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burincburinc
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Merge pull request #138 from burinc/main
fix(galaga): fix collision detection with batched updates
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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 @@
382382
(update :y + (:vy p))
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(update :life dec)))
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%))))
385-
;; Check bullet-enemy collisions
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(doseq [bullet bullets
387-
enemy enemies]
388-
(when (and (collides? :a bullet :b enemy)
389-
(not= (:state enemy) :destroyed))
390-
(let [new-hits (dec (:hits enemy))
391-
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?
414-
(update % :score + (:points enemy))
415-
%))
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;; Add particles if destroyed
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(#(if destroyed?
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(update % :particles
419-
(fn [particles]
420-
(vec (concat particles
421-
(create-particles :x (:x enemy)
422-
:y (:y enemy)
423-
:count 10
424-
:color (:color enemy))))))
425-
%))))))))
385+
;; FIXED: Batch bullet-enemy collision detection
386+
;; Get FRESH bullets and enemies after updates
387+
(let [current-bullets (:bullets @game-state)
388+
current-enemies (:enemies @game-state)
389+
hit-bullets (atom #{})
390+
hit-enemies (atom #{})
391+
damaged-enemies (atom {})
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score-added (atom 0)
393+
new-particles (atom [])]
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;; Collect all collisions (but don't apply yet)
396+
(doseq [bullet current-bullets
397+
:when (not (contains? @hit-bullets bullet))
398+
enemy current-enemies
399+
:when (and (not (contains? @hit-enemies enemy))
400+
(not= (:state enemy) :destroyed))]
401+
(when (collides? :a bullet :b enemy)
402+
;; Mark bullet as hit
403+
(swap! hit-bullets conj bullet)
404+
405+
(let [new-hits (dec (:hits enemy))
406+
destroyed? (<= new-hits 0)]
407+
(if destroyed?
408+
;; Enemy destroyed - mark for removal
409+
(do
410+
(swap! hit-enemies conj enemy)
411+
(swap! score-added + (:points enemy))
412+
(swap! new-particles concat
413+
(create-particles :x (:x enemy)
414+
:y (:y enemy)
415+
:count 10
416+
:color (:color enemy))))
417+
;; Enemy damaged - mark for hit count update
418+
(swap! damaged-enemies assoc enemy new-hits)))))
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;; Apply all collision effects at once (single swap!)
421+
(when (or (seq @hit-bullets) (seq @hit-enemies) (seq @damaged-enemies))
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(when (seq @hit-enemies)
423+
(play-explosion-sound))
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(swap! game-state
426+
(fn [state]
427+
(-> state
428+
;; Remove hit bullets
429+
(update :bullets
430+
(fn [bullets]
431+
(vec (remove #(contains? @hit-bullets %) bullets))))
432+
;; Remove destroyed enemies and update damaged ones
433+
(update :enemies
434+
(fn [enemies]
435+
(vec (keep (fn [e]
436+
(cond
437+
;; Enemy destroyed - remove it
438+
(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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