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@.

20 files changed, 335 insertions, 40 deletions

diff --git a/Makefile b/Makefile
index d2b9a8b5..8ea2b92f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-# $Id: Makefile,v 1.511 2017/05/05 02:06:19 schwarze Exp $
+# $Id: Makefile,v 1.512 2017/05/07 17:31:45 schwarze Exp $
 #
 # Copyright (c) 2010, 2011, 2012 Kristaps Dzonsons <kristaps@bsd.lv>
 # Copyright (c) 2011, 2013-2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -125,6 +125,7 @@ SRCS		 = att.c \
 		   term.c \
 		   term_ascii.c \
 		   term_ps.c \
+		   term_tab.c \
 		   tree.c
 
 DISTFILES	 = INSTALL \
@@ -260,6 +261,7 @@ MANDOC_TERM_OBJS = eqn_term.o \
 		   term.o \
 		   term_ascii.o \
 		   term_ps.o \
+		   term_tab.o \
 		   tbl_term.o
 
 DBM_OBJS	 = dbm.o \
diff --git a/Makefile.depend b/Makefile.depend
index b75befd8..61d967af 100644
--- a/Makefile.depend
+++ b/Makefile.depend
@@ -72,4 +72,5 @@ tbl_term.o: tbl_term.c config.h mandoc.h out.h term.h
 term.o: term.c config.h mandoc.h mandoc_aux.h out.h term.h main.h
 term_ascii.o: term_ascii.c config.h mandoc.h mandoc_aux.h out.h term.h manconf.h main.h
 term_ps.o: term_ps.c config.h mandoc_aux.h out.h term.h manconf.h main.h
+term_tab.o: term_tab.c mandoc_aux.h out.h term.h
 tree.o: tree.c config.h mandoc.h roff.h mdoc.h man.h main.h
diff --git a/man_term.c b/man_term.c
index 41d13fc9..a96974f9 100644
--- a/man_term.c
+++ b/man_term.c
@@ -1,4 +1,4 @@
-/*	$Id: man_term.c,v 1.197 2017/05/05 15:17:32 schwarze Exp $ */
+/*	$Id: man_term.c,v 1.198 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2008-2012 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2015, 2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -142,7 +142,9 @@ terminal_man(void *arg, const struct roff_man *man)
 	p = (struct termp *)arg;
 	p->overstep = 0;
 	p->rmargin = p->maxrmargin = p->defrmargin;
-	p->tabwidth = term_len(p, 5);
+	term_tab_set(p, NULL);
+	term_tab_set(p, "T");
+	term_tab_set(p, ".5i");
 
 	memset(&mt, 0, sizeof(struct mtermp));
 	mt.lmargin[mt.lmargincur] = term_len(p, p->defindent);
diff --git a/mdoc_man.c b/mdoc_man.c
index 04e9c8d8..d926e386 100644
--- a/mdoc_man.c
+++ b/mdoc_man.c
@@ -1,4 +1,4 @@
-/*	$Id: mdoc_man.c,v 1.113 2017/05/05 15:17:32 schwarze Exp $ */
+/*	$Id: mdoc_man.c,v 1.114 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2011-2017 Ingo Schwarze <schwarze@openbsd.org>
  *
@@ -112,6 +112,7 @@ static	void	  pre_sp(DECL_ARGS);
 static	int	  pre_sect(DECL_ARGS);
 static	int	  pre_sy(DECL_ARGS);
 static	void	  pre_syn(const struct roff_node *);
+static	void	  pre_ta(DECL_ARGS);
 static	int	  pre_vt(DECL_ARGS);
 static	int	  pre_xr(DECL_ARGS);
 static	void	  print_word(const char *);
@@ -128,6 +129,7 @@ static	const void_fp roff_manacts[ROFF_MAX] = {
 	pre_ft,
 	pre_ll,
 	pre_sp,
+	pre_ta,
 };
 
 static	const struct manact __manacts[MDOC_MAX - MDOC_Dd] = {
@@ -443,7 +445,6 @@ static void
 print_line(const char *s, int newflags)
 {
 
-	outflags &= ~MMAN_br;
 	outflags |= MMAN_nl;
 	print_word(s);
 	outflags |= newflags;
@@ -1715,6 +1716,15 @@ pre_sy(DECL_ARGS)
 	return 1;
 }
 
+static void
+pre_ta(DECL_ARGS)
+{
+	print_line(".ta", 0);
+	for (n = n->child; n != NULL; n = n->next)
+		print_word(n->string);
+	outflags |= MMAN_nl;
+}
+
 static int
 pre_vt(DECL_ARGS)
 {
diff --git a/mdoc_term.c b/mdoc_term.c
index ab26e280..08620365 100644
--- a/mdoc_term.c
+++ b/mdoc_term.c
@@ -1,4 +1,4 @@
-/*	$Id: mdoc_term.c,v 1.355 2017/05/05 15:17:32 schwarze Exp $ */
+/*	$Id: mdoc_term.c,v 1.356 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010, 2012-2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -261,7 +261,9 @@ terminal_mdoc(void *arg, const struct roff_man *mdoc)
 	p = (struct termp *)arg;
 	p->overstep = 0;
 	p->rmargin = p->maxrmargin = p->defrmargin;
-	p->tabwidth = term_len(p, 5);
+	term_tab_set(p, NULL);
+	term_tab_set(p, "T");
+	term_tab_set(p, ".5i");
 
 	n = mdoc->first->child;
 	if (p->synopsisonly) {
@@ -1128,8 +1130,14 @@ static void
 termp_bl_post(DECL_ARGS)
 {
 
-	if (n->type == ROFFT_BLOCK)
-		term_newln(p);
+	if (n->type != ROFFT_BLOCK)
+		return;
+	term_newln(p);
+	if (n->tok != MDOC_Bl || n->norm->Bl.type != LIST_column)
+		return;
+	term_tab_set(p, NULL);
+	term_tab_set(p, "T");
+	term_tab_set(p, ".5i");
 }
 
 static int
@@ -1272,6 +1280,9 @@ termp_sh_pre(DECL_ARGS)
 		break;
 	case ROFFT_BODY:
 		p->offset = term_len(p, p->defindent);
+		term_tab_set(p, NULL);
+		term_tab_set(p, "T");
+		term_tab_set(p, ".5i");
 		switch (n->sec) {
 		case SEC_DESCRIPTION:
 			fn_prio = 0;
@@ -1322,6 +1333,9 @@ termp_d1_pre(DECL_ARGS)
 		return 1;
 	term_newln(p);
 	p->offset += term_len(p, p->defindent + 1);
+	term_tab_set(p, NULL);
+	term_tab_set(p, "T");
+	term_tab_set(p, ".5i");
 	return 1;
 }
 
@@ -1427,7 +1441,7 @@ termp_fa_pre(DECL_ARGS)
 static int
 termp_bd_pre(DECL_ARGS)
 {
-	size_t			 tabwidth, lm, len, rm, rmax;
+	size_t			 lm, len, rm, rmax;
 	struct roff_node	*nn;
 	int			 offset;
 
@@ -1467,9 +1481,11 @@ termp_bd_pre(DECL_ARGS)
 	    DISP_centered != n->norm->Bd.type)
 		return 1;
 
-	tabwidth = p->tabwidth;
-	if (DISP_literal == n->norm->Bd.type)
-		p->tabwidth = term_len(p, 8);
+	if (n->norm->Bd.type == DISP_literal) {
+		term_tab_set(p, NULL);
+		term_tab_set(p, "T");
+		term_tab_set(p, "8n");
+	}
 
 	lm = p->offset;
 	rm = p->rmargin;
@@ -1493,9 +1509,9 @@ termp_bd_pre(DECL_ARGS)
 		 * notion of selective eoln whitespace is pretty dumb
 		 * anyway, so don't sweat it.
 		 */
+		if (nn->tok < ROFF_MAX)
+			continue;
 		switch (nn->tok) {
-		case ROFF_br:
-		case ROFF_sp:
 		case MDOC_Sm:
 		case MDOC_Bl:
 		case MDOC_D1:
@@ -1513,7 +1529,6 @@ termp_bd_pre(DECL_ARGS)
 		p->flags |= TERMP_NOSPACE;
 	}
 
-	p->tabwidth = tabwidth;
 	p->rmargin = rm;
 	p->maxrmargin = rmax;
 	return 0;
@@ -1584,6 +1599,9 @@ termp_ss_pre(DECL_ARGS)
 		break;
 	case ROFFT_BODY:
 		p->offset = term_len(p, p->defindent);
+		term_tab_set(p, NULL);
+		term_tab_set(p, "T");
+		term_tab_set(p, ".5i");
 		break;
 	default:
 		break;
diff --git a/regress/roff/Makefile b/regress/roff/Makefile
index 96474204..c97b99b1 100644
--- a/regress/roff/Makefile
+++ b/regress/roff/Makefile
@@ -1,7 +1,7 @@
 # $OpenBSD: Makefile,v 1.20 2015/02/06 16:05:51 schwarze Exp $
 
 SUBDIR  = args cond esc scale string
-SUBDIR += br cc de ds ft ig it ll na nr ps rm sp tr
+SUBDIR += br cc de ds ft ig it ll na nr ps rm sp ta tr
 
 .include "../Makefile.sub"
 .include <bsd.subdir.mk>
diff --git a/regress/roff/ta/Makefile b/regress/roff/ta/Makefile
new file mode 100644
index 00000000..656c5174
--- /dev/null
+++ b/regress/roff/ta/Makefile
@@ -0,0 +1,5 @@
+# $OpenBSD$
+
+REGRESS_TARGETS	= basic-mdoc basic-man
+
+.include <bsd.regress.mk>
diff --git a/regress/roff/ta/basic-man.in b/regress/roff/ta/basic-man.in
new file mode 100644
index 00000000..f2f4641d
--- /dev/null
+++ b/regress/roff/ta/basic-man.in
@@ -0,0 +1,19 @@
+.TH TA-BASIC-MAN 1 "May 7, 2014" OpenBSD
+.SH NAME
+ta-basic-man \- setting tabstop positions in man(7)
+.SH DESCRIPTION
+.nf
+default:
+1	2	3
+10n:
+.ta 10n
+1	2	3
+none:
+.ta
+1	2	3
+3n +6n T 4n +2n:
+.ta 3n +6n T 4n +2n
+1	2	3	4	5	6	7	8	9
+default unit:
+.ta 3 +4 12
+1	2	3	4
diff --git a/regress/roff/ta/basic-man.out_ascii b/regress/roff/ta/basic-man.out_ascii
new file mode 100644
index 00000000..37062245
--- /dev/null
+++ b/regress/roff/ta/basic-man.out_ascii
@@ -0,0 +1,22 @@
+TA-BASIC-MAN(1)             General Commands Manual            TA-BASIC-MAN(1)
+
+
+
+NNAAMMEE
+       ta-basic-man - setting tabstop positions in man(7)
+
+DDEESSCCRRIIPPTTIIOONN
+       default:
+       1    2    3
+       10n:
+       1         23
+       none:
+       123
+       3n +6n T 4n +2n:
+       1  2     3   4 5   6 7   8 9
+       default unit:
+       1  2   3    4
+
+
+
+OpenBSD                           May 7, 2014                  TA-BASIC-MAN(1)
diff --git a/regress/roff/ta/basic-mdoc.in b/regress/roff/ta/basic-mdoc.in
new file mode 100644
index 00000000..24c83bd4
--- /dev/null
+++ b/regress/roff/ta/basic-mdoc.in
@@ -0,0 +1,34 @@
+.Dd May 7, 2017
+.Dt TA-BASIC-MDOC 1
+.Os OpenBSD
+.Sh NAME
+.Nm ta-basic-mdoc
+.Nd setting tabstop positions in mdoc(7)
+.Sh DESCRIPTION
+.Bd -unfilled
+default:
+1	2	3
+10n:
+.ta 10n
+1	2	3
+none:
+.ta
+1	2	3
+3n +6n T 4n +2n:
+.ta 3n +6n T 4n +2n
+1	2	3	4	5	6	7	8	9
+.Ed
+.Pp
+literal:
+.Bd -literal -compact
+1	2	3
+.Ed
+.Pp
+after literal:
+.br
+1	2	3
+.Pp
+default unit:
+.br
+.ta 3 +4 12
+1	2	3	4
diff --git a/regress/roff/ta/basic-mdoc.out_ascii b/regress/roff/ta/basic-mdoc.out_ascii
new file mode 100644
index 00000000..83feb627
--- /dev/null
+++ b/regress/roff/ta/basic-mdoc.out_ascii
@@ -0,0 +1,25 @@
+TA-BASIC-MDOC(1)            General Commands Manual           TA-BASIC-MDOC(1)
+
+NNAAMMEE
+     ttaa--bbaassiicc--mmddoocc - setting tabstop positions in mdoc(7)
+
+DDEESSCCRRIIPPTTIIOONN
+     default:
+     1    2    3
+     10n:
+     1         23
+     none:
+     123
+     3n +6n T 4n +2n:
+     1  2     3   4 5   6 7   8 9
+
+     literal:
+     1       2       3
+
+     after literal:
+     1       2       3
+
+     default unit:
+     1  2   3    4
+
+OpenBSD                           May 7, 2017                          OpenBSD
diff --git a/roff.c b/roff.c
index e4253b65..8078fb99 100644
--- a/roff.c
+++ b/roff.c
@@ -1,4 +1,4 @@
-/*	$Id: roff.c,v 1.299 2017/05/05 15:17:32 schwarze Exp $ */
+/*	$Id: roff.c,v 1.300 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2008-2012, 2014 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2015, 2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -181,6 +181,7 @@ static	enum rofferr	 roff_it(ROFF_ARGS);
 static	enum rofferr	 roff_line_ignore(ROFF_ARGS);
 static	void		 roff_man_alloc1(struct roff_man *);
 static	void		 roff_man_free1(struct roff_man *);
+static	enum rofferr	 roff_manyarg(ROFF_ARGS);
 static	enum rofferr	 roff_nr(ROFF_ARGS);
 static	enum rofferr	 roff_onearg(ROFF_ARGS);
 static	enum roff_tok	 roff_parse(struct roff *, char *, int *,
@@ -212,7 +213,7 @@ static	enum rofferr	 roff_userdef(ROFF_ARGS);
 
 const char *__roff_name[MAN_MAX + 1] = {
 	"br",		"ft",		"ll",		"sp",
-	NULL,
+	"ta",		NULL,
 	"ab",		"ad",		"af",		"aln",
 	"als",		"am",		"am1",		"ami",
 	"ami1",		"as",		"as1",		"asciify",
@@ -262,7 +263,7 @@ const char *__roff_name[MAN_MAX + 1] = {
 	"shc",		"shift",	"sizes",	"so",
 	"spacewidth",	"special",	"spreadwarn",	"ss",
 	"sty",		"substring",	"sv",		"sy",
-	"T&",		"ta",		"tc",		"TE",
+	"T&",		"tc",		"TE",
 	"TH",		"ti",		"tkf",		"tl",
 	"tm",		"tm1",		"tmc",		"tr",
 	"track",	"transchar",	"trf",		"trimat",
@@ -322,6 +323,7 @@ static	struct roffmac	 roffs[TOKEN_NONE] = {
 	{ roff_onearg, NULL, NULL, 0 },  /* ft */
 	{ roff_onearg, NULL, NULL, 0 },  /* ll */
 	{ roff_onearg, NULL, NULL, 0 },  /* sp */
+	{ roff_manyarg, NULL, NULL, 0 },  /* ta */
 	{ NULL, NULL, NULL, 0 },  /* ROFF_MAX */
 	{ roff_unsupp, NULL, NULL, 0 },  /* ab */
 	{ roff_line_ignore, NULL, NULL, 0 },  /* ad */
@@ -520,7 +522,6 @@ static	struct roffmac	 roffs[TOKEN_NONE] = {
 	{ roff_line_ignore, NULL, NULL, 0 },  /* sv */
 	{ roff_insec, NULL, NULL, 0 },  /* sy */
 	{ roff_T_, NULL, NULL, 0 },  /* T& */
-	{ roff_unsupp, NULL, NULL, 0 },  /* ta */
 	{ roff_unsupp, NULL, NULL, 0 },  /* tc */
 	{ roff_TE, NULL, NULL, 0 },  /* TE */
 	{ roff_TH, NULL, NULL, 0 },  /* TH */
@@ -2800,6 +2801,29 @@ roff_onearg(ROFF_ARGS)
 }
 
 static enum rofferr
+roff_manyarg(ROFF_ARGS)
+{
+	struct roff_node	*n;
+	char			*sp, *ep;
+
+	roff_elem_alloc(r->man, ln, ppos, tok);
+	n = r->man->last;
+
+	for (sp = ep = buf->buf + pos; *sp != '\0'; sp = ep) {
+		while (*ep != '\0' && *ep != ' ')
+			ep++;
+		while (*ep == ' ')
+			*ep++ = '\0';
+		roff_word_alloc(r->man, ln, sp - buf->buf, sp);
+	}
+
+	n->flags |= NODE_LINE | NODE_VALID | NODE_ENDED;
+	r->man->last = n;
+	r->man->next = ROFF_NEXT_SIBLING;
+	return ROFF_IGN;
+}
+
+static enum rofferr
 roff_br(ROFF_ARGS)
 {
 	roff_elem_alloc(r->man, ln, ppos, ROFF_br);
diff --git a/roff.h b/roff.h
index 64f4eb8a..c4448b9f 100644
--- a/roff.h
+++ b/roff.h
@@ -1,4 +1,4 @@
-/*	$Id: roff.h,v 1.46 2017/05/05 15:17:32 schwarze Exp $	*/
+/*	$Id: roff.h,v 1.47 2017/05/07 17:31:45 schwarze Exp $	*/
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2013, 2014, 2015, 2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -70,6 +70,7 @@ enum	roff_tok {
 	ROFF_ft,
 	ROFF_ll,
 	ROFF_sp,
+	ROFF_ta,
 	ROFF_MAX,
 	ROFF_ab,
 	ROFF_ad,
@@ -271,7 +272,6 @@ enum	roff_tok {
 	ROFF_sv,
 	ROFF_sy,
 	ROFF_T_,
-	ROFF_ta,
 	ROFF_tc,
 	ROFF_TE,
 	ROFF_TH,
diff --git a/roff_html.c b/roff_html.c
index 69e9ce54..5fd65d9f 100644
--- a/roff_html.c
+++ b/roff_html.c
@@ -36,6 +36,7 @@ static	const roff_html_pre_fp roff_html_pre_acts[ROFF_MAX] = {
 	NULL,  /* ft */
 	NULL,  /* ll */
 	roff_html_pre_sp,  /* sp */
+	NULL,  /* ta */
 };
 
 
diff --git a/roff_term.c b/roff_term.c
index ea34208e..9786e0a6 100644
--- a/roff_term.c
+++ b/roff_term.c
@@ -31,12 +31,14 @@ static	void	  roff_term_pre_br(ROFF_TERM_ARGS);
 static	void	  roff_term_pre_ft(ROFF_TERM_ARGS);
 static	void	  roff_term_pre_ll(ROFF_TERM_ARGS);
 static	void	  roff_term_pre_sp(ROFF_TERM_ARGS);
+static	void	  roff_term_pre_ta(ROFF_TERM_ARGS);
 
 static	const roff_term_pre_fp roff_term_pre_acts[ROFF_MAX] = {
 	roff_term_pre_br,  /* br */
 	roff_term_pre_ft,  /* ft */
 	roff_term_pre_ll,  /* ft */
-	roff_term_pre_sp,  /* br */
+	roff_term_pre_sp,  /* sp */
+	roff_term_pre_ta,  /* ta */
 };
 
 
@@ -111,3 +113,11 @@ roff_term_pre_sp(ROFF_TERM_ARGS)
 
 	roff_term_pre_br(p, n);
 }
+
+static void
+roff_term_pre_ta(ROFF_TERM_ARGS)
+{
+	term_tab_set(p, NULL);
+	for (n = n->child; n != NULL; n = n->next)
+		term_tab_set(p, n->string);
+}
diff --git a/roff_validate.c b/roff_validate.c
index e18f3c1a..05df6cb6 100644
--- a/roff_validate.c
+++ b/roff_validate.c
@@ -35,6 +35,7 @@ static	const roff_valid_fp roff_valids[ROFF_MAX] = {
 	roff_valid_ft,  /* ft */
 	NULL,  /* ll */
 	NULL,  /* sp */
+	NULL,  /* ta */
 };
 
 
diff --git a/term.c b/term.c
index 1217d473..f2553d89 100644
--- a/term.c
+++ b/term.c
@@ -1,4 +1,4 @@
-/*	$Id: term.c,v 1.259 2017/01/08 18:16:58 schwarze Exp $ */
+/*	$Id: term.c,v 1.260 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -141,8 +141,8 @@ term_flushln(struct termp *p)
 		 * subsequent tabs into a single huge set of spaces.
 		 */
 		ntab = 0;
-		while (i < p->col && '\t' == p->buf[i]) {
-			vend = (vis / p->tabwidth + 1) * p->tabwidth;
+		while (i < p->col && p->buf[i] == '\t') {
+			vend = term_tab_next(vis);
 			vbl += vend - vis;
 			vis = vend;
 			ntab++;
@@ -192,17 +192,20 @@ term_flushln(struct termp *p)
 			vend -= vis;
 			(*p->endline)(p);
 			p->viscol = 0;
-			if (TERMP_BRIND & p->flags) {
-				vbl = p->rmargin;
-				vend += p->rmargin;
-				vend -= p->offset;
-			} else
-				vbl = p->offset;
 
-			/* use pending tabs on the new line */
+			/* Use pending tabs on the new line. */
+
+			vbl = 0;
+			while (ntab--)
+				vbl = term_tab_next(vbl);
 
-			if (0 < ntab)
-				vbl += ntab * p->tabwidth;
+			/* Re-establish indentation. */
+
+			if (p->flags & TERMP_BRIND) {
+				vbl += p->rmargin;
+				vend += p->rmargin - p->offset;
+			} else
+				vbl += p->offset;
 
 			/*
 			 * Remove the p->overstep width.
diff --git a/term.h b/term.h
index 2121dde6..b69e6715 100644
--- a/term.h
+++ b/term.h
@@ -1,4 +1,4 @@
-/*	$Id: term.h,v 1.119 2017/05/04 22:16:09 schwarze Exp $ */
+/*	$Id: term.h,v 1.120 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2011-2015, 2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -64,7 +64,6 @@ struct	termp {
 	size_t		  maxrmargin;	/* Max right margin. */
 	size_t		  maxcols;	/* Max size of buf. */
 	size_t		  offset;	/* Margin offest. */
-	size_t		  tabwidth;	/* Distance of tab positions. */
 	size_t		  col;		/* Bytes in buf. */
 	size_t		  viscol;	/* Chars on current line. */
 	size_t		  trailspace;	/* See termp_flushln(). */
@@ -130,6 +129,9 @@ int		  term_vspan(const struct termp *, const struct roffsu *);
 size_t		  term_strlen(const struct termp *, const char *);
 size_t		  term_len(const struct termp *, size_t);
 
+void		  term_tab_set(const struct termp *, const char *);
+size_t		  term_tab_next(size_t);
+
 void		  term_fontpush(struct termp *, enum termfont);
 void		  term_fontpop(struct termp *);
 void		  term_fontpopq(struct termp *, int);
diff --git a/term_ascii.c b/term_ascii.c
index df5ff139..d0afafc4 100644
--- a/term_ascii.c
+++ b/term_ascii.c
@@ -1,4 +1,4 @@
-/*	$Id: term_ascii.c,v 1.54 2016/07/31 09:29:13 schwarze Exp $ */
+/*	$Id: term_ascii.c,v 1.55 2017/05/07 17:31:45 schwarze Exp $ */
 /*
  * Copyright (c) 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2014, 2015 Ingo Schwarze <schwarze@openbsd.org>
@@ -68,7 +68,6 @@ ascii_init(enum termenc enc, const struct manoutput *outopts)
 	p = mandoc_calloc(1, sizeof(struct termp));
 
 	p->line = 1;
-	p->tabwidth = 5;
 	p->defrmargin = p->lastrmargin = 78;
 	p->fontq = mandoc_reallocarray(NULL,
 	     (p->fontsz = 8), sizeof(enum termfont));
diff --git a/term_tab.c b/term_tab.c
new file mode 100644
index 00000000..3e6fa5c1
--- /dev/null
+++ b/term_tab.c
@@ -0,0 +1,117 @@
+/*	$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;
+	}
+}