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[Bug-gsl] [bug #51104] gsl_permutation_next efficiency

From: Patrick Alken
Subject: [Bug-gsl] [bug #51104] gsl_permutation_next efficiency
Date: Wed, 24 May 2017 04:46:44 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36

Follow-up Comment #1, bug #51104 (project gsl):

from rsmith =at= whistlin =dot= com

I think Pavel Sinitcyn's code gives a more literal translation of step L3 in
Knuth's description of the algorithm ("Algorithm L" in AoCP vol 4, page 319).
The change decreases the running time by over 10% in my tests.  I haven't
tested for accuracy, though.

Incidentally, the existing version of gsl_permutation_next() in
has a superfluous check in the boolean expression on line 155:

    (p->data[j] > p->data[i]) && (p->data[j] < p->data[k])

The while loop starting on line 141 guarantees that p->data[j] < p->data[k]
i<k<j (Knuth explains this in step L3).  But I think the slow down is caused
the structure of the for loop, which seems awkward compared to the
straightforward textbook version.



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