Re: [Audacity-devel] Floating point crash repeating less than a sample of audio

[Martyn S. <mar...@gm...>]

Thanks Steve for the quick diagnosis of this one!  We should fix this 
post-2.0.5.  Def avoid a divide-by-zero!

Not relevant to this bug, or the release process, and quite academic...

I notice that this section of code (EffectRepeat::PromptUser(), lines 
57-81) is probably pretty redundant.  Written in 2003 when we counted 
samples with a 32-bit int, tracks had a maximum length of 2^31 - 1, 
hence the check to make sure they don't get too long (working out 
maxCount and applying it in lines 97-8).

Since we updated sampleCount to a 64-bit int, the chances of overflow 
are tiny (and I suspect current machines couldn't do that before the 
user get bored!).  And so the constant 2147483647 no longer seems 
relevant in checking availSamples, I think it should be much more.

For these very long lengths I believe that we would start to fail (in 
a number of places) at a number of samples around 2^53, due to the use 
of 'double' to represent 'time' 
http://en.wikipedia.org/wiki/Double-precision_floating-point_format, 
and our assumption that we can convert back and forth from 'time' to 
'sampleCount' without error fails beyond 2^53 samples.  (Did I get 
that right?  I believe Richard is the expert here.)

So we should do something like
-      int availSamples = 2147483647 - trackLen;
+      sampleCount availSamples = 2^52 - trackLen;
(correcting the size of the other variables) if we want people to be 
able to 'repeat' stuff beyond 13 hours @ 44.1kHz sampling rate, or 35 
mins @ 1MHz sampling rate, there may be reasonable use-cases for 
wanting to do that.

Is it worth doing?

TTFN
Martyn

On 13/10/2013 14:20, Steve the Fiddle wrote:
> On 13 October 2013 07:44, Steve the Fiddle <ste...@gm...> wrote:
>> On 13 October 2013 04:49, Gale (Audacity Team) <ga...@au...> wrote:
>>> FWIW, reported on the Forum:
>>> http://forum.audacityteam.org/viewtopic.php?f=48&t=75165 .
>>>
>>> I can make it happen thus on Windows and Linux:
>>>
>>> 1 Generate Tone
>>> 2 Zoom in to maximum
>>> 3 drag-select a small distance in the waveform without drawing
>>> a selection there (that is, the selection will only be in the Timeline)
>>> 4 Effect > Repeat.
>>>
>>> None of the other effects I've tried crash.
>>
>> Certainly not a release blocker imho. The issue is also reproducible
>> on Audacity 1.3.12 so it's been around for at least 5 years but is so
>> obscure that it's not been reported 'till now. I'd say around P4 and
>> mark it "Cherry" (it looks to me like a simple divide by zero error).
>
> Added to bugzilla as bug 669.
>
> Steve
>
>>
>> Steve
>>
>>
>>>
>>>
>>> Gale
>
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