stop a flat top stamping its rim down the plateau

A cliff mound is drawn as a rim over a body: its top edge, then the same
rock the whole way down. The top face cycles the first two drawn rows to
fill its depth, so it laid that rim again every second tile. The mound the
Diglett's Cave mouth is cut into came out with three rim lines across it
instead of one along its north edge.

Where the drawing says the body is all one tile, lay the rim once and hold
the body after it. Art that genuinely repeats keeps cycling: the Safari
Zone's fence alternates two tiles the whole way down, and there the repeat
is what the drawing says.

Answered per column and per region, because each catches what the other
misses. The columns carrying a mound's cave mouth end in the mouth's own
tiles, so per column alone they kept cycling while their neighbours held,
leaving rim stubs above the doorway. A region vote alone silences a real
rim-over-body column standing in a region of repeating art, of which the
Safari Zone has three. A column holds if either says so.

Geometry is untouched: the silhouette is pixel for pixel what it was, and
only the texel a top face wears changes. Of 3088 flat-topped runs, the
1336 rim-over-body ones change and nothing else does.

tests/flat_top_test.lua walks every map and fails if any rim-over-body run
revisits an earlier drawn row.

lib/Structures.lua

@@ -2225,6 +2225,43 @@ function Structures.buildVolume(S, map, tiles)
   -- whether the region's dominant columns are flat repeats (a cliff
   -- mound's plateau) rather than drawn facades (a house's front)
   local modeRepeat = (repeatVotes[modeH] or 0) * 2 > modeN
+
+  -- Whether this REGION's tops are a rim over a uniform body -- what every
+  -- cliff mound is drawn as: a top edge, then the same rock the whole way
+  -- down. The top face may then lay that rim once along its north edge and
+  -- hold the body after it, instead of cycling the rim back every second
+  -- tile and striping a plateau with edges it should not have.
+  --
+  -- Answered per column AND per region, because each catches what the
+  -- other misses. A mound is one structure many columns wide, and the
+  -- columns carrying its cave mouth read differently from their neighbours
+  -- (their drawing ends in the mouth's own tiles): per column alone, those
+  -- kept cycling while the rest held, leaving rim stubs above the doorway.
+  -- But a region vote alone silences a genuine rim-over-body column that
+  -- happens to stand in a region of repeating art -- three of them in the
+  -- Safari Zone. A column holds if EITHER says so.
+  --
+  -- Art that genuinely repeats is not uniform and keeps cycling: the
+  -- Safari Zone's fence alternates two tiles the whole way down, and there
+  -- the repeat IS what the drawing says.
+  local uniformVotes, uniformTotal = 0, 0
+  for _, r in ipairs(runs) do
+    local run = r.run
+    if run.extent > 2 then
+      uniformTotal = uniformTotal + 1
+      local body = map:tileAt(r.tx, run.north + 1)
+      local uniform = true
+      for d = 2, run.extent - 1 do
+        if map:tileAt(r.tx, run.north + d) ~= body then
+          uniform = false
+          break
+        end
+      end
+      run.ownUniform = uniform
+      if uniform then uniformVotes = uniformVotes + 1 end
+    end
+  end
+  local regionUniform = uniformTotal > 0 and uniformVotes * 2 > uniformTotal
   for _, r in ipairs(runs) do
     local run = r.run
     local h = run.unit * 8
@@ -2272,6 +2309,7 @@ function Structures.buildVolume(S, map, tiles)
     run.rise = roofRows * 8
     run.peak = h
     run.h = h - run.rise               -- facade height: what sides build to
+    run.topUniform = run.ownUniform or regionUniform
     for ty = run.north, run.front do
       S.runs[keyOf(r.tx, ty)] = run
     end