Fix array index out of range bug

Closes #25

Gifu.xcodeproj/project.pbxproj

@@ -12,6 +12,7 @@
 		004529921BD82209006493BF /* Runes.swift in Sources */ = {isa = PBXBuildFile; fileRef = 004529911BD82209006493BF /* Runes.swift */; };
 		005656ED1A6F14D6008A0ED1 /* Animator.swift in Sources */ = {isa = PBXBuildFile; fileRef = 005656EC1A6F14D6008A0ED1 /* Animator.swift */; };
 		005656EF1A6F1C26008A0ED1 /* AnimatableImageView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 005656EE1A6F1C26008A0ED1 /* AnimatableImageView.swift */; };
+		007E08441BD95E6200883D0C /* ArrayExtension.swift in Sources */ = {isa = PBXBuildFile; fileRef = 007E08431BD95E6200883D0C /* ArrayExtension.swift */; };
 		009BD1391BBC7F6500FC982B /* GifuTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 009BD1381BBC7F6500FC982B /* GifuTests.swift */; };
 		009BD13B1BBC7F6500FC982B /* Gifu.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 00B8C73E1A364DA400C188E7 /* Gifu.framework */; };
 		009BD1441BBC93C800FC982B /* CGSizeExtension.swift in Sources */ = {isa = PBXBuildFile; fileRef = 009BD1431BBC93C800FC982B /* CGSizeExtension.swift */; };
@@ -38,6 +39,7 @@
 		004529911BD82209006493BF /* Runes.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = Runes.swift; path = Carthage/Checkouts/Runes/Source/Runes.swift; sourceTree = SOURCE_ROOT; };
 		005656EC1A6F14D6008A0ED1 /* Animator.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Animator.swift; sourceTree = "<group>"; };
 		005656EE1A6F1C26008A0ED1 /* AnimatableImageView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = AnimatableImageView.swift; sourceTree = "<group>"; };
+		007E08431BD95E6200883D0C /* ArrayExtension.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ArrayExtension.swift; sourceTree = "<group>"; };
 		009BD1361BBC7F6500FC982B /* GifuTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = GifuTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		009BD1381BBC7F6500FC982B /* GifuTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = GifuTests.swift; sourceTree = "<group>"; };
 		009BD13A1BBC7F6500FC982B /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
@@ -84,6 +86,7 @@
 			children = (
 				009BD1431BBC93C800FC982B /* CGSizeExtension.swift */,
 				EAF49C7E1A3A4DE000B395DF /* UIImageExtension.swift */,
+				007E08431BD95E6200883D0C /* ArrayExtension.swift */,
 			);
 			name = Extensions;
 			sourceTree = "<group>";
@@ -282,6 +285,7 @@
 				EAF49CB11A3B6EEB00B395DF /* AnimatedFrame.swift in Sources */,
 				00B8C75F1A364DCE00C188E7 /* ImageSourceHelpers.swift in Sources */,
 				EAF49C811A3A4FAA00B395DF /* FunctionalHelpers.swift in Sources */,
+				007E08441BD95E6200883D0C /* ArrayExtension.swift in Sources */,
 				EAF49C7F1A3A4DE000B395DF /* UIImageExtension.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;

Source/Animator.swift

@@ -91,9 +91,9 @@ class Animator {
   /// - returns: An optional image at a given frame.
   func updateCurrentFrame(duration: CFTimeInterval) -> Bool {
     timeSinceLastFrameChange += min(maxTimeStep, duration)
-    let frameDuration = animatedFrames[currentFrameIndex].duration
+    guard let frameDuration = animatedFrames[safe:currentFrameIndex]?.duration where
+    frameDuration <= timeSinceLastFrameChange else { return false }
 
-    if timeSinceLastFrameChange >= frameDuration {
     timeSinceLastFrameChange -= frameDuration
     let lastFrameIndex = currentFrameIndex
     currentFrameIndex = ++currentFrameIndex % animatedFrames.count
@@ -103,9 +103,7 @@ class Animator {
       animatedFrames[lastFrameIndex] = prepareFrame(currentPreloadIndex)
       currentPreloadIndex = ++currentPreloadIndex % frameCount
     }
+    
     return true
   }
-
-    return false
-  }
 }

Source/ArrayExtension.swift

@@ -0,0 +1,5 @@
+extension Array {
+  subscript(safe index: Int) -> Element? {
+    return indices ~= index ? self[index] : .None
+  }
+}