handle PingPong animations

Shared/Sprites.cs

@@ -16,15 +16,22 @@ namespace SemiColinGames {
     }
   }
 
+  enum AnimationDirection {
+    Forward,
+    PingPong
+  }
+
   struct SpriteAnimation {
     public readonly int Start;
     public readonly int End;
     public readonly int DurationMs;
+    public readonly AnimationDirection Direction;
 
-    public SpriteAnimation(int start, int end, int durationMs) {
+    public SpriteAnimation(int start, int end, int durationMs, AnimationDirection direction) {
       Start = start;
       End = end;
       DurationMs = durationMs;
+      Direction = direction;
     }
   }
 
@@ -61,17 +68,25 @@ namespace SemiColinGames {
         frames.Add(new Frame(source, durationMs));
       }
 
-      // TODO: handle ping-pong animations.
       JToken frameTags = json.SelectToken("meta.frameTags");
       foreach (JToken child in frameTags.Children()) {
         string name = child.SelectToken("name").Value<string>();
         int start = child.SelectToken("from").Value<int>();
         int end = child.SelectToken("to").Value<int>();
+        string directionString = child.SelectToken("direction").Value<string>();
+        AnimationDirection direction = directionString == "pingpong" ?
+            AnimationDirection.PingPong: AnimationDirection.Forward;
         int durationMs = 0;
         for (int i = start; i <= end; i++) {
           durationMs += frames[i].DurationMs;
         }
-        animations[name] = new SpriteAnimation(start, end, durationMs);
+        // A PingPong animation repeats every frame but the first and last.
+        // Therefore its duration is 2x, minus the duration of the first and last frames.
+        if (direction == AnimationDirection.PingPong) {
+          durationMs = durationMs * 2 - frames[start].DurationMs - frames[end].DurationMs;
+        }
+
+        animations[name] = new SpriteAnimation(start, end, durationMs, direction);
       }
     }
 
@@ -85,6 +100,15 @@ namespace SemiColinGames {
         }
         time -= frameTime;
       }
+      if (animation.Direction == AnimationDirection.PingPong) {
+        for (int i = animation.End - 1; i > animation.Start; i--) {
+          int frameTime = frames[i].DurationMs;
+          if (time < frameTime) {
+            return frames[i].Source;
+          }
+          time -= frameTime;
+        }
+      }
       // We shouldn't get here, but if we did, the last frame is a fine thing to return.
       return frames[animation.End].Source;
     }