How should I do an equality test for 80bit IEEE floating point?

Question

related to:

• comparing ieee floats and doubles for equality
• Should we compare floating point numbers for equality against a relative error
• Most effective way for float and double comparison

However with regard to 80 bit IEEE floats (see section 8.2) on an x86

In particular I like the this implementation using a count of representable values between the operands because it scale by default.

One use case would be for numeric approximations where two values approach each other and I need to check to see if they are close enough.

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p.s. The implementation language will be D but I can translate. Also an implementation that can automatically handle whatever the underlying type is (for instance if only 64bit real was available) would be ideal.

The current code in use:

Answer 1

My current solution is

bool Near(real a, real b, int count = 5)
{
    // Returns the number of mantissa bits which are equal in x and y.
    int i = std.math.feqrel!(real)(a,b);
    return i + count >= real.mant_dig;
}

It gives the number of miss-matched bits in the inputs. I'm not sure how well this will work near powers of 2.

Answer 2

Since D has 80-bit reals built in (as far as I can tell), why would you not just use the standard approach of comparing with an epsilon value. This can be a fixed value if you know the rough range in advance, such as US currency:

if (abs (a - b) < 1e-6) // effectively equal

or an acceptable relative error, such as 1 part per million of the average:

if (abs (a - b) < ((a + b) / 2) / 1e6) // effectively equal

Keep in mind I don't know D, the code above is for demonstrative purposes only.