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bpo-26256: Document algorithm speed for the Decimal module. (#4808)

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Cheryl Sabella 2019-02-02 09:37:39 -05:00 committed by Stefan Krah
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Doc/library/decimal.rst
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@ -2115,3 +2115,23 @@ Alternatively, inputs can be rounded upon creation using the
>>> Context(prec=5, rounding=ROUND_DOWN).create_decimal('1.2345678')
Decimal('1.2345')
Q. Is the CPython implementation fast for large numbers?
A. Yes. In the CPython and PyPy3 implementations, the C/CFFI versions of
the decimal module integrate the high speed `libmpdec
<https://www.bytereef.org/mpdecimal/doc/libmpdec/index.html>`_ library for
arbitrary precision correctly-rounded decimal floating point arithmetic.
``libmpdec`` uses `Karatsuba multiplication
<https://en.wikipedia.org/wiki/Karatsuba_algorithm>`_
for medium-sized numbers and the `Number Theoretic Transform
<https://en.wikipedia.org/wiki/Discrete_Fourier_transform_(general)#Number-theoretic_transform>`_
for very large numbers. However, to realize this performance gain, the
context needs to be set for unrounded calculations.
>>> c = getcontext()
>>> c.prec = MAX_PREC
>>> c.Emax = MAX_EMAX
>>> c.Emin = MIN_EMIN
.. versionadded:: 3.3