Implement automatic Pac-Man movement with queued direction turning

Pac-Man now moves automatically in its current direction every
pacman-speed-ms (200ms). Arrow keys queue a desired turn direction
instead of moving directly. Each movement tick:

1. Try the queued direction — if passable, turn and move that way
2. Otherwise keep moving in the current direction
3. Stop only when hitting a wall (no direction change)

New internal state:
- queued-direction: the direction the player wants to turn next
- movement-timer: accumulates delta-time, triggers move at interval

New helper:
- can-move?: checks if a direction is passable (no wall/locked door)

Changed behavior:
- key-press! now sets queued-direction instead of calling move-pacman!
- update! now drives movement via advance-pacman! on a timer
- move-pacman! no longer checks time-up? (advance-pacman! handles it)

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

pacman-project/adt/level.rkt

@@ -4,9 +4,11 @@
 ;;                                Level ADT                                  ;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-;; Contains all game logic: Pac-Man movement, collision detection, coin/key
-;; pickup, door opening, teleportation, pause, and time management.
-;; Contains NO graphics code.
+;; Contains all game logic: automatic Pac-Man movement, collision detection,
+;; coin/key pickup, door opening, teleportation, pause, and time management.
+;; Pac-Man moves automatically in its current direction. Arrow keys queue a
+;; desired turn direction, which is applied at the next movement tick if the
+;; path is clear. Contains NO graphics code.
 
 (define-library (pacman-project adt level)
   (import (scheme base)
@@ -29,7 +31,9 @@
             (key #f)
             (score (make-score))
             (timer (make-timer))
-            (paused? #f))
+            (paused? #f)
+            (queued-direction #f)
+            (movement-timer 0))
 
         ;; Initialize key after maze is created.
         (set! key (make-key maze))
@@ -47,6 +51,23 @@
                 ((eq? direction 'down) (cons 1 0))
                 (else (cons 0 0))))
 
+        ;; can-move? :: symbol -> boolean
+        ;; Checks if Pac-Man can move in the given direction (no wall or
+        ;; locked door blocking the way).
+        (define (can-move? direction)
+          (let* ((delta (direction->delta direction))
+                 (current-pos (pacman 'position))
+                 (next-row (+ (current-pos 'row) (car delta)))
+                 (next-col (+ (current-pos 'col) (cdr delta))))
+            (cond
+              ;; Teleportation tunnels are always passable.
+              ((or (< next-col 0) (>= next-col (maze 'cols))) #t)
+              ;; Walls block.
+              (((maze 'wall?) next-row next-col) #f)
+              ;; Doors block unless the key has been taken.
+              (((maze 'door?) next-row next-col) (key 'taken?))
+              (else #t))))
+
         ;;
         ;; Coin logic
         ;;
@@ -88,9 +109,9 @@
         ;;
 
         ;; move-pacman! :: symbol -> /
-        ;; Moves Pac-Man in the given direction with all game rules.
+        ;; Moves Pac-Man one step in the given direction, handling collisions,
+        ;; teleportation, and item pickup.
         (define (move-pacman! direction)
-          (when (not ((timer 'time-up?)))
           (let* ((delta (direction->delta direction))
                  (delta-row (car delta))
                  (delta-col (cdr delta))
@@ -98,7 +119,7 @@
                  (next-row (+ (current-pos 'row) delta-row))
                  (next-col (+ (current-pos 'col) delta-col)))
 
-              ;; Update direction for the draw layer.
+            ;; Update facing direction for the draw layer.
             ((pacman 'direction!) direction)
 
             (cond
@@ -106,7 +127,7 @@
               ((or (< next-col 0) (>= next-col (maze 'cols)))
                (teleport-horizontal! next-row next-col))
 
-                ;; Door: only open if key has been taken.
+              ;; Door: open it if key has been taken.
               (((maze 'door?) next-row next-col)
                (when (key 'taken?)
                  ((maze 'remove-door!) next-row next-col)))
@@ -120,7 +141,22 @@
                    (((maze 'key?) next-row next-col)
                     (pick-up-key! next-row next-col))
                    (((maze 'coin?) next-row next-col)
-                      (eat-coin! next-row next-col)))))))))
+                    (eat-coin! next-row next-col))))))))
+
+        ;; advance-pacman! :: -> /
+        ;; Called every movement tick. Tries the queued direction first; if
+        ;; that path is blocked, continues in the current direction.
+        (define (advance-pacman!)
+          (when (not ((timer 'time-up?)))
+            (let ((current-dir (pacman 'direction)))
+              ;; Try the queued direction first.
+              (cond
+                ((and queued-direction (can-move? queued-direction))
+                 (move-pacman! queued-direction)
+                 (set! queued-direction #f))
+                ;; Otherwise keep moving in the current direction.
+                ((can-move? current-dir)
+                 (move-pacman! current-dir))))))
 
         ;;
         ;; Pause logic
@@ -135,26 +171,31 @@
         ;;
 
         ;; key-press! :: symbol -> /
-        ;; Processes a key press.
+        ;; Processes a key press. Arrow keys queue a desired direction.
         (define (key-press! pressed-key)
           (cond
             ((eq? pressed-key 'escape) (toggle-pause!))
             ((not paused?)
              (cond
-               ((eq? pressed-key 'right) (move-pacman! 'right))
-               ((eq? pressed-key 'left) (move-pacman! 'left))
-               ((eq? pressed-key 'up) (move-pacman! 'up))
-               ((eq? pressed-key 'down) (move-pacman! 'down))))))
+               ((eq? pressed-key 'right) (set! queued-direction 'right))
+               ((eq? pressed-key 'left) (set! queued-direction 'left))
+               ((eq? pressed-key 'up) (set! queued-direction 'up))
+               ((eq? pressed-key 'down) (set! queued-direction 'down))))))
 
         ;;
         ;; Update (game loop function)
         ;;
 
         ;; update! :: number -> /
-        ;; Called each frame with elapsed milliseconds.
+        ;; Called each frame with elapsed milliseconds. Advances the movement
+        ;; timer and moves Pac-Man automatically when the interval elapses.
         (define (update! delta-time)
           (when (not paused?)
-            ((timer 'decrease!) delta-time)))
+            ((timer 'decrease!) delta-time)
+            (set! movement-timer (+ movement-timer delta-time))
+            (when (>= movement-timer pacman-speed-ms)
+              (advance-pacman!)
+              (set! movement-timer 0))))
 
         ;;
         ;; Dispatch

pacman-project/constants.rkt

@@ -33,6 +33,9 @@
           sprite-scale-key
           sprite-scale-key-ui
 
+          ;; Movement
+          pacman-speed-ms
+
           ;; Animation
           animation-interval-ms
 
@@ -93,6 +96,9 @@
     (define sprite-scale-key 1.5)
     (define sprite-scale-key-ui 3)
 
+    ;; Movement speed: time between automatic movement ticks
+    (define pacman-speed-ms 200)
+
     ;; Animation timing
     (define animation-interval-ms 100)