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Change UTF-8 combining to inspect the previous character at the cursor
position rather than keeping the last character from the input stream,
this is how most terminals work and fixes problems with displaying these
characters in vim. GitHub issue 3600.

/* $OpenBSD: utf8-combined.c,v 1.3 2023/09/15 15:49:05 nicm Exp $ */

/*
 * Copyright (c) 2023 Nicholas Marriott <nicholas.marriott@gmail.com>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
 * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>

#include <stdlib.h>
#include <string.h>
#include <wchar.h>

#include "tmux.h"

static const wchar_t utf8_modifier_table[] = {
	0x1F1E6,
	0x1F1E7,
	0x1F1E8,
	0x1F1E9,
	0x1F1EA,
	0x1F1EB,
	0x1F1EC,
	0x1F1ED,
	0x1F1EE,
	0x1F1EF,
	0x1F1F0,
	0x1F1F1,
	0x1F1F2,
	0x1F1F3,
	0x1F1F4,
	0x1F1F5,
	0x1F1F6,
	0x1F1F7,
	0x1F1F8,
	0x1F1F9,
	0x1F1FA,
	0x1F1FB,
	0x1F1FC,
	0x1F1FD,
	0x1F1FE,
	0x1F1FF,
	0x1F3FB,
	0x1F3FC,
	0x1F3FD,
	0x1F3FE,
	0x1F3FF
};

/* Has this got a zero width joiner at the end? */
int
utf8_has_zwj(const struct utf8_data *ud)
{
	if (ud->size < 3)
		return (0);
	return (memcmp(ud->data + ud->size - 3, "\342\200\215", 3) == 0);
}

/* Is this a zero width joiner? */
int
utf8_is_zwj(const struct utf8_data *ud)
{
	if (ud->size != 3)
		return (0);
	return (memcmp(ud->data, "\342\200\215", 3) == 0);
}

/* Is this a variation selector? */
int
utf8_is_vs(const struct utf8_data *ud)
{
	if (ud->size != 3)
		return (0);
	return (memcmp(ud->data, "\357\270\217", 3) == 0);
}

/* Is this in the modifier table? */
int
utf8_is_modifier(const struct utf8_data *ud)
{
	wchar_t	wc;

	if (utf8_towc(ud, &wc) != UTF8_DONE)
		return (0);
	if (!utf8_in_table(wc, utf8_modifier_table,
	    nitems(utf8_modifier_table)))
		return (0);
	return (1);
}