Merge branch 'worktree-agent-af31a2138071f5c19' into revise/edge-cases-and-sources

course/03-death-step/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/03-death-step/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/03-death-step/reference/rust/src/engine/queue.rs

@@ -59,8 +59,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -167,7 +167,9 @@ fn react(world: &mut World, events: &[Event]) {
             on: trigger.on,
         });
         let position = match *event {
-            Event::Destroyed { id, position } if id == source => Some(position),
+            Event::Destroyed { id, before } if id == source => {
+                Some(position_left_of(world, world.owner_of(source), before))
+            }
             _ => world.board_index(source),
         };
         let ctx = Ctx {
@@ -185,6 +187,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/03-death-step/reference/rust/src/engine/world.rs

@@ -276,10 +276,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/03-death-step/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/03-death-step/reference/ts/src/engine/event.ts

@@ -14,7 +14,9 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number };

course/03-death-step/reference/ts/src/engine/queue.ts

@@ -38,8 +38,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -129,13 +129,27 @@ function react(world: World, events: Event[]): void {
   // The Queue is now locked: nothing that happens below adds to it.
   for (const [source, trigger, event] of queue) {
     world.record({ type: "Triggered", source, on: trigger.on });
-    const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+    const position =
+      event.type === "Destroyed" && event.id === source
+        ? positionLeftOf(world, world.ownerOf(source), event.before)
+        : world.boardIndex(source);
     const actions = trigger.effect(new Ctx(world, world.ownerOf(source), source, undefined, position));
     // Depth-first: each action, and every reaction it causes, finishes before the next.
     for (const action of actions) perform(world, action);
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/03-death-step/reference/ts/src/engine/world.ts

@@ -208,10 +208,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/03-death-step/scenarios.md

@@ -54,3 +54,25 @@ A minion is removed from play only in the Death Creation Step, which runs when t
   - `health(G)` is 3: G was at full Health when it was removed.
   - `zoneOf(G)` is graveyard.
   - `nameOf(D)` is `"Damaged Golem"`.
+
+### test_03_summon_position_after_simultaneous_deaths
+
+- Setup: `newGame` with a deck of 5 `Test Filler` for player 0. Summon `Loot Hoarder` (id H), then `Harvest Golem` (id G), then `Test Reactor` (id R). `dealDamage(G, 2)`, which leaves G at 1 Health.
+- Action: player 0 casts `Whirlwind`.
+- Assertions:
+  - `hand(0)` has 1 card; call it C. The log has exactly one `Summoned` event; call its id D.
+  - The log is exactly, in this order: `SpellCast{card: "Whirlwind"}`, `Damaged{target: H, amount: 1}`, `Damaged{target: G, amount: 1}`, `Damaged{target: R, amount: 1}`, `Destroyed{id: H}`, `Destroyed{id: G}`, `DeathPhase{round: 1}`, `Triggered{source: H, trigger: "deathrattle"}`, `Drew{player: 0, id: C}`, `Triggered{source: G, trigger: "deathrattle"}`, `Summoned{id: D}`. H and G leave play in the same Death Creation Step.
+  - `nameOf(D)` is `"Damaged Golem"`.
+  - `board(0)` is `[D, R]`: the Damaged Golem stands where G stood, left of R. G was second from the left when the step began, but H, to its left, left play in the same step, so that board index no longer points at G's place.
+  - `zoneOf(H)` and `zoneOf(G)` are graveyard.
+
+### test_03_simultaneous_summons_keep_board_order
+
+- Setup: an empty game. Summon `Harvest Golem` (id A), then another `Harvest Golem` (id B), then `Test Reactor` (id R). `dealDamage(A, 2)` and `dealDamage(B, 2)`, which leave both at 1 Health.
+- Action: player 0 casts `Whirlwind`.
+- Assertions:
+  - The log has exactly two `Summoned` events; call their ids DA and DB, in log order.
+  - The log is exactly, in this order: `SpellCast{card: "Whirlwind"}`, `Damaged{target: A, amount: 1}`, `Damaged{target: B, amount: 1}`, `Damaged{target: R, amount: 1}`, `Destroyed{id: A}`, `Destroyed{id: B}`, `DeathPhase{round: 1}`, `Triggered{source: A, trigger: "deathrattle"}`, `Summoned{id: DA}`, `Triggered{source: B, trigger: "deathrattle"}`, `Summoned{id: DB}`. A's Deathrattle resolves first, by order of play, so DA is A's Damaged Golem and DB is B's.
+  - `nameOf(DA)` and `nameOf(DB)` are `"Damaged Golem"`.
+  - `board(0)` is `[DA, DB, R]`: the two Damaged Golems stand in the order A and B stood, both left of R.
+  - `zoneOf(A)` and `zoneOf(B)` are graveyard.

course/03-death-step/tests/rust/test_03.rs

@@ -22,6 +22,16 @@ fn whirlwind() -> GameEvent {
     }
 }
 
+/// The ids of the `Summoned` events in `log`, in order.
+fn summoned_ids(log: &[GameEvent]) -> Vec<Id> {
+    log.iter()
+        .filter_map(|e| match e {
+            GameEvent::Summoned { id } => Some(*id),
+            _ => None,
+        })
+        .collect()
+}
+
 #[test]
 fn test_03_removed_only_after_sequence() {
     let mut g = new_game(GameOpts {
@@ -155,3 +165,82 @@ fn test_03_destroy_marks_pending() {
     assert_eq!(g.zone_of(golem), Zone::Graveyard);
     assert_eq!(g.name_of(d), "Damaged Golem");
 }
+
+#[test]
+fn test_03_summon_position_after_simultaneous_deaths() {
+    let mut g = new_game(GameOpts {
+        decks: Some([fillers(5), vec![]]),
+    });
+    let h = g.summon("Loot Hoarder", 0);
+    let golem = g.summon("Harvest Golem", 0);
+    let r = g.summon("Test Reactor", 0);
+    g.deal_damage(golem, 2);
+    g.clear_log();
+
+    g.cast("Whirlwind", 0, None);
+
+    let hand = g.hand(0);
+    assert_eq!(hand.len(), 1);
+    let c = hand[0];
+    let summoned = summoned_ids(&g.log());
+    assert_eq!(summoned.len(), 1);
+    let d = summoned[0];
+    assert_eq!(
+        g.log(),
+        vec![
+            whirlwind(),
+            damaged(h, 1),
+            damaged(golem, 1),
+            damaged(r, 1),
+            GameEvent::Destroyed { id: h },
+            GameEvent::Destroyed { id: golem },
+            GameEvent::DeathPhase { round: 1 },
+            triggered(h, "deathrattle"),
+            GameEvent::Drew { player: 0, id: c },
+            triggered(golem, "deathrattle"),
+            GameEvent::Summoned { id: d },
+        ]
+    );
+    assert_eq!(g.name_of(d), "Damaged Golem");
+    assert_eq!(g.board(0), vec![d, r]);
+    assert_eq!(g.zone_of(h), Zone::Graveyard);
+    assert_eq!(g.zone_of(golem), Zone::Graveyard);
+}
+
+#[test]
+fn test_03_simultaneous_summons_keep_board_order() {
+    let mut g = new_game(GameOpts::default());
+    let a = g.summon("Harvest Golem", 0);
+    let b = g.summon("Harvest Golem", 0);
+    let r = g.summon("Test Reactor", 0);
+    g.deal_damage(a, 2);
+    g.deal_damage(b, 2);
+    g.clear_log();
+
+    g.cast("Whirlwind", 0, None);
+
+    let summoned = summoned_ids(&g.log());
+    assert_eq!(summoned.len(), 2);
+    let (da, db) = (summoned[0], summoned[1]);
+    assert_eq!(
+        g.log(),
+        vec![
+            whirlwind(),
+            damaged(a, 1),
+            damaged(b, 1),
+            damaged(r, 1),
+            GameEvent::Destroyed { id: a },
+            GameEvent::Destroyed { id: b },
+            GameEvent::DeathPhase { round: 1 },
+            triggered(a, "deathrattle"),
+            GameEvent::Summoned { id: da },
+            triggered(b, "deathrattle"),
+            GameEvent::Summoned { id: db },
+        ]
+    );
+    assert_eq!(g.name_of(da), "Damaged Golem");
+    assert_eq!(g.name_of(db), "Damaged Golem");
+    assert_eq!(g.board(0), vec![da, db, r]);
+    assert_eq!(g.zone_of(a), Zone::Graveyard);
+    assert_eq!(g.zone_of(b), Zone::Graveyard);
+}

course/03-death-step/tests/ts/test_03.test.ts

@@ -111,3 +111,72 @@ test("test_03_destroy_marks_pending", () => {
   expect(g.zoneOf(golem)).toBe("graveyard");
   expect(g.nameOf(d)).toBe("Damaged Golem");
 });
+
+test("test_03_summon_position_after_simultaneous_deaths", () => {
+  const g = newGame({ decks: [fillers(5), []] });
+  const h = g.summon("Loot Hoarder", 0);
+  const golem = g.summon("Harvest Golem", 0);
+  const r = g.summon("Test Reactor", 0);
+  g.dealDamage(golem, 2);
+  g.clearLog();
+
+  g.cast("Whirlwind", 0);
+
+  const hand = g.hand(0);
+  expect(hand).toHaveLength(1);
+  const c = hand[0];
+  const summoned = g.log().flatMap((e) => (e.type === "Summoned" ? [e.id] : []));
+  expect(summoned).toHaveLength(1);
+  const d = summoned[0];
+  expect(g.log()).toEqual([
+    { type: "SpellCast", card: "Whirlwind" },
+    { type: "Damaged", target: h, amount: 1 },
+    { type: "Damaged", target: golem, amount: 1 },
+    { type: "Damaged", target: r, amount: 1 },
+    { type: "Destroyed", id: h },
+    { type: "Destroyed", id: golem },
+    { type: "DeathPhase", round: 1 },
+    { type: "Triggered", source: h, trigger: "deathrattle" },
+    { type: "Drew", player: 0, id: c },
+    { type: "Triggered", source: golem, trigger: "deathrattle" },
+    { type: "Summoned", id: d },
+  ]);
+  expect(g.nameOf(d)).toBe("Damaged Golem");
+  expect(g.board(0)).toEqual([d, r]);
+  expect(g.zoneOf(h)).toBe("graveyard");
+  expect(g.zoneOf(golem)).toBe("graveyard");
+});
+
+test("test_03_simultaneous_summons_keep_board_order", () => {
+  const g = newGame();
+  const a = g.summon("Harvest Golem", 0);
+  const b = g.summon("Harvest Golem", 0);
+  const r = g.summon("Test Reactor", 0);
+  g.dealDamage(a, 2);
+  g.dealDamage(b, 2);
+  g.clearLog();
+
+  g.cast("Whirlwind", 0);
+
+  const summoned = g.log().flatMap((e) => (e.type === "Summoned" ? [e.id] : []));
+  expect(summoned).toHaveLength(2);
+  const [da, db] = summoned;
+  expect(g.log()).toEqual([
+    { type: "SpellCast", card: "Whirlwind" },
+    { type: "Damaged", target: a, amount: 1 },
+    { type: "Damaged", target: b, amount: 1 },
+    { type: "Damaged", target: r, amount: 1 },
+    { type: "Destroyed", id: a },
+    { type: "Destroyed", id: b },
+    { type: "DeathPhase", round: 1 },
+    { type: "Triggered", source: a, trigger: "deathrattle" },
+    { type: "Summoned", id: da },
+    { type: "Triggered", source: b, trigger: "deathrattle" },
+    { type: "Summoned", id: db },
+  ]);
+  expect(g.nameOf(da)).toBe("Damaged Golem");
+  expect(g.nameOf(db)).toBe("Damaged Golem");
+  expect(g.board(0)).toEqual([da, db, r]);
+  expect(g.zoneOf(a)).toBe("graveyard");
+  expect(g.zoneOf(b)).toBe("graveyard");
+});

course/04-interceptors/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/04-interceptors/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/04-interceptors/reference/rust/src/engine/queue.rs

@@ -69,8 +69,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -205,7 +205,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let ctx = Ctx {
@@ -224,6 +226,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/04-interceptors/reference/rust/src/engine/world.rs

@@ -295,10 +295,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/04-interceptors/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/04-interceptors/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/04-interceptors/reference/ts/src/engine/queue.ts

@@ -45,8 +45,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -161,7 +161,10 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       const actions = trigger.effect(new Ctx(world, world.ownerOf(source), source, undefined, position));
       // Depth-first: each action, and every reaction it causes, finishes before the next.
       for (const action of actions) perform(world, action);
@@ -169,6 +172,17 @@ function react(world: World, events: Event[]): void {
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/04-interceptors/reference/ts/src/engine/world.ts

@@ -225,10 +225,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/05-assemble/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/05-assemble/reference/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/05-assemble/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/05-assemble/reference/rust/src/engine/queue.rs

@@ -86,8 +86,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -241,7 +241,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -250,6 +252,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/05-assemble/reference/rust/src/engine/world.rs

@@ -350,10 +350,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/05-assemble/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/05-assemble/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/05-assemble/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/05-assemble/reference/ts/src/engine/queue.ts

@@ -55,8 +55,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -188,12 +188,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/05-assemble/reference/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/06-parser/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/06-parser/reference/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/06-parser/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/06-parser/reference/rust/src/engine/queue.rs

@@ -86,8 +86,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -241,7 +241,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -250,6 +252,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/06-parser/reference/rust/src/engine/world.rs

@@ -350,10 +350,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/06-parser/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/06-parser/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/06-parser/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/06-parser/reference/ts/src/engine/queue.ts

@@ -55,8 +55,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -188,12 +188,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/06-parser/reference/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/07-rule-resolution/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/07-rule-resolution/reference/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/07-rule-resolution/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/07-rule-resolution/reference/rust/src/engine/queue.rs

@@ -86,8 +86,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -241,7 +241,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -250,6 +252,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/07-rule-resolution/reference/rust/src/engine/world.rs

@@ -350,10 +350,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/07-rule-resolution/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/07-rule-resolution/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/07-rule-resolution/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/07-rule-resolution/reference/ts/src/engine/queue.ts

@@ -55,8 +55,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -188,12 +188,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/07-rule-resolution/reference/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/08-push-chains/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/08-push-chains/reference/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/08-push-chains/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/08-push-chains/reference/rust/src/engine/queue.rs

@@ -86,8 +86,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -241,7 +241,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -250,6 +252,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/08-push-chains/reference/rust/src/engine/world.rs

@@ -350,10 +350,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/08-push-chains/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/08-push-chains/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/08-push-chains/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/08-push-chains/reference/ts/src/engine/queue.ts

@@ -55,8 +55,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -188,12 +188,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/08-push-chains/reference/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/09-turn-pipeline/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/09-turn-pipeline/reference/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/09-turn-pipeline/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/09-turn-pipeline/reference/rust/src/engine/queue.rs

@@ -86,8 +86,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -241,7 +241,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -250,6 +252,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/09-turn-pipeline/reference/rust/src/engine/world.rs

@@ -350,10 +350,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/09-turn-pipeline/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/09-turn-pipeline/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/09-turn-pipeline/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };

course/09-turn-pipeline/reference/ts/src/engine/queue.ts

@@ -55,8 +55,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -188,12 +188,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/09-turn-pipeline/reference/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {

course/10-spellgrid/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}

course/10-spellgrid/reference/rust/src/engine/effect.rs

@@ -85,7 +85,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),

course/10-spellgrid/reference/rust/src/engine/event.rs

@@ -36,10 +36,12 @@ pub enum Event {
         card: String,
     },
     /// A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-    /// `position` is the board index it had when the step began; its Deathrattle summons there.
+    /// `before` is the nearest minion to its right that did not die in the same step, if any:
+    /// its Deathrattle summons immediately left of that minion, or at the right end. It is a
+    /// handle because a board index goes stale when a minion further left dies in the same step.
     Destroyed {
         id: EntityId,
-        position: usize,
+        before: Option<EntityId>,
     },
     DeathPhase {
         round: u32,

course/10-spellgrid/reference/rust/src/engine/queue.rs

@@ -110,8 +110,8 @@ pub struct Ctx<'a> {
     pub source: Option<EntityId>,
     /// The chosen target, if any.
     pub target: Option<EntityId>,
-    /// The source's board position. For a Deathrattle, the source has left play,
-    /// and this is the position its Death Event remembered.
+    /// The source's board position. For a Deathrattle, the source has left play, and
+    /// this is where it stood: immediately left of the minion its Death Event remembers.
     pub position: Option<usize>,
 }
 
@@ -270,7 +270,9 @@ fn react(world: &mut World, events: &[Event]) {
                 on: trigger.on,
             });
             let position = match *event {
-                Event::Destroyed { id, position } if id == source => Some(position),
+                Event::Destroyed { id, before } if id == source => {
+                    Some(position_left_of(world, world.owner_of(source), before))
+                }
                 _ => world.board_index(source),
             };
             let owner = world.owner_of(source);
@@ -279,6 +281,16 @@ fn react(world: &mut World, events: &[Event]) {
     }
 }
 
+/// The board index, right now, immediately left of `before` on `owner`'s board; the
+/// right end if `before` is `None` or no longer there. A Death Event remembers the
+/// minion, not an index, so this still holds after other minions have left or arrived.
+fn position_left_of(world: &World, owner: PlayerId, before: Option<EntityId>) -> usize {
+    let board = world.zone(owner, Zone::Board);
+    before
+        .and_then(|before| board.iter().position(|&id| id == before))
+        .unwrap_or(board.len())
+}
+
 /// Who may respond to `event`, in order of play: every minion in play and, for a
 /// Death Event, the minion that died. It has left play, but its Deathrattle still answers.
 fn listeners(world: &World, event: &Event) -> Vec<EntityId> {

course/10-spellgrid/reference/rust/src/engine/world.rs

@@ -383,10 +383,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {

course/10-spellgrid/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+  // Event remembers where its minion stood as a handle, the minion that stays to its
+  // right, not as a board index: an index goes stale as soon as a minion further left
+  // leaves play in the same step (Module 1: hold handles, not indexes).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}

course/10-spellgrid/reference/ts/src/engine/effect.ts

@@ -60,7 +60,7 @@ function targets(target: Target, ctx: Ctx): EntityId[] {
 
 /**
  * Where a summon goes: to the right of a source still on the board; where a dead
- * source stood (its Death Event's position); at the right end for a spell.
+ * source stood (left of the minion its Death Event remembers); at the right end for a spell.
  */
 function summonPosition(ctx: Ctx): number | undefined {
   const index = ctx.source === undefined ? undefined : ctx.world.boardIndex(ctx.source);

course/10-spellgrid/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId }

course/10-spellgrid/reference/ts/src/engine/queue.ts

@@ -75,8 +75,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -219,12 +219,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.

course/10-spellgrid/reference/ts/src/engine/world.ts

@@ -308,10 +308,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {