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math.combinatorics: use cond in k-permutations.

db4
John Benediktsson 2013-05-15 15:09:25 -07:00
parent c1e91316d2
commit 917ae3e410
1 changed files with 8 additions and 6 deletions
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basis/math/combinatorics/combinatorics.factor
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@ -1,9 +1,9 @@
! Copyright (c) 2007-2010 Slava Pestov, Doug Coleman, Aaron Schaefer, John Benediktsson. ! Copyright (c) 2007-2010 Slava Pestov, Doug Coleman, Aaron Schaefer, John Benediktsson.
! See http://factorcode.org/license.txt for BSD license. ! See http://factorcode.org/license.txt for BSD license.
USING: accessors arrays assocs binary-search classes.tuple fry USING: accessors arrays assocs binary-search classes.tuple
hints kernel locals math math.order math.ranges memoize combinators fry hints kernel locals math math.order math.ranges
namespaces sequences sequences.private sorting ; memoize namespaces sequences sequences.private sorting ;
FROM: sequences => change-nth ; FROM: sequences => change-nth ;
IN: math.combinatorics IN: math.combinatorics
@ -70,9 +70,11 @@ TUPLE: k-permutations length skip k seq ;
:: <k-permutations> ( seq k -- permutations ) :: <k-permutations> ( seq k -- permutations )
seq length :> n seq length :> n
n k nPk :> len n k nPk :> len
len zero? k zero? or [ { } ] [ {
len n factorial over /i k seq k-permutations boa { [ len k [ zero? ] either? ] [ { } ] }
] if ; { [ n k = ] [ seq <permutations> ] }
[ len n factorial over /i k seq k-permutations boa ]
} cond ;
M: k-permutations length length>> ; inline M: k-permutations length length>> ; inline
M: k-permutations nth-unsafe M: k-permutations nth-unsafe