Credit cards may slip through a lock when cursed or hero is fumbling

Note by copperwater: Another slight nerf to credit cards as an unlocking
tool. In the original concept of the idea this was possible to happen
for a non-cursed credit card when the hero wasn't fumbling, but I
figured it should only really punish when there's some negative
circumstance mucking things up.

As implemented this patch had a flat 5% chance if cursed or fumbling; I
modified it back to the original idea which is Dex based. I also noticed
that the new code was inserted at the wrong point, before the "still
busy" check, meaning this check would have triggered every turn of
unlocking, rather than once per the unlock attempt. Moved it so that it
triggers after the unlock attempt resolves, which is not perfect
(ideally it would have a chance of happening at any point during the
process) but good enough.

doc/xnh-changelog-9.0.md

@@ -26,6 +26,8 @@ changes:
 - Every lock in the game may be unopenable by using a credit card. Boxes have a
   40% chance, and doors have a chance that starts at 10% and increases with
   depth.
+- Also, if the card is cursed or you are fumbling, the card may slip through the
+  lock, ending up inside a box or on the other side of a door.
 - Cockatrice nests may contain chickatrices and cockatrice eggs.
 - Eating troll meat provides 2d6 turns of intrinsic regneneration.
 - Drinking from a magic fountain when either HP or Pw is below 60% may give a

src/lock.c

@@ -67,7 +67,9 @@ lock_action(void)
 static int
 picklock(void)
 {
-    coordxy doorx = u.ux + u.dx, doory = u.uy + u.dy;
+    coordxy doorx = u.ux + u.dx, doory = u.uy + u.dy, behind_x, behind_y;
+    boolean credit_card_slipped;
+
     if (gx.xlock.box) {
         if (gx.xlock.box->where != OBJ_FLOOR
             || gx.xlock.box->ox != u.ux || gx.xlock.box->oy != u.uy) {
@@ -129,6 +131,30 @@ picklock(void)
     if (rn2(100) >= gx.xlock.chance)
         return 1; /* still busy */
 
+    if (gx.xlock.picktyp == CREDIT_CARD && (Fumbling || gx.xlock.pick->cursed)
+        /* 1/(Dex/4) chance if Fumbling; 1/2*Dex chance otherwise */
+        && !rn2(Fumbling ? ((ACURR(A_DEX) + 3) / 4)
+                         : ACURR(A_DEX) * 2)) {
+        if (gx.xlock.door) {
+            behind_x = doorx + (gx.xlock.door->horizontal ? 0 : u.dx);
+            behind_y = doory + (gx.xlock.door->horizontal ? u.dy : 0);
+            credit_card_slipped = isok(behind_x, behind_y)
+                                  && SPACE_POS(levl[behind_x][behind_y].typ);
+        } else {
+            credit_card_slipped = TRUE;
+        }
+        if (credit_card_slipped) {
+            Your("%s slips through the crack!", xname(gx.xlock.pick));
+            obj_extract_self(gx.xlock.pick);
+            if (gx.xlock.door) {
+                place_object(gx.xlock.pick, behind_x, behind_y);
+            } else {
+                add_to_container(gx.xlock.box, gx.xlock.pick);
+            }
+            return ((gx.xlock.usedtime = 0));
+        }
+    }
+
     /* using the Master Key of Thievery finds traps if its bless/curse
        state is adequate (non-cursed for rogues, blessed for others;
        checked when setting up 'xlock') */