[Qt-interest] strange qFuzzyCompare implementation

[Konrad Rosenbaum]

On Monday 13 April 2009, Stephan Rose wrote:
> There actually is another way to do a float compare like this that does
> *not* rely on that type of hardcoded value.
>
> #define MaxError 5

But it relies on another type of hardcoded value.

> bool compareFloat(float p1, float p2)
> {
>   int a, b;
>   memcpy(&a, &p1, sizeof(float));
>   memcpy(&b, &p2, sizeof(float));
>
>   a = a < 0 ? 0x80000000 - a : a;
>   b = b < 0 ? 0x80000000 - b : b;
>
>   return qabs(a - b) <= MaxError;
> }

Your algorithm compares the absolutes of two numbers instead of the two 
numbers and I'm not entirely sure whether its behaviour for negative 
numbers is what you intended (floats encode the absolute plus sign, whereas 
ints encode negatives as inverted bits).

I think your conversion of negatives should be

a &= 0x7fffffff;

Of course the rules for when to convert at all are difficult to find if you 
want consistent results for large numbers and very small numbers close to 
zero (esp. a very small negative and a very small positive number that 
should compare to "sufficiently equal", while eg. -200 and +200 should 
compare to "not equal").

> If there is no error, a-b = 0.
> The greater the error is the larger a-b will be. You simply set MaxError
> to what you want the greatest deviation in your code to be.

I wouldn't exactly call that "simply set MaxError", since it requires some 
expertise to calculate a good starting point for your kind of calculation.


	Konrad

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