Xapian devel - Jul 2008 - [Xapian-commits] 10821: trunk/xapian-core/ trunk/xapian-core/api/

On Sun, Jul 06, 2008 at 11:57:40PM +0100, richard wrote:
> api/omenquire.cc: When calculating percentages, round to the
> nearest integer, rather than rounding down.  There was a FIXME
> about this, but no explanation of why it hadn't already been
> done, and I can see no bad side effects so far.  The most obvious
> positive effect is that queries which should get precisely 100%
> will no longer be assigned 99% due to rounding errors.
Well, one issue is that queries which shouldn't get precisely 100% now
can...

I don't know how common an issue that is, but then I don't know how
common the issue you mention is either.

Cheers,
    Olly

Olly Betts wrote:
> On Sun, Jul 06, 2008 at 11:57:40PM +0100, richard wrote:
>> api/omenquire.cc: When calculating percentages, round to the
>> nearest integer, rather than rounding down.  There was a FIXME
>> about this, but no explanation of why it hadn't already been
>> done, and I can see no bad side effects so far.  The most obvious
>> positive effect is that queries which should get precisely 100%
>> will no longer be assigned 99% due to rounding errors.
> 
> Well, one issue is that queries which shouldn't get precisely 100% now
> can...
> 
> I don't know how common an issue that is, but then I don't know how
> common the issue you mention is either.
The test case I committed yesterday suffered from this problem for me, 
and I've certainly seen it before (generally with large queries), but I 
couldn't guess at a rate at which it occurs.

I don't think it's unreasonable to return 100% for a document which 
matches well enough to get 99.5%; and it's certainly more reasonable 
than returning 99% for a document which actually got 99.999999%.

I suppose we could instead round up only very slightly, so that a 
document needed to get at least 99.9999% or so to be returned with 100%. 
  I'm not sure whether that would be better or worse than rounding to 
nearest, but either is better than the rounding down which we had.

-- 
Richard