[Issue 838] Update ASCeilPixelValue and ASRoundPixelValue

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## master
 * Add your own contributions to the next release on the line below this with your name.
+- User FLT_EPSILON in ASCeilPixelValue and ASFloorPixelValue to help with floating point precision errors when computing layouts for 3x devices. [Ricky Cancro](https://github.com/rcancro) [#838](https://github.com/TextureGroup/Texture/pull/864)
 - Disable interface colescing and match to pre-colescing interface update behavior [Max Wang](https://github.com/wsdwsd0829) [#862](https://github.com/TextureGroup/Texture/pull/862) 
 - [ASDisplayNode] Add safeAreaInsets, layoutMargins and related properties to ASDisplayNode, with full support for older OS versions [Yevgen Pogribnyi](https://github.com/ypogribnyi) [#685](https://github.com/TextureGroup/Texture/pull/685)
 - [ASPINRemoteImageDownloader] Allow cache to provide animated image. [Max Wang](https://github.com/wsdwsd0829) [#850](https://github.com/TextureGroup/Texture/pull/850)

Source/Private/ASInternalHelpers.m

@@ -177,10 +177,23 @@ CGSize ASFloorSizeValues(CGSize s)
   return CGSizeMake(ASFloorPixelValue(s.width), ASFloorPixelValue(s.height));
 }
 
+// See ASCeilPixelValue for a more thoroguh explanation of (f + FLT_EPSILON),
+// but here is some quick math:
+//
+// Imagine a layout that comes back with a height of 100.66666666663
+// for a 3x deice:
+// 100.66666666663 * 3 = 301.99999999988995
+// floor(301.99999999988995) = 301
+// 301 / 3 = 100.333333333
+//
+// If we add FLT_EPSILON to normalize the garbage at the end we get:
+// po (100.66666666663 + FLT_EPSILON) * 3 = 302.00000035751782
+// floor(302.00000035751782) = 302
+// 302/3 = 100.66666666
 CGFloat ASFloorPixelValue(CGFloat f)
 {
   CGFloat scale = ASScreenScale();
-  return floor(f * scale) / scale;
+  return floor((f + FLT_EPSILON) * scale) / scale;
 }
 
 CGPoint ASCeilPointValues(CGPoint p)
@@ -193,10 +206,29 @@ CGSize ASCeilSizeValues(CGSize s)
   return CGSizeMake(ASCeilPixelValue(s.width), ASCeilPixelValue(s.height));
 }
 
+// With 3x devices layouts will often to compute to pixel bounds but
+// include garbage values beyond the precision of a float/double.
+// This garbage can result in a pixel value being rounded up when it isn't
+// necessary.
+//
+// For example, imagine a layout that comes back with a height of 100.666666666669
+// for a 3x device:
+// 100.666666666669 * 3 = 302.00000000000699
+// ceil(302.00000000000699) = 303
+// 303/3 = 101
+//
+// If we use FLT_EPSILON to get rid of the garbage at the end of the value,
+// things work as expected:
+// (100.666666666669 - FLT_EPSILON) * 3 = 301.99999964237912
+// ceil(301.99999964237912) = 302
+// 302/3 = 100.666666666
+//
+// For even more conversation around this, see:
+// https://github.com/TextureGroup/Texture/issues/838
 CGFloat ASCeilPixelValue(CGFloat f)
 {
   CGFloat scale = ASScreenScale();
-  return ceil(f * scale) / scale;
+  return ceil((f - FLT_EPSILON) * scale) / scale;
 }
 
 CGFloat ASRoundPixelValue(CGFloat f)