Get animation working

src/Presentation.cake

@@ -239,6 +239,8 @@
   (var current-slide-index int 0)
   (var slide-start-ticks Uint64 0)
 
+  (var animation-start-ticks Uint64 (SDL_GetPerformanceCounter))
+
   (var exit-reason (* (const char)) null)
   (while true
     (var event SDL_Event)
@@ -334,16 +336,29 @@
 	;; 	return 1;
 	;; }#"#)
 
+    (var wizard-width int 161)
+    (var wizard-height int 106)
+    (var animation-time-seconds float
+      (/ (- (SDL_GetPerformanceCounter) animation-start-ticks)
+         (type-cast (SDL_GetPerformanceFrequency) float)))
+    (var loop-rate float 0.7f)
+    (var num-frames int 6)
+    (var-cast-to current-frame int
+                 (interpolate-range
+                  0.f (type-cast num-frames float)
+                  0.f loop-rate
+                  (- animation-time-seconds
+                     (* loop-rate (truncf (/ animation-time-seconds loop-rate))))))
     (when (> current-slide-index 0)
       (var source-rectangle SDL_Rect
-        (array 38 0
-               106
-               107))
+        (array (* current-frame wizard-width) 0
+               wizard-width
+               wizard-height))
       (var destination-rectangle SDL_Rect
         (array (type-cast (* window-width 0.08f) int)
                (type-cast (* window-height 0.77f) int)
-               (* 2 106)
-               (* 2 107)))
+               (* 2 wizard-width)
+               (* 2 wizard-height)))
       (unless (= 0 (SDL_RenderCopyEx renderer wizard-texture
                                      (addr source-rectangle) (addr destination-rectangle)
                                      0.f null ;; no rotation