Implement nose coupling so run-arounds work, and stop coupling wiping the district

2026-08-01 15:15:28 -04:00
parent f52ff0e9ac
commit e23d81eade
12 changed files with 366 additions and 23 deletions
@@ -10,7 +10,43 @@ The target is 20 Revenue over 5 Days.

 ## Unreleased

-Nothing yet.
+### Teaching the bot to use a siding
+
+**Nose coupling, which the rules had and the engine did not.** §A.3: "engines also have couplers on
+the front end, so a train can pick cars up onto its nose". Coupling always appended to the back, so
+which end cars landed on did not exist — and that is precisely what a run-around is for. Cars met
+running FORWARD now couple in front, cars met BACKING couple behind, so the approach decides which
+car is next off the tail:
+
+    running forward -> reefer, boxcar, hopper   next off: hopper
+    backing up      -> boxcar, hopper, reefer   next off: reefer
+
+The bot prefers a run-around to setting a car down, since it keeps the car — but only when the drop
+can actually follow. Without that guard it ran the loop for its own sake (8 a game, 63 Moves), which
+the shuttling regression correctly failed.
+
+**A serious bug found on the way.** The `carsCoupled` reducer cleared **every card in the Office
+Area**, not the ones the crew ran over — its own comment said "every card along the path" while the
+code iterated the whole grid. One coupling anywhere deleted every standing car and every industry
+track in the district, so loads worked over several Stages vanished when a crew picked up an
+unrelated boxcar somewhere else. The event now names the cards it lifted from.
+
+**A test replaced rather than relaxed.** "Under 60 Moves a game" started failing. That threshold was
+calibrated when the crew coupled ~0.4 cars a game; it now couples ~8 and drops ~11, and a run-around
+is *supposed* to cost several Moves. Counting Moves was always a proxy for aimlessness, so the test
+now measures the thing itself — Moves per drop-or-coupling — which still fails when the bot is made
+to wander.
+
+| | before sidings | now |
+| --- | --- | --- |
+| revenue | 3.9 | 3.2 |
+| freight | 0.9 | 1.0 |
+| drops per game | 3.3 | **11.1** |
+| couplings per game | ~0.4 | **7.9** |
+
+Switching is transformed; revenue is not. The crew now does real work — roughly 4 Moves per
+productive act, which is what running a loop costs — but that work is not yet converting into
+Revenue. Where it goes next is an open question rather than a known fix.

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