v0.4.5 - fix bug with not being able to drop cars

src/engine/apply.ts

@@ -138,6 +138,23 @@ function farPort(card: TrackCard | undefined, entry: Port): Port {
   return (card ? exitsFrom(card, entry)[0] : undefined) ?? opposite(entry);
 }
 
+/**
+ * THE PORT A TRAIN BACKS OUT BY — the other end of the card it is standing on.
+ *
+ * Not `opposite(facing)`. A train stands on a two-port card (never a turnout: §A.1 forbids
+ * finishing a Move on one), and backing up means leaving by whichever of those two ports it is not
+ * facing. On a straight those coincide; on a curve they never do, because a curve joins ADJACENT
+ * edges — a crew facing north on a north-west curve backs out through WEST, and `opposite('n')` is
+ * a south port the card does not have.
+ *
+ * The consequence of getting this wrong is total: `exploreMoves` returns nothing at all from a port
+ * the card lacks, so the crew simply cannot back up. It could round a curve and never come off it,
+ * which is enough to make a siding unreachable and setting out a cut impossible.
+ */
+function reversePort(s: GameState, player: PlayerIndex, from: GridCoord, facing: Port): Port {
+  return farPort(areaOf(s, player).grid.get(coordKey(from)), facing);
+}
+
 /** A tray's facing, expressed as the port it would leave by going forward. */
 function facingPort(s: GameState, trayId: TrayId): Port {
   const tray = s.trays.get(trayId);
@@ -1118,11 +1135,7 @@ function destinationsFor(
 ) {
   const tray = s.trays.get(trayId)!;
   const facing = facingPort(s, trayId);
-  // Reversing is the OPPOSITE port, whichever it is. This was hardcoded to flip between east and
-  // west, so a crew facing north or south reversed to 'e' — a port a north-south card does not
-  // have — and could never back out of a district spur. Combined with a facing that was itself
-  // derived from an east/west direction, it stranded 29 of 62 leftover crews on north-south track.
-  const exit: Port = reverse ? opposite(facing) : facing;
+  const exit: Port = reverse ? reversePort(s, player, from, facing) : facing;
   return reachableDestinations(
     {
       area: areaOf(s, player),
@@ -1161,7 +1174,10 @@ export function movesFor(
   };
   const facing = facingPort(s, trayId);
   const forward = exploreMoves(ctx, from, facing);
-  const back = exploreMoves(ctx, from, opposite(facing));
+  // The card's other end, not the compass opposite — see `reversePort`. This is what the board
+  // highlights, so getting it wrong hides half the crew's legal moves rather than merely refusing
+  // one: the squares behind the train never light up at all.
+  const back = exploreMoves(ctx, from, reversePort(s, player, from, facing));
 
   const to = new Map<string, GridCoord>();
   for (const d of [...forward.destinations, ...back.destinations]) to.set(coordKey(d.coord), d.coord);