Closed Bug 702412 Opened 11 years ago Closed 11 years ago

[layers] float comparisons should use an epsilon


(Firefox for Android Graveyard :: General, defect, P2)



(firefox11 fixed, fennec11+)

Tracking Status
firefox11 --- fixed
fennec 11+ ---


(Reporter: kats, Assigned: kats)




(2 files, 1 obsolete file)

Floating point comparisons in java should generally use an epsilon rather than being compared to each other directly. Doing (foo == 0.0f) may fail when even though (Math.abs(foo) < 1e-7) passes, due to floating point rounding issues.

Code that uses == for float comparisons exists in and a few other classes in the layers code.
Priority: -- → P2
Attached patch Add epsilon to float compares (obsolete) — Splinter Review
Turns out the other places this happened were in PanZoomPosition, which existed only briefly in the patch queue.
Attachment #574962 - Flags: review?(pwalton)
Oops, wrong version of patch
Attachment #574962 - Attachment is obsolete: true
Attachment #574962 - Flags: review?(pwalton)
Attachment #574963 - Flags: review?(pwalton)
Comment on attachment 574963 [details] [diff] [review]
Add epsilon to float compares

Review of attachment 574963 [details] [diff] [review]:

getExcess() actually explicitly returns 0.0f, so I'd have thought the comparison would be ok (I assume that 0.0f == 0.0f holds - but perhaps it doesn't after assignment?). Does this fix the crash you were seeing?
getExcess() explicitly returns 0.0f, but it could also return things like viewportPos or getViewportEnd() - mPageLength, and those values might be really really close to zero without actually being zero.

I've attached a test case that shows how rounding error can result in values just slightly off because of floating point representation.

I thought that this might have caused the crash in bug 700559 by sending the state into WAITING_TO_SNAP instead of STOPPED, but after looking through the logic again I don't think that's possible.
Comment on attachment 574963 [details] [diff] [review]
Add epsilon to float compares

Review of attachment 574963 [details] [diff] [review]:

Fine by me, with the method name renamed.

::: embedding/android/ui/
@@ +333,5 @@
>      private float computeElasticity(float excess, float viewportLength) {
>          return 1.0f - excess / (viewportLength * SNAP_LIMIT);
>      }
> +    private static boolean floatsEqual(float a, float b) {

I'd call this floatsNearlyEqual() or something -- floatsEqual() isn't accurate.
Attachment #574963 - Flags: review?(pwalton) → review+
Closed: 11 years ago
Resolution: --- → FIXED
tracking-fennec: --- → 11+
Product: Firefox for Android → Firefox for Android Graveyard
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