valid_backtrack (const char  *utf8,
                                      size_t       utf16_len)
{
  return utf8_to_utf16_make_valid_backtrack (utf8, utf16_len);
}

/* SIMD-based UTF-8 validation originates in the c-utf8 project from
 * https://github.com/c-util/c-utf8/ from the following authors:
 *
 *   David Rheinsberg <david@readahead.eu>
 *   Evgeny Vereshchagin <evvers@ya.ru>
 *   Jan Engelhardt <jengelh@inai.de>
 *   Tom Gundersen <teg@jklm.no>
 *
 * It has been adapted for portability and integration.
 * The original code is dual-licensed Apache-2.0 or LGPLv2.1+
 */

#define align_to(_val, _to) (((_val) + (_to) - 1) & ~((_to) - 1))

static inline guint8
load_u8 (gconstpointer memory,
         gsize         offset)
{
  return ((const guint8 *)memory)[offset];
}

#if G_GNUC_CHECK_VERSION(4,8) || defined(__clang__)
# define _attribute_aligned(n) __attribute__((aligned(n)))
#elif defined(_MSC_VER)
# define _attribute_aligned(n) __declspec(align(n))
#else
# define _attribute_aligned(n)
#endif

static inline gsize
load_word (gconstpointer memory,
           gsize         offset)
{
#if GLIB_SIZEOF_VOID_P == 8
  _attribute_aligned(8) const guint8 *m = ((const guint8 *)memory) + offset;

  return ((guint64)m[0] <<  0) | ((guint64)m[1] <<  8) |
         ((guint64)m[2] << 16) | ((guint64)m[3] << 24) |
         ((guint64)m[4] << 32) | ((guint64)m[5] << 40) |
         ((guint64)m[6] << 48) | ((guint64)m[7] << 56);
#else
  _attribute_aligned(4) const guint8 *m = ((const guint8 *)memory) + offset;

  return ((guint)m[0] <<  0) | ((guint)m[1] <<  8) |
         ((guint)m[2] << 16) | ((guint)m[3] << 24);
#endif
}

/* The following constants are truncated on 32-bit machines */
#define UTF8_ASCII_MASK ((gsize)0x8080808080808080L)
#define UTF8_ASCII_SUB  ((gsize)0x0101010101010101L)

static inline int
utf8_word_is_ascii (gsize word)
{
  /* True unless any byte is NULL or has the MSB set. */
  return ((((word - UTF8_ASCII_SUB) | word) & UTF8_ASCII_MASK) == 0);
}

static void
utf8_verify_ascii (const char **strp,
                   gsize       *lenp)
{
  const char *str = *strp;
  gsize len = lenp ? *lenp : strlen (str);

  while (len > 0 && load_u8 (str, 0) < 128)
    {
      if ((gpointer) align_to ((guintptr) str, sizeof (gsize)) == str)
        {
          while (len >= 2 * sizeof (gsize))
            {
              if (!utf8_word_is_ascii (load_word (str, 0)) ||
                  !utf8_word_is_ascii (load_word (str, sizeof (gsize))))
                break;

              str += 2 * sizeof(gsize);
              len -= 2 * sizeof(gsize);
            }

          while (len > 0 && load_u8 (str, 0) < 128)
            {
              if G_UNLIKELY (load_u8 (str, 0) == 0x00)
                goto out;

              ++str;
              --len;
            }
        }
      else
        {
          if G_UNLIKELY (load_u8 (str, 0) == 0x00)
            goto out;

          ++str;
          --len;
        }
    }

out:
  *strp = str;

  if (lenp)
    *lenp = len;
}

#define UTF8_CHAR_IS_TAIL(_x) (((_x) & 0xC0) == 0x80)

static void
utf8_verify (const char **strp,
             gsize       *lenp)
{
  const char *str = *strp;
  gsize len = lenp ? *lenp : strlen (str);

  /* See Unicode 10.0.0, Chapter 3, Section D92 */

  while (len > 0)
    {
      guint8 b = load_u8 (str, 0);

      if (b == 0x00)
        goto out;

      else if (b <= 0x7F)
        {
          /*
           * Special-case and optimize the ASCII case.
           */
          utf8_verify_ascii ((const char **)&str, &len);
        }

      else if (b >= 0xC2 && b <= 0xDF)
        {
          if G_UNLIKELY (len < 2)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 1)))
            goto out;

          str += 2;
          len -= 2;

        }

      else if (b == 0xE0)
        {
          if G_UNLIKELY (len < 3)
            goto out;
          if G_UNLIKELY (load_u8 (str, 1) < 0xA0 || load_u8 (str, 1) > 0xBF)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;

          str += 3;
          len -= 3;
        }

      else if (b >= 0xE1 && b <= 0xEC)
        {
          if G_UNLIKELY (len < 3)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 1)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;

          str += 3;
          len -= 3;
        }

      else if (b == 0xED)
        {
          if G_UNLIKELY (len < 3)
            goto out;
          if G_UNLIKELY (load_u8 (str, 1) < 0x80 || load_u8 (str, 1) > 0x9F)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;

          str += 3;
          len -= 3;
        }

      else if (b >= 0xEE && b <= 0xEF)
        {
          if G_UNLIKELY (len < 3)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 1)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;

          str += 3;
          len -= 3;
        }

      else if (b == 0xF0)
        {
          if G_UNLIKELY (len < 4)
            goto out;
          if G_UNLIKELY (load_u8 (str, 1) < 0x90 || load_u8 (str, 1) > 0xBF)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 3)))
            goto out;

          str += 4;
          len -= 4;
        }

      else if (b >= 0xF1 && b <= 0xF3)
        {
          if G_UNLIKELY (len < 4)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 1)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 3)))
            goto out;

          str += 4;
          len -= 4;
        }

      else if (b == 0xF4)
        {
          if G_UNLIKELY (len < 4)
            goto out;
          if G_UNLIKELY (load_u8 (str, 1) < 0x80 || load_u8 (str, 1) > 0x8F)
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 2)))
            goto out;
          if G_UNLIKELY (!UTF8_CHAR_IS_TAIL (load_u8 (str, 3)))
            goto out;

          str += 4;
          len -= 4;
        }

      else goto out;
    }

out:
  *strp = str;

  if (lenp)
    *lenp = len;
}

/**
 * g_utf8_validate:
 * @str: (array length=max_len) (element-type guint8): a pointer to character data
 * @max_len: max bytes to validate, or `-1` to go until nul
 * @end: (out) (optional) (transfer none) (array zero-terminated=1) (element-type guint8): return location for end of valid data
 * 
 * Validates UTF-8 encoded text.
 *
 * @str is the text to validate; if @str is nul-terminated, then @max_len can be
 * `-1`, otherwise @max_len should be the number of bytes to validate.
 *
 * If @end is non-`NULL`, then the end of the valid range will be stored there.
 * This is the first byte of the first invalid character if some bytes were
 * invalid, or the end of the text being validated otherwise — either the
 * trailing nul byte, or the first byte beyond @max_len (if it’s positive).
 *
 * Note that `g_utf8_validate()` returns `FALSE` if @max_len is  positive and
 * any of the @max_len bytes are nul.
 *
 * Returns `TRUE` if all of @str was valid. Many GLib and GTK
 * routines require valid UTF-8 as input; so data read from a file
 * or the network should be checked with `g_utf8_validate()` before
 * doing anything else with it.
 * 
 * Returns: `TRUE` if the text was valid UTF-8
 */
gboolean
g_utf8_validate (const char   *str,
                 gssize        max_len,
                 const gchar **end)
{
  size_t max_len_unsigned = (max_len >= 0) ? (size_t) max_len : strlen (str);

  return g_utf8_validate_len (str, max_len_unsigned, end);
}

/**
 * g_utf8_validate_len:
 * @str: (array length=max_len) (element-type guint8): a pointer to character data
 * @max_len: max bytes to validate
 * @end: (out) (optional) (transfer none) (array zero-terminated=1) (element-type guint8): return location for end of valid data
 *
 * Validates UTF-8 encoded text.
 *
 * As with [func@GLib.utf8_validate], but @max_len must be set, and hence this
 * function will always return `FALSE` if any of the bytes of @str are nul.
 *
 * Returns: `TRUE` if the text was valid UTF-8
 * Since: 2.60
 */
gboolean
g_utf8_validate_len (const char   *str,
                     gsize         max_len,
                     const gchar **end)

{
  utf8_verify (&str, &max_len);

  if (end != NULL)
    *end = str;

  return max_len == 0;
}

/**
 * g_str_is_ascii:
 * @str: a string
 *
 * Determines if a string is pure ASCII. A string is pure ASCII if it
 * contains no bytes with the high bit set.
 *
 * Returns: true if @str is ASCII
 *
 * Since: 2.40
 */
gboolean
g_str_is_ascii (const gchar *str)
{
  utf8_verify_ascii (&str, NULL);

  return *str == 0;
}

/**
 * g_unichar_validate:
 * @ch: a Unicode character
 * 
 * Checks whether @ch is a valid Unicode character.
 *
 * Some possible integer values of @ch will not be valid. U+0000 is considered a
 * valid character, though it’s normally a string terminator.
 * 
 * Returns: `TRUE` if @ch is a valid Unicode character
 **/
gboolean
g_unichar_validate (gunichar ch)
{
  return UNICODE_VALID (ch);
}

/**
 * g_utf8_strreverse:
 * @str: a UTF-8 encoded string
 * @len: the maximum length of @str to use, in bytes. If @len is negative,
 *   then the string is nul-terminated.
 *
 * Reverses a UTF-8 string.
 *
 * @str must be valid UTF-8 encoded text. (Use [func@GLib.utf8_validate] on all
 * text before trying to use UTF-8 utility functions with it.)
 *
 * This function is intended for programmatic uses of reversed strings.
 * It pays no attention to decomposed characters, combining marks, byte 
 * order marks, directional indicators (LRM, LRO, etc) and similar 
 * characters which might need special handling when reversing a string 
 * for display purposes.
 *
 * Note that unlike [func@GLib.strreverse], this function returns
 * newly-allocated memory, which should be freed with [func@GLib.free] when
 * no longer needed. 
 *
 * Returns: (transfer full): a newly-allocated string which is the reverse of @str
 *
 * Since: 2.2
 */
gchar *
g_utf8_strreverse (const gchar *str,
		   gssize       len)
{
  gchar *r, *result;
  const gchar *p;

  if (len < 0)
    len = strlen (str);

  result = g_new (gchar, len + 1);
  r = result + len;
  p = str;
  while (r > result)
    {
      gchar *m, skip = g_utf8_skip[*(guchar*) p];
      r -= skip;
      g_assert (r >= result);
      for (m = r; skip; skip--)
        *m++ = *p++;
    }
  result[len] = 0;

  return result;
}

/**
 * g_utf8_make_valid:
 * @str: string to coerce into UTF-8
 * @len: the maximum length of @str to use, in bytes. If @len is negative,
 *   then the string is nul-terminated.
 *
 * If the provided string is valid UTF-8, return a copy of it. If not,
 * return a copy in which bytes that could not be interpreted as valid Unicode
 * are replaced with the Unicode replacement character (U+FFFD).
 *
 * For example, this is an appropriate function to use if you have received
 * a string that was incorrectly declared to be UTF-8, and you need a valid
 * UTF-8 version of it that can be logged or displayed to the user, with the
 * assumption that it is close enough to ASCII or UTF-8 to be mostly
 * readable as-is.
 *
 * Returns: (transfer full): a valid UTF-8 string whose content resembles @str
 *
 * Since: 2.52
 */
gchar *
g_utf8_make_valid (const gchar *str,
                   gssize       len)
{
  GString *string;
  const gchar *remainder, *invalid;
  gsize remaining_bytes, valid_bytes;

  g_return_val_if_fail (str != NULL, NULL);

  if (len < 0)
    len = strlen (str);

  string = NULL;
  remainder = str;
  remaining_bytes = len;

  while (remaining_bytes != 0) 
    {
      if (g_utf8_validate (remainder, remaining_bytes, &invalid)) 
	break;
      valid_bytes = invalid - remainder;
    
      if (string == NULL) 
	string = g_string_sized_new (remaining_bytes);

      g_string_append_len (string, remainder, valid_bytes);
      /* append U+FFFD REPLACEMENT CHARACTER */
      g_string_append (string, "\357\277\275");
      
      remaining_bytes -= valid_bytes + 1;
      remainder = invalid + 1;
    }
  
  if (string == NULL)
    return g_strndup (str, len);
  
  g_string_append_len (string, remainder, remaining_bytes);
  g_string_append_c (string, '\0');

  g_assert (g_utf8_validate (string->str, -1, NULL));

  return g_string_free (string, FALSE);
}