Give shafts in corridor ends a good chance of being preknown.

3 files changed, 18 insertions, 7 deletions

diff --git a/crawl-ref/source/dungeon.cc b/crawl-ref/source/dungeon.cc
index 05bd1b9fdd..153091bbf6 100644
--- a/crawl-ref/source/dungeon.cc
+++ b/crawl-ref/source/dungeon.cc
@@ -2989,6 +2989,8 @@ static builder_rc_type _builder_basic(int level_number)
             ts.pos.x = xend;
             ts.pos.y = yend;
             grd[xend][yend] = DNGN_UNDISCOVERED_TRAP;
+            if (shaft_known(level_number, false))
+                ts.reveal();
 #ifdef DEBUG_DIAGNOSTICS
             mprf(MSGCH_DIAGNOSTICS, "Trail ends in shaft.");
 #endif
@@ -3173,14 +3175,9 @@ static void _place_traps(int level_number)
             }
         }
 
-        if (ts.type == TRAP_SHAFT &&  // Shafts can be generated visible
-            coinflip() && // Starts about 50% of the time
-            random2(level_number) < 3) // And gets less frequent
-        {
+        grd(ts.pos) = DNGN_UNDISCOVERED_TRAP;
+        if (ts.type == TRAP_SHAFT && shaft_known(level_number, true))
             ts.reveal();
-        } else {
-            grd(ts.pos) = DNGN_UNDISCOVERED_TRAP;
-        }
         ts.prepare_ammo();
     }
 }
diff --git a/crawl-ref/source/traps.cc b/crawl-ref/source/traps.cc
index 824f6fc4bd..e3d1c0ddc7 100644
--- a/crawl-ref/source/traps.cc
+++ b/crawl-ref/source/traps.cc
@@ -1315,6 +1315,19 @@ bool is_valid_shaft_level(const level_id &place)
     return ((branch.depth - place.depth) >= min_delta);
 }
 
+// Shafts can be generated visible.
+// 
+// Starts about 50% of the time and approaches 0% for randomly
+// placed traps, and starts at 100% and approaches 50% for
+// others (e.g. at end of corridor).
+bool shaft_known(int depth, bool randomly_placed)
+{
+    if (randomly_placed)
+        return (coinflip() && x_chance_in_y(3, depth));
+    else
+        return (coinflip() || x_chance_in_y(3, depth));
+}
+
 level_id generic_shaft_dest(level_pos lpos, bool known = false)
 {
     level_id  lid   = lpos.id;
diff --git a/crawl-ref/source/traps.h b/crawl-ref/source/traps.h
index 7d761e85d2..c39b70482c 100644
--- a/crawl-ref/source/traps.h
+++ b/crawl-ref/source/traps.h
@@ -34,6 +34,7 @@ trap_def* find_trap(const coord_def& where);
 trap_type get_trap_type(const coord_def& where);
 
 bool     is_valid_shaft_level(const level_id &place = level_id::current());
+bool     shaft_known(int depth, bool randomly_placed);
 level_id generic_shaft_dest(coord_def pos, bool known);
 void     handle_items_on_shaft(const coord_def& where, bool open_shaft);