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Initial, still somewhat experimental implementation to leverage
less(1) -T and :t ctags(1)-like functionality to jump to the
definitions of various terms inside manual pages.
To be polished in the tree, so bear with me and report issues.

Technically, if less(1) is used as a pager, information is collected
by the mdoc(7) terminal formatter, first stored using the ohash
library, then ultimately written to a temporary file which is passed
to less via -T.  No change intended for other output formatters or
when running without a pager.

Based on an idea from Kristaps using feedback from many, in particular
phessler@ nicm@ millert@ halex@ doug@ kspillner@ deraadt@.

/*      $Id: tag.c,v 1.1 2015/07/17 22:38:29 schwarze Exp $    */
/*
 * Copyright (c) 2015 Ingo Schwarze <schwarze@openbsd.org>
 *
 * 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 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 <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#if HAVE_OHASH
#include <ohash.h>
#else
#include "compat_ohash.h"
#endif

#include "mandoc_aux.h"
#include "tag.h"

struct tag_entry {
	size_t	 line;
	char	 s[];
};

static	void	*tag_alloc(size_t, void *);
static	void	 tag_free(void *, void *);
static	void	*tag_calloc(size_t, size_t, void *);

static struct ohash	 tag_data;
static char		*tag_fn = NULL;
static int		 tag_fd = -1;


/*
 * Set up the ohash table to collect output line numbers
 * where various marked-up terms are documented and create
 * the temporary tags file, saving the name for the pager.
 */
void
tag_init(void)
{
	struct ohash_info	 tag_info;

	tag_fn = mandoc_strdup("/tmp/man.XXXXXXXXXX");
	if ((tag_fd = mkstemp(tag_fn)) == -1) {
		free(tag_fn);
		tag_fn = NULL;
		return;
	}

	tag_info.alloc = tag_alloc;
	tag_info.calloc = tag_calloc;
	tag_info.free = tag_free;
	tag_info.key_offset = offsetof(struct tag_entry, s);
	tag_info.data = NULL;
	ohash_init(&tag_data, 4, &tag_info);
}

char *
tag_filename(void)
{

	return(tag_fn);
}

/*
 * Return the line number where a term is defined,
 * or 0 if the term is unknown.
 */
size_t
tag_get(const char *s, size_t len)
{
	struct tag_entry	*entry;
	const char		*end;
	unsigned int		 slot;

	if (tag_fd == -1)
		return(0);
	if (len == 0)
		len = strlen(s);
	end = s + len;
	slot = ohash_qlookupi(&tag_data, s, &end);
	entry = ohash_find(&tag_data, slot);
	return(entry == NULL ? 0 : entry->line);
}

/*
 * Set the line number where a term is defined.
 */
void
tag_put(const char *s, size_t len, size_t line)
{
	struct tag_entry	*entry;
	const char		*end;
	unsigned int		 slot;

	if (tag_fd == -1)
		return;
	if (len == 0)
		len = strlen(s);
	end = s + len;
	slot = ohash_qlookupi(&tag_data, s, &end);
	entry = ohash_find(&tag_data, slot);
	if (entry == NULL) {
		entry = mandoc_malloc(sizeof(*entry) + len + 1);
		memcpy(entry->s, s, len);
		entry->s[len] = '\0';
		ohash_insert(&tag_data, slot, entry);
	}
	entry->line = line;
}

/*
 * Write out the tags file using the previously collected
 * information and clear the ohash table while going along.
 */
void
tag_write(void)
{
	FILE			*stream;
	struct tag_entry	*entry;
	unsigned int		 slot;

	if (tag_fd == -1)
		return;
	stream = fdopen(tag_fd, "w");
	entry = ohash_first(&tag_data, &slot);
	while (entry != NULL) {
		if (stream != NULL)
			fprintf(stream, "%s - %zu\n", entry->s, entry->line);
		free(entry);
		entry = ohash_next(&tag_data, &slot);
	}
	ohash_delete(&tag_data);
	if (stream != NULL)
		fclose(stream);
}

void
tag_unlink(void)
{

	if (tag_fn != NULL)
		unlink(tag_fn);
}

/*
 * Memory management callback functions for ohash.
 */
static void *
tag_alloc(size_t sz, void *arg)
{

	return(mandoc_malloc(sz));
}

static void *
tag_calloc(size_t nmemb, size_t sz, void *arg)
{

	return(mandoc_calloc(nmemb, sz));
}

static void
tag_free(void *p, void *arg)
{

	free(p);
}