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/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;
 }