Rev 19: a dust cloud blinds whoever stands in it (CPU7)

The automaton stood in Kestrel's DUST CLOUD and filled a square with
stone three rows away. The cloud blocked only sight lines passing
through it; the square a line began or ended in was never asked. The
FAQ is plain: no LOS spell may be cast by a wizard in a cloud, nor at
one, though a spell upon oneself still works, and VISIONSTONE does
not see through dust. From rev 19 the engine's sight and the view's —
the targeting aid the client and the automaton both read — refuse a
line with dust at either end. Older games keep their sight.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jm2auWk6RP71CjaAb4FMoG

packages/engine/src/game.ts

@@ -238,7 +238,7 @@ export interface CastParams {
 }
 
 /** The revision new games are dealt under; GameConfig.deckRev pins it per game. */
-export const CURRENT_RULES_REV = 18;
+export const CURRENT_RULES_REV = 19;
 
 /** Every rulings revision since the baseline, newest last — the entries a
  * game's deckRev freezes it before or after. Shown to players as the house
@@ -261,6 +261,7 @@ export const RULES_REVISIONS: { rev: number; note: string }[] = [
   { rev: 16, note: "BUTT-HEAD's charge is measured as far as the goat's legs reach. Older games searched only six steps of corridor and refused a longer ram as unreachable, even with the legs to make it." },
   { rev: 17, note: "A warp mouth on a pit's rim is a way off it like any square: a bare step beside one square and one mouth is a fork the walker names. In every game the mouth may be named, and is taken when no square offers; only an older game's bare step still lands on the square unasked." },
   { rev: 18, note: "REFLECTION's returning half is an attack on the caster in its own right, with the caster's own counteraction window — an ABSORB or a BLUNT meets it as it would any blow. Before, the half landed the instant the spell resolved." },
+  { rev: 19, note: "DUST CLOUD blinds whoever stands in it: no LOS spell may be cast from inside a cloud, nor at anyone standing in one, and VISIONSTONE does not see through it. Spells cast on oneself still work. Before, the cloud blocked only sight lines passing through it." },
 ];
 
 export interface GameConfig {
@@ -438,7 +439,18 @@ function sightThroughOneEdge(
   return throughOneEdge(board, (b) => sightBetween(b, from, to, blockers));
 }
 
+/** Rev 19: DUST CLOUD blinds whoever stands in it and hides them — "an
+ * LOS spell cannot be cast upon a player in DUST CLOUD, or by a player
+ * who is in a DUST CLOUD" (official FAQ); a spell cast upon oneself
+ * still works, and VISIONSTONE does not see through dust either. Before
+ * it the cloud blocked only sight lines passing through. */
+function dustAtEnd(state: GameState, from: Cell, to: Cell): boolean {
+  if ((state.config.deckRev ?? 1) < 19 || cellKey(from) === cellKey(to)) return false;
+  return state.squareContents[cellKey(from)]?.kind === "dust" || state.squareContents[cellKey(to)]?.kind === "dust";
+}
+
 function losWith(state: GameState, from: Cell, to: Cell, viewerId: PlayerId | undefined, stone: boolean): boolean {
+  if (dustAtEnd(state, from, to)) return false;
   const board = doorsAjar(state, viewerId, openedDoors(state, boardView(state)));
   const blockers = losBlockers(state);
   if (sightBetween(board, from, to, blockers)) return true;