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Merge pull request #139 from burinc/fix-collision-bugs
Fix asteroids collision bug
2 parents 964e143 + 589ffa5 commit e01d7a5

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‎src/scittle/games/asteroids.cljs‎

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@@ -443,18 +443,21 @@
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:when (> (:life new-p) 0)]
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new-p))))
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;; Check bullet-asteroid collisions (FIXED to prevent duplicate hits)
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(let [hit-bullets (atom #{})
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;; Check bullet-asteroid collisions (FIXED to use current state)
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(let [current-bullets (:bullets @game-state)
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current-asteroids (:asteroids @game-state)
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current-ufos (:ufos @game-state)
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hit-bullets (atom #{})
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hit-asteroids (atom #{})
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new-asteroids (atom [])
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score-added (atom 0)
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new-particles (atom [])]
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;; Find all collisions (but don't apply yet)
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(doseq [bullet bullets
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(doseq [bullet current-bullets
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:when (and (not (:from-ufo bullet))
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(not (contains? @hit-bullets bullet)))
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asteroid asteroids
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asteroid current-asteroids
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:when (not (contains? @hit-asteroids asteroid))]
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(when (check-collision :obj1 bullet :obj2 asteroid
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:radius1 2 :radius2 (:size asteroid))
@@ -485,16 +488,18 @@
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(swap! game-state update :particles
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#(vec (take max-particles (concat % @new-particles))))))
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;; Check bullet-UFO collisions (FIXED to prevent duplicate hits)
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(let [hit-bullets (atom #{})
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;; Check bullet-UFO collisions (FIXED to use current state)
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(let [current-bullets (:bullets @game-state)
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current-ufos (:ufos @game-state)
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hit-bullets (atom #{})
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hit-ufos (atom #{})
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score-added (atom 0)
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new-particles (atom [])]
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(doseq [bullet bullets
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(doseq [bullet current-bullets
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:when (and (not (:from-ufo bullet))
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(not (contains? @hit-bullets bullet)))
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ufo ufos
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ufo current-ufos
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:when (not (contains? @hit-ufos ufo))]
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(when (check-collision :obj1 bullet :obj2 ufo
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:radius1 2 :radius2 ufo-size)

‎src/scittle/games/asteroids_article.clj‎

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@@ -816,6 +816,149 @@
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;; 5. **Optimize particle counts**: Fewer particles with better placement looks just as good
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;; 6. **Profile on mobile**: Desktop performance doesn't predict mobile behavior
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;; ### The Stale Data Bug - Collision Detection with Old Positions
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;; After fixing the exponential explosion bug, we discovered another critical issue: bullets weren't destroying asteroids reliably! This was the **exact same bug** we encountered in our Galaga implementation.
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;; #### The Problem: Using Captured State
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;; The collision detection code was using bullets and asteroids captured at the START of the `update-game!` function, but checking collisions AFTER updating their positions:
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;; ```clojure
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;; ;; ❌ BROKEN: Stale data causes missed collisions
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;; (defn update-game! []
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;; (when (= (:game-status @game-state) :playing)
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;; (let [{:keys [bullets asteroids ufos]} @game-state] ; ← Captured at START
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;;
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;; ;; Update bullet positions
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;; (swap! game-state update :bullets
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;; (fn [bs]
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;; (vec (for [b bs
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;; :let [new-b (-> b
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;; (update :x #(wrap-position ...))
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;; (update :y #(wrap-position ...)))]
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;; :when (> (:life new-b) 0)]
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;; new-b))))
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;;
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;; ;; Update asteroid positions
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;; (swap! game-state update :asteroids
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;; (fn [as]
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;; (vec (for [a as]
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;; (-> a
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;; (update :x #(wrap-position ...))
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;; (update :y #(wrap-position ...)))))))
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;;
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;; ;; Check collisions - BUG: Uses OLD positions from line 2!
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;; (doseq [bullet bullets ; ← OLD positions before movement
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;; asteroid asteroids] ; ← OLD positions before movement
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;; (when (check-collision bullet asteroid)
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;; ...))
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;; ```
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;; **Why This Fails:**
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;;
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;; 1. Frame starts: Bullet at x=100, Asteroid at x=105 (not colliding)
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;; 2. Bullet moves to x=108 (NOW colliding with asteroid!)
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;; 3. Asteroid moves to x=110
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;; 4. Collision check uses OLD positions (100 vs 105) - NO collision detected!
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;; 5. Result: Bullet passes right through asteroid
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;; #### The Fix: Fresh State Captures
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;; Capture the CURRENT state right before collision detection, AFTER all position updates:
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;; ```clojure
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;; ;; ✅ FIXED: Use current state after position updates
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;; (defn update-game! []
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;; (when (= (:game-status @game-state) :playing)
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;; (let [{:keys [bullets asteroids ufos]} @game-state]
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;;
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;; ;; Update bullet positions
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;; (swap! game-state update :bullets ...)
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;;
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;; ;; Update asteroid positions
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;; (swap! game-state update :asteroids ...)
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;;
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;; ;; Capture FRESH state after all updates
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;; (let [current-bullets (:bullets @game-state) ; ← FRESH after updates
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;; current-asteroids (:asteroids @game-state) ; ← FRESH after updates
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;; current-ufos (:ufos @game-state) ; ← FRESH after updates
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;; hit-bullets (atom #{})
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;; hit-asteroids (atom #{})]
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;;
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;; ;; Now collision detection uses CURRENT positions
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;; (doseq [bullet current-bullets ; ← Current frame positions
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;; :when (not (contains? @hit-bullets bullet))
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;; asteroid current-asteroids] ; ← Current frame positions
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;; (when (check-collision bullet asteroid)
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;; ...))
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;;
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;; ;; UFO collisions also use current state
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;; (doseq [bullet current-bullets ; ← Reuse fresh capture
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;; :when (not (contains? @hit-bullets bullet))
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;; ufo current-ufos] ; ← Current frame positions
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;; (when (check-collision bullet ufo)
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;; ...))
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;; ```
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;; #### The Key Insight
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;; The `let` binding at the function start creates a **snapshot** of the game state. Any subsequent `swap!` calls modify the atom, but the `let` variables still point to the old data. For collision detection to work correctly, we must capture fresh state AFTER all position updates complete.
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;; #### Scope Considerations
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;; Initially, we made a mistake: we defined `current-bullets` and `current-ufos` in the first collision detection block (asteroids) and tried to use them in the second block (UFOs). This failed because each `let` block has its own scope!
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;; **The solution:** Each collision detection block captures its own fresh data:
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;; ```clojure
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;; ;; Bullet-Asteroid collisions
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;; (let [current-bullets (:bullets @game-state)
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;; current-asteroids (:asteroids @game-state)
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;; current-ufos (:ufos @game-state) ; ← Also captured for UFO collisions
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;; ...]
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;; ...)
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;;
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;; ;; Bullet-UFO collisions (separate scope!)
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;; (let [current-bullets (:bullets @game-state) ; ← Fresh capture again
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;; current-ufos (:ufos @game-state) ; ← Fresh capture again
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;; ...]
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;; ...)
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;; ```
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;; #### Learning from Galaga
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;; This was the identical bug we fixed in our Galaga game! The pattern is common in game loops:
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;;
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;; 1. Capture state at function start for reference
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;; 2. Update multiple collections via `swap!`
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;; 3. Check interactions between updated objects
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;; 4. **Mistake**: Using initial captures instead of current state
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;;
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;; The fix is always the same: capture fresh state right before collision detection.
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;; #### Impact After Fix
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;; **Before:**
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;; - Bullets often passed through asteroids
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;; - Fast-moving bullets especially problematic
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;; - UFOs seemed invulnerable
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;; - Frustrating gameplay experience
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;; **After:**
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;; - Precise collision detection
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;; - Bullets reliably destroy asteroids
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;; - UFOs react properly to hits
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;; - Satisfying, responsive gameplay
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;; #### Additional Lessons
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;; 7. **Timing matters**: State captured at start vs. end of update cycle matters!
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;; 8. **Test collision detection**: Easy to miss stale data bugs during casual play
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;; 9. **Reuse patterns**: Same bug/fix across multiple games suggests a general principle
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;; 10. **Scope awareness**: Remember that `let` blocks don't share bindings
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;; 11. **Think in terms of frames**: What state exists at each point in the game loop?
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;; ## Retro Sound Effects with Web Audio API
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;; No arcade game is complete without sound! We added authentic retro sound effects using the Web Audio API, following the same pattern used in our Galaga implementation.

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