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

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skrah merged 4 commits into from
Feb 2, 2019
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bpo-26256: Document algorithm speed for the Decimal module. #4808

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bpo-26256: Document algorithm speed for the Decimal module.
csabella Dec 12, 2017
f01cd86
Fix typo
csabella Dec 22, 2017
4e8a63a
Integrate requested changes from code review
csabella Jan 28, 2019
d504981
Clarify C/CFFI versions of module
csabella Feb 2, 2019
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20 changes: 20 additions & 0 deletions Doc/library/decimal.rst
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Expand Up @@ -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')

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csabella marked this conversation as resolved.
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