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Author fredrikj
Recipients fredrikj
Date 2008-07-27.01:11:01
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A few weeks ago, I blogged about taking advantage of Karatsuba
multiplication and Newton's method to divide big integers quickly (some
of you may have read it, as it was posted to Daily Python-URL among
other places):

To summarize, this method blows the builtin division out of the water 
already at ~(2000 digits)/(1000 digits).

The primary catch is that the result of the Newton division may be
slightly wrong (typically 1 ulp). However, a single extra multiplication
and a subtraction at the end allows one to compute a remainder, and
since the remainder must satisfy 0 <= r < q, the error is easily
corrected. From a quick test, the cost of the extra multiplication seems
to move the break-even point with the builtin division up to around
5000/2500 digits.

A pure Python implementation of divmod, with error correction based on
the remainder, can be found in this file:

(See the function idivmod)

Of particular note is that fast divmod gives a fast way to do radix
conversion, by recursively splitting the number in half. The function
numeral (see same .py file) does this, e.g:

>>> from time import clock
>>> a = 2**1257787-1
>>> t1=clock(); s1=str(a); t2=clock(); t2-t1
>>> t1=clock(); s2=numeral(a); t2=clock(); t2-t1
>>> s1 == s2

(This recursive algorithm, by the way, is actually faster than str()
even with the slow builtin divmod.)

Would there be any interest in porting these algorithms to C and using
them in the standard Python long implementation?

There are likely some problems that I have overlooked. A critical review
will be most welcome.
Date User Action Args
2008-07-27 01:11:06fredrikjsetrecipients: + fredrikj
2008-07-27 01:11:05fredrikjsetmessageid: <>
2008-07-27 01:11:04fredrikjlinkissue3451 messages
2008-07-27 01:11:02fredrikjcreate