Closed Bug 702412 Opened 8 years ago Closed 8 years ago

[layers] float comparisons should use an epsilon

Categories

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

All
Android
defect

Tracking

()

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

People

(Reporter: kats, Assigned: kats)

References

Details

Attachments

(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 PanZoomPosition.java 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Attachment #574962 - Attachment is obsolete: true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/PanZoomController.java
@@ +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+
http://hg.mozilla.org/projects/birch/rev/e056708c00aa
Status: ASSIGNED → RESOLVED
Closed: 8 years ago
Resolution: --- → FIXED
tracking-fennec: --- → 11+
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