Use the most precise version of "next float". (#185)

Windows.UI.Composition key frames need to have distinct progress values. Sometimes during translation we need to place a key frame as close as possible to a previous key frame. Before this change we did that by adding a small constant however that doesn't always give the smallest possible increment (because the increment depends on the exponent of the float, and because the constant we chose was conservative). With this change we actually calculate the next float value, so it is guaranteed to be as close as possible to the previous key frame.

source/LottieToWinComp/Float32.cs

@@ -27,7 +27,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
         /// Does not handle NaN or Infinity.
         /// </summary>
         /// <returns>The largest value which is less than <paramref name="value"/>.</returns>
-        internal static float Decremented(float value)
+        internal static float PreviousSmallerThan(float value)
         {
             var temp = new Float32(value);
 
@@ -41,7 +41,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
         /// Does not handle NaN or Infinity.
         /// </summary>
         /// <returns>The smallest value which is larger than <paramref name="value"/>.</returns>
-        internal static float Incremented(float value)
+        internal static float NextLargerThan(float value)
         {
             var temp = new Float32(value);
 

source/LottieToWinComp/LottieToWinCompTranslator.cs

@@ -60,10 +60,6 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
 #endif
     sealed class LottieToWinCompTranslator : IDisposable
     {
-        // Very small animation progress increment used to place keyframes as close as possible
-        // to each other.
-        const float KeyFrameProgressEpsilon = 0.0000001F;
-
         // The name used to bind to the property set that contains the Progress property.
         const string RootName = "_";
         readonly LottieComposition _lc;
@@ -3671,7 +3667,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
             // Insert the keyframes with the progress adjusted so the first keyframe is at 0 and the remaining
             // progress values are scaled appropriately.
             var previousValue = firstKeyFrame.Value;
-            var previousProgress = 0.0 - KeyFrameProgressEpsilon;
+            var previousProgress = Float32.PreviousSmallerThan(0);
             var rootReferenceRequired = false;
             var previousKeyFrameWasExpression = false;
             string progressMappingProperty = null;
@@ -3736,7 +3732,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
                         {
                             // Ensure the previous expression doesn't continue being evaluated during the current keyframe.
                             // This is necessary because the expression is only defined from the previous progress to the current progress.
-                            insertKeyFrame(compositionAnimation, (float)previousProgress + KeyFrameProgressEpsilon, previousValue, _c.CreateStepThenHoldEasingFunction());
+                            insertKeyFrame(compositionAnimation, Float32.NextLargerThan(previousProgress), previousValue, _c.CreateStepThenHoldEasingFunction());
                         }
 
                         // The easing for a keyframe at 0 is unimportant, so always use Hold.
@@ -3753,16 +3749,16 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
                         // so that there is room to add a key frame just after this to hold
                         // the final value. This is necessary so that the expression we're about
                         // to add won't get evaluated during the following segment.
-                        if (adjustedProgress > 0)
+                        if ((float)adjustedProgress > 0)
                         {
-                            adjustedProgress -= KeyFrameProgressEpsilon;
+                            Float32.PreviousSmallerThan((float)adjustedProgress);
                         }
 
 #if !LinearEasingOnSpatialBeziers
                         // Add an animation to map from progress to t over the range of this key frame.
                         if (previousProgress > 0)
                         {
-                            progressMappingAnimation.InsertKeyFrame((float)previousProgress + KeyFrameProgressEpsilon, 0, _c.CreateStepThenHoldEasingFunction());
+                            progressMappingAnimation.InsertKeyFrame(Float32.NextLargerThan(previousProgress), 0, _c.CreateStepThenHoldEasingFunction());
                         }
 
                         progressMappingAnimation.InsertKeyFrame((float)adjustedProgress, 1, _c.CreateCompositionEasingFunction(keyFrame.Easing));
@@ -3784,7 +3780,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
                     if (previousKeyFrameWasExpression)
                     {
                         // Ensure the previous expression doesn't continue being evaluated during the current keyframe.
-                        insertKeyFrame(compositionAnimation, (float)previousProgress + KeyFrameProgressEpsilon, previousValue, _c.CreateStepThenHoldEasingFunction());
+                        insertKeyFrame(compositionAnimation, Float32.NextLargerThan(previousProgress), previousValue, _c.CreateStepThenHoldEasingFunction());
                     }
 
                     insertKeyFrame(compositionAnimation, (float)adjustedProgress, keyFrame.Value, _c.CreateCompositionEasingFunction(keyFrame.Easing));
@@ -3792,14 +3788,14 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
                 }
 
                 previousValue = keyFrame.Value;
-                previousProgress = adjustedProgress;
+                previousProgress = (float)adjustedProgress;
             }
 
             if (previousKeyFrameWasExpression && previousProgress < 1)
             {
                 // Add a keyframe to hold the final value. Otherwise the expression on the last keyframe
                 // will get evaluated outside the bounds of its keyframe.
-                insertKeyFrame(compositionAnimation, (float)previousProgress + KeyFrameProgressEpsilon, (T)(object)previousValue, _c.CreateStepThenHoldEasingFunction());
+                insertKeyFrame(compositionAnimation, Float32.NextLargerThan(previousProgress), (T)(object)previousValue, _c.CreateStepThenHoldEasingFunction());
             }
 
             // Add a reference to the root Visual if needed (i.e. if an expression keyframe was added).