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fix(galaga): fix collision detection with batched updates
Critical bug fix: Enemies weren't being destroyed when hit by bullets. This was caused by two interrelated issues in collision detection. Problems Fixed: 1. STALE DATA: bullets and enemies were captured at the start of update-game!, but collision checks happened AFTER bullet position updates. Result: checking collisions with old positions. 2. RACE CONDITIONS: Each collision in doseq triggered separate swap! calls, but all iterations used the ORIGINAL collections. Multiple collisions on same bullet/enemy caused state corruption. Solution: Collision Batching - Capture FRESH bullets/enemies right before collision detection - Collect all collisions in sets (preventing duplicate processing) - Apply ALL effects in a single atomic swap! at the end - Use :when guards to skip already-processed objects Benefits: - Enemies now properly destroyed when shot - Boss enemies correctly track 2-hit damage - No race conditions or state corruption - Single re-render per collision batch - Predictable, deterministic behavior This uses the same collision batching pattern from Asteroids, inspired by Erik Assum's feedback on atomic state updates. Also updated article documentation to explain the bug and solution with detailed code examples and credit to Erik Assum.
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Lines changed: 174 additions & 48 deletions

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‎src/scittle/games/galaga.clj‎

Lines changed: 105 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -284,18 +284,116 @@
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;; (/ (+ (:width a) (:width b)) 2))
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;; (< (Math/abs (- (:y a) (:y b)))
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;; (/ (+ (:height a) (:height b)) 2))))
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;;
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;; ;; Check bullet-enemy collisions
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;; ```
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;; ### Critical Bug: Collision Detection Race Conditions
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;; During development, we discovered a critical bug that made enemies invulnerable! The problem was subtle but devastating:
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;; **The Bug:**
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;; ```clojure
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;; ;; ❌ BROKEN - Enemies won't die!
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;; (doseq [bullet bullets
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;; enemy enemies]
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;; (when (collides? :a bullet :b enemy)
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;; ;; Damage or destroy enemy
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;; (let [new-hits (dec (:hits enemy))]
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;; (if (<= new-hits 0)
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;; (destroy-enemy! :enemy enemy)
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;; (damage-enemy! :enemy enemy :hits new-hits)))))
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;; ;; Each collision triggers separate swap!
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;; (swap! game-state update :bullets remove-bullet)
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;; (swap! game-state update :enemies remove-enemy)
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;; ...))
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;; ```
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;; **Two problems here:**
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;; 1. **Stale Data**: `bullets` and `enemies` were captured at the start of `update-game!`, but we check collisions AFTER updating bullet positions. So we're checking collisions with OLD positions!
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;; 2. **Race Conditions**: Each collision in `doseq` triggers a separate `swap!`, but all iterations use the ORIGINAL collections. If bullet A hits enemies 1 and 2, both iterations try to remove bullet A, causing state corruption.
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;; **The Solution: Collision Batching**
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;; We adopted the same pattern used in Asteroids (per [Erik Assum](https://github.com/slipset)'s excellent feedback):
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;; ```clojure
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;; ;; ✅ FIXED - Batch collision detection
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;; ;; Get FRESH bullets and enemies after updates
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;; (let [current-bullets (:bullets @game-state)
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;; current-enemies (:enemies @game-state)
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;; hit-bullets (atom #{})
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;; hit-enemies (atom #{})
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;; damaged-enemies (atom {})
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;; score-added (atom 0)
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;; new-particles (atom [])]
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;;
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;; ;; Collect all collisions (but don't apply yet)
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;; (doseq [bullet current-bullets
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;; :when (not (contains? @hit-bullets bullet))
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;; enemy current-enemies
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;; :when (and (not (contains? @hit-enemies enemy))
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;; (not= (:state enemy) :destroyed))]
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;; (when (collides? :a bullet :b enemy)
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;; ;; Mark bullet as hit
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;; (swap! hit-bullets conj bullet)
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;;
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;; (let [new-hits (dec (:hits enemy))
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;; destroyed? (<= new-hits 0)]
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;; (if destroyed?
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;; ;; Enemy destroyed - mark for removal
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;; (do
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;; (swap! hit-enemies conj enemy)
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;; (swap! score-added + (:points enemy))
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;; (swap! new-particles concat
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;; (create-particles :x (:x enemy)
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;; :y (:y enemy)
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;; :count 10
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;; :color (:color enemy))))
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;; ;; Enemy damaged - mark for hit count update
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;; (swap! damaged-enemies assoc enemy new-hits)))))
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;;
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;; ;; Apply all collision effects at once (single atomic swap!)
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;; (when (or (seq @hit-bullets) (seq @hit-enemies) (seq @damaged-enemies))
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;; (when (seq @hit-enemies)
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;; (play-explosion-sound))
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;;
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;; (swap! game-state
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;; (fn [state]
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;; (-> state
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;; ;; Remove hit bullets
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;; (update :bullets
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;; (fn [bullets]
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;; (vec (remove #(contains? @hit-bullets %) bullets))))
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;; ;; Remove destroyed enemies and update damaged ones
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;; (update :enemies
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;; (fn [enemies]
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;; (vec (keep (fn [e]
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;; (cond
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;; ;; Enemy destroyed - remove it
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;; (contains? @hit-enemies e)
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;; nil
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;;
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;; ;; Enemy damaged - update hits
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;; (contains? @damaged-enemies e)
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;; (assoc e :hits (get @damaged-enemies e))
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;;
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;; ;; Enemy not hit - keep as is
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;; :else e))
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;; enemies))))
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;; ;; Add score for destroyed enemies
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;; (update :score + @score-added)
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;; ;; Add particles for destroyed enemies
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;; (update :particles
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;; (fn [particles]
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;; (vec (concat particles @new-particles)))))))))
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;; ```
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;; **Benefits of the fix:**
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;; 1. **Fresh Data**: Capture bullets/enemies AFTER position updates
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;; 2. **No Race Conditions**: Sets track which objects are already hit
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;; 3. **Single Atomic Update**: One `swap!` applies all effects at once
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;; 4. **Proper Boss Damage**: Correctly tracks multi-hit enemies
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;; 5. **Predictable Behavior**: State updates are deterministic
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;; This same issue affected our Asteroids game, and the batched approach solved both!
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;; ## Visual Effects
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;; ### Particle System

‎src/scittle/games/galaga.cljs‎

Lines changed: 69 additions & 41 deletions
Original file line numberDiff line numberDiff line change
@@ -382,47 +382,75 @@
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(update :y + (:vy p))
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(update :life dec)))
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%))))
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;; Check bullet-enemy collisions
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(doseq [bullet bullets
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enemy enemies]
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(when (and (collides? :a bullet :b enemy)
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(not= (:state enemy) :destroyed))
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(let [new-hits (dec (:hits enemy))
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destroyed? (<= new-hits 0)]
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(when destroyed?
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(play-explosion-sound)) ; Play explosion sound
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;; Single atomic update for all collision effects
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(swap! game-state
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(fn [state]
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(-> state
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;; Remove bullet
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(update :bullets #(vec (remove (fn [b] (= b bullet)) %)))
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;; Handle enemy destruction or damage
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(update :enemies
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(fn [enemies]
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(if destroyed?
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;; Destroy enemy
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(vec (remove (fn [e] (= e enemy)) enemies))
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;; Damage enemy
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(mapv (fn [e]
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(if (= e enemy)
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(assoc e :hits new-hits)
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e))
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enemies))))
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;; Add score if destroyed
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(#(if destroyed?
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(update % :score + (:points enemy))
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%))
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;; Add particles if destroyed
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(#(if destroyed?
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(update % :particles
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(fn [particles]
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(vec (concat particles
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(create-particles :x (:x enemy)
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:y (:y enemy)
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:count 10
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:color (:color enemy))))))
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%))))))))
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;; FIXED: Batch bullet-enemy collision detection
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;; Get FRESH bullets and enemies after updates
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(let [current-bullets (:bullets @game-state)
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current-enemies (:enemies @game-state)
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hit-bullets (atom #{})
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hit-enemies (atom #{})
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damaged-enemies (atom {})
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score-added (atom 0)
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new-particles (atom [])]
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;; Collect all collisions (but don't apply yet)
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(doseq [bullet current-bullets
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:when (not (contains? @hit-bullets bullet))
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enemy current-enemies
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:when (and (not (contains? @hit-enemies enemy))
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(not= (:state enemy) :destroyed))]
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(when (collides? :a bullet :b enemy)
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;; Mark bullet as hit
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(swap! hit-bullets conj bullet)
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(let [new-hits (dec (:hits enemy))
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destroyed? (<= new-hits 0)]
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(if destroyed?
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;; Enemy destroyed - mark for removal
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(do
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(swap! hit-enemies conj enemy)
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(swap! score-added + (:points enemy))
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(swap! new-particles concat
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(create-particles :x (:x enemy)
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:y (:y enemy)
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:count 10
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:color (:color enemy))))
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;; Enemy damaged - mark for hit count update
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(swap! damaged-enemies assoc enemy new-hits)))))
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;; Apply all collision effects at once (single swap!)
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(when (or (seq @hit-bullets) (seq @hit-enemies) (seq @damaged-enemies))
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(when (seq @hit-enemies)
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(play-explosion-sound))
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(swap! game-state
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(fn [state]
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(-> state
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;; Remove hit bullets
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(update :bullets
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(fn [bullets]
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(vec (remove #(contains? @hit-bullets %) bullets))))
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;; Remove destroyed enemies and update damaged ones
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(update :enemies
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(fn [enemies]
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(vec (keep (fn [e]
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(cond
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;; Enemy destroyed - remove it
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(contains? @hit-enemies e)
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nil
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;; Enemy damaged - update hits
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(contains? @damaged-enemies e)
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(assoc e :hits (get @damaged-enemies e))
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;; Enemy not hit - keep as is
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:else e))
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enemies))))
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;; Add score for destroyed enemies
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(update :score + @score-added)
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;; Add particles for destroyed enemies
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(update :particles
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(fn [particles]
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(vec (concat particles @new-particles)))))))))
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;; Check enemy bullet-player collisions
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(doseq [bullet enemy-bullets]
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(when (collides? :a bullet :b player)

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