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Basic implementation of the roff(7) .ta (define tab stops) request.
This is the first feature made possible by the parser reorganization.
Improves the formatting of the SYNOPSIS in many Xenocara GL manuals.
Also important for ports, as reported by many, including naddy@.

/*	$OpenBSD: term.c,v 1.119 2017/01/08 18:08:44 schwarze Exp $ */
/*
 * Copyright (c) 2017 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 "mandoc_aux.h"
#include "out.h"
#include "term.h"

struct tablist {
	size_t	*t;	/* Allocated array of tab positions. */
	size_t	 s;	/* Allocated number of positions. */
	size_t	 n;	/* Currently used number of positions. */
};

static struct {
	struct tablist	 a;	/* All tab positions for lookup. */
	struct tablist	 p;	/* Periodic tab positions to add. */
	size_t		 d;	/* Default tab width in units of n. */
} tabs;


void
term_tab_set(const struct termp *p, const char *arg)
{
	static int	 recording_period;

	struct roffsu	 su;
	struct tablist	*tl;
	size_t		 pos;
	int		 add;

	/* Special arguments: clear all tabs or switch lists. */

	if (arg == NULL) {
		tabs.a.n = tabs.p.n = 0;
		recording_period = 0;
		if (tabs.d == 0) {
			a2roffsu(".8i", &su, SCALE_IN);
			tabs.d = term_hspan(p, &su) / 24;
		}
		return;
	}
	if (arg[0] == 'T' && arg[1] == '\0') {
		recording_period = 1;
		return;
	}

	/* Parse the sign, the number, and the unit. */

	if (*arg == '+') {
		add = 1;
		arg++;
	} else
		add = 0;
	if (a2roffsu(arg, &su, SCALE_EM) == 0)
		return;

	/* Select the list, and extend it if it is full. */

	tl = recording_period ? &tabs.p : &tabs.a;
	if (tl->n >= tl->s) {
		tl->s += 8;
		tl->t = mandoc_reallocarray(tl->t, tl->s, sizeof(*tl->t));
	}

	/* Append the new position. */

	pos = term_hspan(p, &su);
	tl->t[tl->n] = pos;
	if (add && tl->n)
		tl->t[tl->n] += tl->t[tl->n - 1];
	tl->n++;
}

size_t
term_tab_next(size_t prev)
{
	size_t	 i, j;

	for (i = 0;; i++) {
		if (i == tabs.a.n) {
			if (tabs.p.n == 0)
				return prev;
/*
				return i ? prev :
				    (prev / tabs.d + 1) * tabs.d;
 */
			tabs.a.n += tabs.p.n;
			if (tabs.a.s < tabs.a.n) {
				tabs.a.s = tabs.a.n;
				tabs.a.t = mandoc_reallocarray(tabs.a.t,
				    tabs.a.s, sizeof(*tabs.a.t));
			}
			for (j = 0; j < tabs.p.n; j++)
				tabs.a.t[i + j] = tabs.p.t[j] +
				    (i ? tabs.a.t[i - 1] : 0);
		}
		if (prev < tabs.a.t[i] / 24)
			return tabs.a.t[i] / 24;
	}
}