long to float conversion not choosing closest float

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp, python
Domain
backend

Research direction

Start with the two Python reproductions in the issue and run the suspected CPython.test_statistics test_shift_data_exact test. Trace the implementation of long-to-float conversion and true division in the IronPython runtime. Done means the conversion chooses the closest float and both reproductions and the referenced statistics test pass.

Written by the indexing model from the issue text.

Description

It looks like it always chooses previous float, for example (this should fail!):

for i in range(100):
    x = (1 << 56) + i
    assert int(float(x)) <= x

This is causing true division of long values to yield imprecise results. For example, the following (see https://github.com/IronLanguages/ironpython3/issues/844#issuecomment-652968559) fails:

assert 151246562239730615230750785535999 / 7130534626283790383418955530240 == 21.211111111111112

This is probably causing the failures in test_shift_data_exact of CPython.test_statistics.

Dominant language
C#
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Merged PRs (30d)
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