The \h escape sequence provides another method for moving backwards,

and after that, previously written output gets overwritten, but
overwriting with blanks does *not* erase previously written content.
Yes, manual pages exist that are crazy enough to rely on that...

diff --git a/usr.bin/mandoc/term.c b/usr.bin/mandoc/term.c
index b6ca145..a7316d1 100644 (file)
--- a/usr.bin/mandoc/term.c
+++ b/usr.bin/mandoc/term.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: term.c,v 1.124 2017/06/04 22:43:50 schwarze Exp $ */
+/*     $OpenBSD: term.c,v 1.125 2017/06/07 02:13:52 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -115,13 +115,13 @@ term_flushln(struct termp *p)
        vis = vend = 0;
        i = 0;
 
-       while (i < p->col) {
+       while (i < p->lastcol) {
                /*
                 * Handle literal tab characters: collapse all
                 * subsequent tabs into a single huge set of spaces.
                 */
                ntab = 0;
-               while (i < p->col && p->buf[i] == '\t') {
+               while (i < p->lastcol && p->buf[i] == '\t') {
                        vend = term_tab_next(vis);
                        vbl += vend - vis;
                        vis = vend;
@@ -136,7 +136,7 @@ term_flushln(struct termp *p)
                 * space is printed according to regular spacing rules).
                 */
 
-               for (j = i, jhy = 0; j < p->col; j++) {
+               for (j = i, jhy = 0; j < p->lastcol; j++) {
                        if (' ' == p->buf[j] || '\t' == p->buf[j])
                                break;
 
@@ -191,14 +191,14 @@ term_flushln(struct termp *p)
                }
 
                /* Write out the [remaining] word. */
-               for ( ; i < p->col; i++) {
+               for ( ; i < p->lastcol; i++) {
                        if (vend > bp && jhy > 0 && i > jhy)
                                break;
                        if ('\t' == p->buf[i])
                                break;
                        if (' ' == p->buf[i]) {
                                j = i;
-                               while (i < p->col && ' ' == p->buf[i])
+                               while (i < p->lastcol && ' ' == p->buf[i])
                                        i++;
                                dv = (i - j) * (*p->width)(p, ' ');
                                vbl += dv;
@@ -241,7 +241,7 @@ term_flushln(struct termp *p)
        else
                vis = 0;
 
-       p->col = 0;
+       p->col = p->lastcol = 0;
        p->minbl = p->trailspace;
        p->flags &= ~(TERMP_BACKAFTER | TERMP_BACKBEFORE | TERMP_NOPAD);
 
@@ -285,7 +285,7 @@ term_newln(struct termp *p)
 {
 
        p->flags |= TERMP_NOSPACE;
-       if (p->col || p->viscol)
+       if (p->lastcol || p->viscol)
                term_flushln(p);
 }
 
@@ -554,10 +554,12 @@ term_word(struct termp *p, const char *word)
                                }
                        }
                        /* Trim trailing backspace/blank pair. */
-                       if (p->col > 2 &&
-                           (p->buf[p->col - 1] == ' ' ||
-                            p->buf[p->col - 1] == '\t'))
-                               p->col -= 2;
+                       if (p->lastcol > 2 &&
+                           (p->buf[p->lastcol - 1] == ' ' ||
+                            p->buf[p->lastcol - 1] == '\t'))
+                               p->lastcol -= 2;
+                       if (p->col > p->lastcol)
+                               p->col = p->lastcol;
                        continue;
                default:
                        continue;
@@ -602,7 +604,10 @@ bufferc(struct termp *p, char c)
        }
        if (p->col + 1 >= p->maxcols)
                adjbuf(p, p->col + 1);
-       p->buf[p->col++] = c;
+       if (p->lastcol <= p->col || (c != ' ' && c != ASCII_NBRSP))
+               p->buf[p->col] = c;
+       if (p->lastcol < ++p->col)
+               p->lastcol = p->col;
 }
 
 /*
@@ -644,7 +649,10 @@ encode1(struct termp *p, int c)
                        p->buf[p->col++] = c;
                p->buf[p->col++] = 8;
        }
-       p->buf[p->col++] = c;
+       if (p->lastcol <= p->col || (c != ' ' && c != ASCII_NBRSP))
+               p->buf[p->col] = c;
+       if (p->lastcol < ++p->col)
+               p->lastcol = p->col;
        if (p->flags & TERMP_BACKAFTER) {
                p->flags |= TERMP_BACKBEFORE;
                p->flags &= ~TERMP_BACKAFTER;
@@ -670,7 +678,10 @@ encode(struct termp *p, const char *word, size_t sz)
                    isgraph((unsigned char)word[i]))
                        encode1(p, word[i]);
                else {
-                       p->buf[p->col++] = word[i];
+                       if (p->lastcol <= p->col ||
+                           (word[i] != ' ' && word[i] != ASCII_NBRSP))
+                               p->buf[p->col] = word[i];
+                       p->col++;
 
                        /*
                         * Postpone the effect of \z while handling
@@ -684,6 +695,8 @@ encode(struct termp *p, const char *word, size_t sz)
                        }
                }
        }
+       if (p->lastcol < p->col)
+               p->lastcol = p->col;
 }
 
 void

diff --git a/usr.bin/mandoc/term.h b/usr.bin/mandoc/term.h
index b625a23..d9bd128 100644 (file)
--- a/usr.bin/mandoc/term.h
+++ b/usr.bin/mandoc/term.h
@@ -1,4 +1,4 @@
-/*     $OpenBSD: term.h,v 1.67 2017/06/04 22:43:50 schwarze Exp $ */
+/*     $OpenBSD: term.h,v 1.68 2017/06/07 02:13:52 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2011-2015, 2017 Ingo Schwarze <schwarze@openbsd.org>
@@ -62,7 +62,8 @@ struct        termp {
        size_t            maxrmargin;   /* Max right margin. */
        size_t            maxcols;      /* Max size of buf. */
        size_t            offset;       /* Margin offest. */
-       size_t            col;          /* Bytes in buf. */
+       size_t            col;          /* Byte position in buf. */
+       size_t            lastcol;      /* Bytes in buf. */
        size_t            viscol;       /* Chars on current line. */
        size_t            trailspace;   /* See termp_flushln(). */
        size_t            minbl;        /* Minimum blanks before next field. */