v0.4.8 — the Limits bound the whole district, the nine spots really are nine, and an action points at its square

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YQAJ4dND7enLj54eLyiF2C

src/engine/apply.ts

@@ -77,6 +77,7 @@ import {
   opposite,
   reachableDestinations,
   variantsFor,
+  withinLimits,
 } from './track.ts';
 
 export type ApplyResult =
@@ -1105,6 +1106,9 @@ function checkPlay(
         if (placement.row === area.runningRow && !carriesThroughTrack(proto)) {
           return 'BREAKS_RUNNING_TRACK';
         }
+        // Same reasoning: a siding reaching past your own sign JOINS perfectly well, and is refused
+        // because it leaves your territory (§2.1). `canPlaceAt` enforces it too — this only names it.
+        if (!withinLimits(area, placement)) return 'OUTSIDE_LIMITS';
         return canPlaceAt(area, placement, proto) ? null : 'NOT_CONNECTED';
       }
 
@@ -1123,6 +1127,10 @@ function checkPlay(
        * main line.
        */
       if (placement.row === area.runningRow) return 'ON_RUNNING_TRACK';
+      // A Facility CARRIES TRACK — "placing a Facility places track" (§11.2) — so it is bounded by
+      // the Limits exactly as a siding is. An industry outside them is how a district used to leave
+      // its own territory.
+      if (!withinLimits(area, placement)) return 'OUTSIDE_LIMITS';
       // Q4 — a lockout prevents BUILDING both in one district: no duplicate, and never a producer
       // alongside the consumer of the same commodity.
       if (isLockedOut(area, card.kind.facility)) return 'FACILITY_LOCKED';
@@ -1133,6 +1141,18 @@ function checkPlay(
     case 'modifier': {
       if (!placement) return 'NO_PLACEMENT';
       if (area.grid.has(coordKey(placement))) return 'NOT_CONNECTED';
+      /**
+       * NOT ON THE RUNNING TRACK ROW — AND NOT BOUNDED BY THE LIMITS EITHER. Jesse's call, both
+       * halves.
+       *
+       * A Modifier is not track (§9), so unlike a siding it may hang outside the Limits: a Facility
+       * standing at the limit has three of its nine spots out there, and refusing them would make
+       * the card unplayable exactly where the district ends. What it may NOT do is stand in the row
+       * the Running Track grows along. Inside the Limits that row is always full, so this bites only
+       * beyond the sign — which is the ground the main extends onto, and a Modifier parked there
+       * would block your own sign from moving outward (§2.1) with nothing on screen to warn you.
+       */
+      if (placement.row === area.runningRow) return 'ON_RUNNING_TRACK';
       // One of a kind per Office Area, as with industries (Q4) — no district gets two Ice Houses.
       if (hasModifierInArea(area, card.kind.modifier)) return 'FACILITY_LOCKED';
       // §9 — a Modifier is not track. It must sit adjacent to a Facility THAT CAN HOST IT (one of

src/engine/intents.ts

@@ -143,6 +143,11 @@ export type RejectionCode =
   | 'ON_RUNNING_TRACK'
   /** A card with no east-west road would dead-end the Running Track it was laid in. */
   | 'BREAKS_RUNNING_TRACK'
+  /**
+   * §2.1 — track, and the Facilities that carry it, stay inside your own Limits at every row. A
+   * Modifier is not track and is exempt (§9's nine spots), which is why this is not shared with it.
+   */
+  | 'OUTSIDE_LIMITS'
   /**
    * A turnout may be laid on top of a straight, or of a curve on the same arc as its diverging leg
    * — but only those, and only while the square is idle. See `checkTurnoutUpgrade`.

src/engine/legal.ts

@@ -313,5 +313,35 @@ function placementCandidates(s: GameState, player: PlayerIndex): GridCoord[] {
       out.push(c);
     }
   }
+
+  /**
+   * THE NINE-SPOT MODIFIER NEIGHBOURHOOD (§9), which the four-square walk above cannot reach.
+   *
+   * A Modifier goes "adjacent to a Facility, on any of the nine nearby spots" — diagonals included —
+   * and `check` has always accepted all eight. It was the CANDIDATES that were orthogonal-only, so a
+   * diagonal square with no orthogonal neighbour was legal and never offered: reported by Jesse as
+   * being unable to place a Modifier to the south-east of his industry.
+   *
+   * Only around a Facility, and only in a second pass. Track has to JOIN, and a diagonal shares no
+   * edge, so a Modifier is the only card these squares can ever take — generating diagonals around
+   * every card instead costs 54% more simulation time for candidates `check` then rejects. Appending
+   * rather than interleaving leaves the existing order untouched, which matters because the bot
+   * breaks ties by first-best: measured over 200 seeded games, 0 played differently.
+   */
+  for (const [key, card] of area.grid) {
+    if (!card.facility) continue;
+    const [row, col] = key.split(',').map(Number);
+    for (const c of [
+      { row: row! + 1, col: col! + 1 },
+      { row: row! + 1, col: col! - 1 },
+      { row: row! - 1, col: col! + 1 },
+      { row: row! - 1, col: col! - 1 },
+    ]) {
+      const k = `${c.row},${c.col}`;
+      if (seen.has(k) || area.grid.has(k)) continue;
+      seen.add(k);
+      out.push(c);
+    }
+  }
   return out;
 }

src/engine/track.ts

@@ -559,6 +559,31 @@ export function carriesThroughTrack(card: TrackCard): boolean {
  * a TURNOUT laid on the Running Track — the Office no longer carries a stub of its own, because no
  * office or industry card on the printed sheet does.
  */
+/**
+ * §2.1 — IS THIS SQUARE INSIDE THE DISTRICT AT ALL?
+ *
+ * The Limits sign denotes "the limit of your control area", and §3 defines Secondary Track as "all
+ * tracks IN YOUR LIMITS that are not the Running Track" — so the bound belongs to the district, not
+ * to the one row the sign happens to stand in. This used to guard the Running Track alone, on the
+ * reasoning that the rows above and below were unbounded; the consequence was a siding snaking east
+ * past your own sign, taking industries with it, and a player switching cars on track outside the
+ * territory §8.1 and §10 reason about. Reported by Jesse: sidings must not be built outside the
+ * Limits.
+ *
+ * INCLUSIVE OF THE SIGN'S OWN COLUMN. The sign stands ON the boundary rather than beyond it, and the
+ * opening district is signs at ±1 around the Office — so the strict reading would leave exactly one
+ * buildable column and break §11.3's promise that both Secondary rows, and the nine-spot Modifier
+ * neighbourhood, are usable from the first Stage.
+ *
+ * MODIFIERS ARE NOT SUBJECT TO THIS, and are not track: §9 places one on any of the nine spots
+ * around a Facility, and a Facility standing at the limit has three of its nine outside them.
+ * Jesse's call. `check` bars them from the Running Track ROW instead, which is the ground the main
+ * grows onto.
+ */
+export function withinLimits(area: OfficeArea, coord: GridCoord): boolean {
+  return coord.col >= area.limitsWest.col && coord.col <= area.limitsEast.col;
+}
+
 export function canPlaceAt(area: OfficeArea, coord: GridCoord, card: TrackCard): boolean {
   const existing = cardAt(area, coord);
   // A Limits sign on the Running Track is the GROWTH POINT, not an obstacle. Extending means laying
@@ -575,14 +600,12 @@ export function canPlaceAt(area: OfficeArea, coord: GridCoord, card: TrackCard):
   // the sign itself is the only growth point. Allowing a placement beyond it built track on the far
   // side of the sign and then planted a SECOND sign further out, leaving the board reading
   //   limits · Whistle Post · limits · straight · limits
-  // with a Limits card stranded mid-track. The district below the Running Track is unbounded.
-  if (coord.row === area.runningRow) {
-    if (coord.col < area.limitsWest.col || coord.col > area.limitsEast.col) return false;
-    // And it must not BREAK the Running Track. A curve laid here has no east-west road, so the main
-    // stops dead at it — the bot did exactly this, turning the west end of its own Running Track
-    // into a stub and cutting the Office off from the Limits.
-    if (!carriesThroughTrack(card)) return false;
-  }
+  // with a Limits card stranded mid-track.
+  if (!withinLimits(area, coord)) return false;
+  // And it must not BREAK the Running Track. A curve laid here has no east-west road, so the main
+  // stops dead at it — the bot did exactly this, turning the west end of its own Running Track
+  // into a stub and cutting the Office off from the Limits.
+  if (coord.row === area.runningRow && !carriesThroughTrack(card)) return false;
 
   const ports: Port[] = ['n', 's', 'e', 'w'];
   for (const p of ports) {