Spawn ghost gems where gems used to be.

modules/dom/canvas.scm

@@ -23,6 +23,7 @@
   #:use-module (scheme base)
   #:use-module (hoot ffi)
   #:export (get-context
+            set-global-alpha!
             set-fill-color!
             set-font!
             set-text-align!
@@ -40,6 +41,9 @@
   (ref extern) (ref string) -> (ref extern))
 
 ;; CanvasRenderingContext2D
+(define-foreign set-global-alpha!
+  "canvas" "setGlobalAlpha"
+  (ref extern) f64 -> none)
 (define-foreign set-fill-color!
   "canvas" "setFillColor"
   (ref extern) (ref string) -> none)

modules/game/actors.scm

@@ -9,6 +9,7 @@
             ^floor-switch
             ^gate
             ^gem
+            ^ghost-gem
             ^and-gate
             ^xor-gate
             ^or-gate
@@ -268,6 +269,21 @@
      (when (eq? ($ other 'type) 'player)
        ($ picked-up? #t)))))
 
+(define (^ghost-gem bcom x y)
+  (define position (vector x y 1))
+  (match-lambda*
+    (('type) 'ghost-gem)
+    (('position) position)
+    (('tick grid-info) #f)
+    (('post-tick grid-info) #f)
+    (('enter obj grid-info) #f)
+    (('exit obj grid-info) #f)
+    (('wire-state grid-info) #f)
+    (('update-wire-state grid-info) #f)
+    (('alive?) #t)
+    (('describe) `(ghost-gem ,position))
+    (('collide other offset grid-info) #f)))
+
 (define (^gate bcom x y)
   (define position (vector x y 1))
   (define open? (spawn ^cell))
@@ -461,6 +477,7 @@
               ($ event '(bump)))))
        ('switch ($ event '(switch)))
        ('gem ($ event '(gem)))
+       ('ghost-gem #t)
        ('gate
         (unless ($ other 'open?)
           (reverse-move)

modules/game/level.scm

@@ -37,7 +37,7 @@
   (actor level-actor)
   (player level-player))
 
-(define (make-level width height background objects spawn-gem?)
+(define (make-level width height background objects collected-gem?)
   (let ((level* (spawn ^level width height))
         (background* (make-vector (* width height))))
     ;; Unpack background tile data.
@@ -70,7 +70,9 @@
                         (8 (let ((target-x (bytevector-u8-ref objects (+ i 3)))
                                  (target-y (bytevector-u8-ref objects (+ i 4))))
                              (spawn ^floor-switch x y target-x target-y)))
-                        (9 (and spawn-gem? (spawn ^gem x y)))
+                        (9 (if collected-gem?
+                               (spawn ^ghost-gem x y)
+                               (spawn ^gem x y)))
                         (10 (spawn ^gate x y))
                         (11 (spawn ^and-gate x y))
                         (12 (let ((target-x (bytevector-u8-ref objects (+ i 3)))