From 987be7e8e55eca327530629c625f43fa67178223 Mon Sep 17 00:00:00 2001
From: nhmall <nhmall@nethack.org>
Date: Sat, 16 Dec 2023 11:38:55 -0500
Subject: [PATCH] a pair of domove_core() blocks

Following line 2425 of hack.c, in domove_core():
    mtmp = m_at(x, y);
mtmp can be null.

There were two if blocks following that, both of which
only make sense when mtmp is not null.

One of them was explicitly checking for mtmp being non-null,
and the other was avoiding catastrophe by relying on a
hidden check buried within an _is_safepet(mon) macro.

Place both of those blocks into an
    if (mtmp) { }
block.

99% of the diff is just indentation.
---
 src/hack.c | 79 +++++++++++++++++++++++++++++-------------------------
 1 file changed, 42 insertions(+), 37 deletions(-)

diff --git a/src/hack.c b/src/hack.c
index d314e2c2f..1f18b41fe 100644
--- a/src/hack.c
+++ b/src/hack.c
@@ -2557,6 +2557,8 @@ domove_core(void)
         return;
 
     mtmp = m_at(x, y);
+    /* mtmp can be null at this point */
+
     /* tentatively move the hero plus steed; leave CLIPPING til later */
     u.ux += u.dx;
     u.uy += u.dy;
@@ -2567,47 +2569,50 @@ domove_core(void)
         exercise_steed(); /* train riding skill */
     }
 
-    if (displaceu && mtmp) {
-        boolean noticed_it = (canspotmon(mtmp) || glyph_is_invisible(glyph)
-                              || glyph_is_warning(glyph));
+    if (mtmp) {
+        if (displaceu) {
+            boolean noticed_it = (canspotmon(mtmp) || glyph_is_invisible(glyph)
+                                  || glyph_is_warning(glyph));
 
-        remove_monster(u.ux, u.uy);
-        place_monster(mtmp, u.ux0, u.uy0);
-        newsym(u.ux, u.uy);
-        newsym(u.ux0, u.uy0);
-        /* monst still knows where hero is */
-        mtmp->mux = u.ux, mtmp->muy = u.uy;
+            remove_monster(u.ux, u.uy);
+            place_monster(mtmp, u.ux0, u.uy0);
+            newsym(u.ux, u.uy);
+            newsym(u.ux0, u.uy0);
+            /* monst still knows where hero is */
+            mtmp->mux = u.ux, mtmp->muy = u.uy;
 
-        pline("%s swaps places with you...",
-              !noticed_it ? Something : Monnam(mtmp));
-        if (!canspotmon(mtmp))
-            map_invisible(u.ux0, u.uy0);
-        /* monster chose to swap places; hero doesn't get any credit
-           or blame if something bad happens to it */
-        gc.context.mon_moving = 1;
-        if (!minliquid(mtmp))
-            (void) mintrap(mtmp, NO_TRAP_FLAGS);
-        gc.context.mon_moving = 0;
+            pline("%s swaps places with you...",
+                  !noticed_it ? Something : Monnam(mtmp));
+            if (!canspotmon(mtmp))
+                map_invisible(u.ux0, u.uy0);
+            /* monster chose to swap places; hero doesn't get any credit
+               or blame if something bad happens to it */
+            gc.context.mon_moving = 1;
+            if (!minliquid(mtmp))
+                (void) mintrap(mtmp, NO_TRAP_FLAGS);
+            gc.context.mon_moving = 0;
 
-    /*
-     * If safepet at destination then move the pet to the hero's
-     * previous location using the same conditions as in do_attack().
-     * there are special extenuating circumstances:
-     * (1) if the pet dies then your god angers,
-     * (2) if the pet gets trapped then your god may disapprove.
-     *
-     * Ceiling-hiding pets are skipped by this section of code, to
-     * be caught by the normal falling-monster code.
-     */
-    } else if (is_safemon(mtmp)
-               && !(is_hider(mtmp->data) && mtmp->mundetected)) {
-        if (!domove_swap_with_pet(mtmp, x, y)) {
-            u.ux = u.ux0, u.uy = u.uy0; /* didn't move after all */
-            /* could skip this bit since we're about to call u_on_newpos() */
-            if (u.usteed)
-                u.usteed->mx = u.ux, u.usteed->my = u.uy;
+        /*
+         * If safepet at destination then move the pet to the hero's
+         * previous location using the same conditions as in do_attack().
+         * there are special extenuating circumstances:
+         * (1) if the pet dies then your god angers,
+         * (2) if the pet gets trapped then your god may disapprove.
+         *
+         * Ceiling-hiding pets are skipped by this section of code, to
+         * be caught by the normal falling-monster code.
+         */
+        } else if (is_safemon(mtmp)
+                   && !(is_hider(mtmp->data) && mtmp->mundetected)) {
+            if (!domove_swap_with_pet(mtmp, x, y)) {
+                u.ux = u.ux0, u.uy = u.uy0; /* didn't move after all */
+                /* could skip this bit since we're about to call u_on_newpos() */
+                if (u.usteed)
+                    u.usteed->mx = u.ux, u.usteed->my = u.uy;
+            }
         }
-    }
+    }  /* mtmp != NULL */
+
     /* tentative move above didn't handle CLIPPING, in case there was a
        monster in the way and the move attempt ended up being blocked;
        do a full re-position now, possibly back to where hero started */