fix #H30 - rn2(0) from off by 1 bug in special level door creation

From a bug report, the placement
of random doors by the code that loads special levels would attempt to
evaluate rn2(0) and either get a divide by zero crash (normal build) or an
impossible warning (DEBUG enabled when compiing rnd.c, done automatically
when BETA is defined).  The problem was only noticable for random door in
a 1x1 room; none of our distributed levels specify such a thing so regular
users won't have encountered this bug.  It's a one line fix.

     Altar placement in temples also had a quirk of a similar nature.  It
wouldn't trigger rn2(0) problems but would always place the altar to left
of mid-point in rooms with even width and above the center point in ones
with even height.  Now the placement is randomized so that sometimes it'll
be to the right and/or below mid-point in such cases.

     This also simplifies a couple other instances of similar expressions
that I spotted.

doc/fixes34.4

@@ -181,6 +181,8 @@ only count successful statue creations against the monster limit in sp_lev.c
 unseen wand of striking zapped by unseen monster became known if it hit a door
 tweak knight quest messages
 guidebook grammar bits
+special level loader wasn't able to place random door in 1x1 room; could
+	trigger divide-by-0 crash for user-developed custom levels
 
 
 Platform- and/or Interface-Specific Fixes

src/mklev.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)mklev.c	3.5	2001/11/29	*/
+/*	SCCS Id: @(#)mklev.c	3.5	2006/01/28	*/
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -83,8 +83,8 @@ xchar xl,yl,xh,yh;
 {
 	register xchar x, y;
 
-	x = (xl == xh) ? xl : (xl + rn2(xh-xl+1));
-	y = (yl == yh) ? yl : (yl + rn2(yh-yl+1));
+	x = rn1(xh - xl + 1, xl);
+	y = rn1(yh - yl + 1, yl);
 	if(okdoor(x, y))
 		goto gotit;
 

src/mkroom.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)mkroom.c	3.5	2005/03/12	*/
+/*	SCCS Id: @(#)mkroom.c	3.5	2006/01/28	*/
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -483,11 +483,19 @@ shrine_pos(roomno)
 int roomno;
 {
 	static coord buf;
+	int delta;
 	struct mkroom *troom = &rooms[roomno - ROOMOFFSET];
 
-	buf.x = troom->lx + ((troom->hx - troom->lx) / 2);
-	buf.y = troom->ly + ((troom->hy - troom->ly) / 2);
-	return(&buf);
+	/* if width and height are odd, placement will be the exact center;
+	   if either or both are even, center point is a hypothetical spot
+	   between map locations and placement will be adjacent to that */
+	delta = troom->hx - troom->lx;
+	buf.x = troom->lx + delta / 2;
+	if ((delta % 2) && rn2(2)) buf.x++;
+	delta = troom->hy - troom->ly;
+	buf.y = troom->ly + delta / 2;
+	if ((delta % 2) && rn2(2)) buf.y++;
+	return &buf;
 }
 
 STATIC_OVL void
@@ -555,14 +563,14 @@ int
 somex(croom)
 register struct mkroom *croom;
 {
-	return rn2(croom->hx-croom->lx+1) + croom->lx;
+	return rn1(croom->hx - croom->lx + 1, croom->lx);
 }
 
 int
 somey(croom)
 register struct mkroom *croom;
 {
-	return rn2(croom->hy-croom->ly+1) + croom->ly;
+	return rn1(croom->hy - croom->ly + 1, croom->ly);
 }
 
 boolean

src/sp_lev.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)sp_lev.c	3.5	2006/01/02	*/
+/*	SCCS Id: @(#)sp_lev.c	3.5	2006/01/28	*/
 /*	Copyright (c) 1989 by Jean-Christophe Collet */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -620,7 +620,8 @@ struct mkroom *broom;
 		dpos = dd->pos;
 		if (dpos == -1)	/* The position is RANDOM */
 		    dpos = rn2((dwall == W_WEST || dwall == W_EAST) ?
-			    (broom->hy - broom->ly) : (broom->hx - broom->lx));
+			       (broom->hy - broom->ly + 1) :
+			       (broom->hx - broom->lx + 1));
 
 		/* Convert wall and pos into an absolute coordinate! */