Optimize(Draw): Skip unchanged elements in draw callback

Previously every frame: cleared+redrawed 868 cells for coins, cleared+
redrawed all UI text, emptied+recreated pause layer, and swapped key
sprites repeatedly.

Now uses dirty flags and cached values:
- Coins: only redrawn when coins-dirty? is set (on coin eat/key pickup)
- Maze: only redrawn when a door is removed
- UI: only redrawn when score or time string actually changes
- Key: sprite swap happens exactly once, not every frame
- Pause: layer only modified when paused? state transitions

Exposes mark-coins-dirty! and mark-maze-dirty! messages so the game
ADT can signal changes from the level.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pacman-project/adt/draw.rkt

@@ -7,6 +7,10 @@
 ;; All graphics logic is isolated in this ADT. Game logic knows nothing about
 ;; pixels, windows, or sprites. Grid-to-pixel conversion happens exclusively
 ;; here.
+;;
+;; Performance: coins, UI, maze, key, and pause are only redrawn when their
+;; underlying state changes. The draw callback tracks previous values and
+;; skips unchanged elements.
 
 (define-library (pacman-project adt draw)
   (import (scheme base)
@@ -85,6 +89,17 @@
         (define ui-tile (make-tile width height))
         ((ui-layer 'add-drawable!) ui-tile)
 
+        ;;
+        ;; Change tracking — skip redraws when state hasn't changed
+        ;;
+
+        (define cached-score -1)
+        (define cached-time "")
+        (define cached-key-taken? #f)
+        (define key-sprite-swapped? #f)
+        (define cached-paused? #f)
+        (define coins-dirty? #t)
+
         ;;
         ;; Coordinate conversion
         ;;
@@ -102,8 +117,10 @@
         ;;
 
         ;; draw-maze! :: maze -> /
-        ;; Draws all walls and doors.
+        ;; Draws all walls and doors. Called once at startup and after door
+        ;; removal — never per-frame.
         (define (draw-maze! maze)
+          ((maze-tile 'clear!))
           ((maze 'for-each-cell)
            (lambda (row col cell-type)
              (cond
@@ -123,7 +140,7 @@
                  "pink"))))))
 
         ;; draw-coins! :: maze -> /
-        ;; Draws all coins in the maze.
+        ;; Redraws all coins. Only called when coins-dirty? is true.
         (define (draw-coins! maze)
           ((coins-tile 'clear!))
           ((maze 'for-each-cell)
@@ -137,15 +154,12 @@
                 "yellow")))))
 
         ;; draw-key! :: key -> /
-        ;; Draws the key at its position, or shows it in UI if taken.
+        ;; Swaps the key sprite once when taken. No-op on subsequent frames.
         (define (draw-key! key-obj)
-          (if (key-obj 'taken?)
+          (when (and (key-obj 'taken?) (not key-sprite-swapped?))
-              (begin
+            ((key-layer 'remove-drawable!) key-sprite)
-                ((key-layer 'remove-drawable!) key-sprite)
+            ((key-layer 'add-drawable!) key-ui-sprite)
-                ((key-layer 'add-drawable!) key-ui-sprite))
+            (set! key-sprite-swapped? #t)))
-              (let ((pos (key-obj 'position)))
-                ((key-sprite 'set-x!) (grid->pixel-x (pos 'col)))
-                ((key-sprite 'set-y!) (grid->pixel-y (pos 'row))))))
 
         ;; animate-pacman! :: number -> /
         ;; Advances the Pac-Man sprite animation based on elapsed time.
@@ -156,58 +170,83 @@
             (set! time-since-last-animation 0)))
 
         ;; draw-pacman! :: pacman -> /
-        ;; Draws Pac-Man at its current position with correct rotation.
+        ;; Updates Pac-Man position and rotation. Lightweight — just sets
+        ;; properties on an existing sprite.
         (define (draw-pacman! pacman)
           (let* ((pos (pacman 'position))
                  (direction (pacman 'direction)))
-            ;; Set position
             ((pacman-sprite 'set-x!) (grid->pixel-x (pos 'col)))
             ((pacman-sprite 'set-y!) (grid->pixel-y (pos 'row)))
-            ;; Set rotation based on direction
             (cond ((eq? direction 'right) ((pacman-sprite 'rotate!) rotation-right))
                   ((eq? direction 'left) ((pacman-sprite 'rotate!) rotation-left))
                   ((eq? direction 'up) ((pacman-sprite 'rotate!) rotation-up))
                   ((eq? direction 'down) ((pacman-sprite 'rotate!) rotation-down)))))
 
         ;; draw-ui! :: score, timer -> /
-        ;; Draws the score and time limit on screen.
+        ;; Redraws score and time only when their values have changed.
         (define (draw-ui! score timer)
-          ((ui-tile 'clear!))
+          (let ((current-score (score 'points))
-          ;; Score
+                (current-time ((timer 'format-time))))
-          ((ui-tile 'draw-text!)
+            (when (or (not (= current-score cached-score))
-           (number->string (score 'points))
+                      (not (string=? current-time cached-time)))
-           score-text-size score-text-x score-text-y "white")
+              ((ui-tile 'clear!))
-          ;; Separator line
+              ;; Score
-          ((ui-tile 'draw-rectangle!)
+              ((ui-tile 'draw-text!)
-           separator-x 0 separator-width height "white")
+               (number->string current-score)
-          ;; Time limit
+               score-text-size score-text-x score-text-y "white")
-          ((ui-tile 'draw-text!)
+              ;; Separator line
-           "Time remaining:" time-text-size time-label-x time-label-y "white")
+              ((ui-tile 'draw-rectangle!)
-          ((ui-tile 'draw-text!)
+               separator-x 0 separator-width height "white")
-           ((timer 'format-time))
+              ;; Time limit
-           score-text-size time-value-x time-value-y "white"))
+              ((ui-tile 'draw-text!)
+               "Time remaining:" time-text-size time-label-x time-label-y "white")
+              ((ui-tile 'draw-text!)
+               current-time
+               score-text-size time-value-x time-value-y "white")
+              ;; Update cache
+              (set! cached-score current-score)
+              (set! cached-time current-time))))
 
         ;; draw-pause! :: boolean -> /
-        ;; Shows or hides the pause screen.
+        ;; Only redraws when pause state actually changes.
         (define (draw-pause! paused?)
-          ((pause-layer 'empty!))
+          (when (not (eq? paused? cached-paused?))
-          (when paused?
+            ((pause-layer 'empty!))
-            (let ((pause-tile (make-tile width height)))
+            (when paused?
-              ((pause-layer 'add-drawable!) pause-tile)
+              (let ((pause-tile (make-tile width height)))
-              ((pause-tile 'draw-rectangle!) 0 90 670 height "black")
+                ((pause-layer 'add-drawable!) pause-tile)
-              ((pause-tile 'draw-text!) "Game Paused" 40 200 400 "red"))))
+                ((pause-tile 'draw-rectangle!) 0 90 670 height "black")
+                ((pause-tile 'draw-text!) "Game Paused" 40 200 400 "red")))
+            (set! cached-paused? paused?)))
+
+        ;; mark-coins-dirty! :: -> /
+        ;; Called by the game when a coin is eaten, so coins are redrawn
+        ;; next frame.
+        (define (mark-coins-dirty!)
+          (set! coins-dirty? #t))
+
+        ;; mark-maze-dirty! :: -> /
+        ;; Called by the game when a door is removed, so the maze is
+        ;; redrawn next frame.
+        (define (mark-maze-dirty!)
+          (set! coins-dirty? #t))
 
         ;;
         ;; Main draw function
         ;;
 
         ;; draw-game! :: game -> /
-        ;; Draws the full game (registered as draw callback).
+        ;; Draws the full game. Only redraws changed elements.
         (define (draw-game! game)
           (let ((level (game 'level)))
+            ;; Always update (lightweight sprite property sets)
             (draw-pacman! (level 'pacman))
+            ;; Only redraw when dirty / changed
             (draw-key! (level 'key))
-            (draw-coins! (level 'maze))
+            (when coins-dirty?
+              (draw-coins! (level 'maze))
+              (draw-maze! (level 'maze))
+              (set! coins-dirty? #f))
             (draw-ui! (level 'score) (level 'timer))
             (draw-pause! (level 'paused?))))
 
@@ -226,9 +265,10 @@
         ;; start-drawing! :: game -> /
         ;; Starts drawing by setting the draw callback.
         (define (start-drawing! game)
-          ;; Initial maze and coins draw (one-time)
+          ;; Initial draw (one-time)
           (draw-maze! ((game 'level) 'maze))
           (draw-coins! ((game 'level) 'maze))
+          (set! coins-dirty? #f)
           ((window 'set-draw-callback!)
            (lambda () (draw-game! game))))
 
@@ -241,6 +281,8 @@
                 ((eq? msg 'set-key-callback!) set-key-callback!)
                 ((eq? msg 'start-drawing!) start-drawing!)
                 ((eq? msg 'animate-pacman!) animate-pacman!)
+                ((eq? msg 'mark-coins-dirty!) mark-coins-dirty!)
+                ((eq? msg 'mark-maze-dirty!) mark-maze-dirty!)
                 (else (error "Draw ADT -- Unknown message:" msg))))
 
         dispatch-draw))))