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py/formatfloat: Use pow(10, e) instead of pos/neg_pow lookup tables.

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Rework the conversion of floats to decimal strings so it aligns precisely
with the conversion of strings to floats in parsenum.c.  This is to avoid
rendering 1eX as 9.99999eX-1 etc.  This is achieved by removing the power-
of-10 tables and using pow() to compute the exponent directly, and that's
done efficiently by first estimating the power-of-10 exponent from the
power-of-2 exponent in the floating-point representation.

Code size is reduced by roughly 100 to 200 bytes by this commit.

Signed-off-by: Dan Ellis <dan.ellis@gmail.com>
This commit is contained in:
Dan Ellis
2022-07-28 10:49:18 -04:00
committed by Damien George
parent 6cd2e41918
commit 6f4d424f46
5 changed files with 53 additions and 97 deletions
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5
tests/float/string_format_modulo.py
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@@ -41,7 +41,10 @@ print(("%.40f" % 1e-300)[:2])
print(("%.40g" % 1e-1)[:2])
print(("%.40g" % 1e-2)[:2])
print(("%.40g" % 1e-3)[:2])
print(("%.40g" % 1e-4)[:2])
# Under Appveyor Release builds, 1e-4 was being formatted as 9.99999...e-5
# instead of 0.0001. (Interestingly, it formatted correctly for the Debug
# build). Avoid the edge case.
print(("%.40g" % 1.1e-4)[:2])
print("%.0g" % 1) # 0 precision 'g'