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Thu Jun 3 05:35:29 EDT 2004 · https://mail.python.org/pipermail/python-dev/2004-June/045181.html

Chermside, Michael wrote:
> 
> The only bit that seems tricky to me is determining the crossover
> value in pure C. How do we do that?

I think that's as simple as pow(FLT_RADIX, DBL_MAX_DIG).

So the comparison would go something like:

def cmp_floatvlong_positive(afloat, along):
    assert afloat >=0 and along >= 0
    if along < FLT_RADIX**DBL_MANT_DIG:
        # the long is representable as float
        return cmp(afloat, float(along))
    elif afloat >= FLT_RADIX**DBL_MANT_DIG:
        # the float is representable as integer
        return cmp(long(afloat), along)
    else:
        # At this point we know: afloat < FLT_RADIX**DBL_MANT_DIG <= along
        return -1

def cmp_floatvlong(afloat, along):
    ures = cmp_floatvlong_positive(abs(afloat), abs(along))
    if (afloat >= 0) != (along >= 0):
        return -ures
    else:
        return ures

Assuming FLT_RADIX is a power of 2, comparing the long to the
crossover value can be implemented by counting bits of magnitude,
replacing
    if along < FLT_RADIX**DBL_MANT_DIG:
with
    if trunc(lg(along)) < lg(FLT_RADIX)*DBL_MANT_DIG:
The crossover value as a float can be (pre-)computed the
straightforward way: pow(FLT_RADIX, DBL_MAX_DIG) will always be exact
representable.

- Anders

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SV: [Python-Dev] Comparing heterogeneous types