The "value" argument to the roff(7) .nr requests ends right before

the first non-digit character.  While here, implement and document
an optional sign, requesting increment or decrement, as documented
in the Ossanna/Kernighan/Ritter troff manual and supported by groff.

Reported by bentley@ on discuss@.

1 files changed, 17 insertions, 4 deletions

diff --git a/roff.c b/roff.c
index c8382154..2f3d1498 100644
--- a/roff.c
+++ b/roff.c
@@ -1360,7 +1360,7 @@ roff_ds(ROFF_ARGS)
 }
 
 void
-roff_setreg(struct roff *r, const char *name, int val)
+roff_setreg(struct roff *r, const char *name, int val, char sign)
 {
 	struct roffreg	*reg;
 
@@ -1375,11 +1375,17 @@ roff_setreg(struct roff *r, const char *name, int val)
 		reg = mandoc_malloc(sizeof(struct roffreg));
 		reg->key.p = mandoc_strdup(name);
 		reg->key.sz = strlen(name);
+		reg->val = 0;
 		reg->next = r->regtab;
 		r->regtab = reg;
 	}
 
-	reg->val = val;
+	if ('+' == sign)
+		reg->val += val;
+	else if ('-' == sign)
+		reg->val -= val;
+	else
+		reg->val = val;
 }
 
 int
@@ -1426,14 +1432,21 @@ roff_nr(ROFF_ARGS)
 {
 	const char	*key;
 	char		*val;
+	size_t		 sz;
 	int		 iv;
+	char		 sign;
 
 	val = *bufp + pos;
 	key = roff_getname(r, &val, ln, pos);
 
-	iv = mandoc_strntoi(val, strlen(val), 10);
+	sign = *val;
+	if ('+' == sign || '-' == sign)
+		val++;
+
+	sz = strspn(val, "0123456789");
+	iv = sz ? mandoc_strntoi(val, sz, 10) : 0;
 
-	roff_setreg(r, key, iv);
+	roff_setreg(r, key, iv, sign);
 
 	return(ROFF_IGN);
 }