_darcs/pristine/doc.txt

# * Permutations.plt: Generating and manipulating permutations *-* outline *-*

```* _Permutations.plt_: Generating and manipulating permutations   *-* outline *-*

** Introduction
The permutations.plt PlaneT package provides utilities to generate and
manipulate permutations.  To include these functions in your programs,
issue the command

(require (planet "permutations.ss" ("wmfarr" "permutations.plt" 1 2)))

The permutations package is maintained by Will M. Farr <farr@mit.edu>
and is released under the GPL (see the License section of this
bug reports to Will at the above email address.

** Permutation Functions

> (permutation? obj) : #f unless obj is a permutation: a vector of n
elements which contains each of the numbers 0, 1, ..., n-1 exactly
once.  Note that this predicate is O(n) in the length of the vector
given!

> (permutation-identity n) : (vector 0 1 ... (- n 1))

> (pvector-ref p v i) : References the (vector-ref p i)-th element of
the vector v.

> (pvector-set! p v i x) : Sets the (vector-ref p i)-th element of v
to x.

> (permutation-next! v) : Assuming v is a permutation, generates the
next permutation in _lexigraphical order_, and stores the result in
v.  Returns the modified v.  If v is the last permutation in a
lexical sequence of permutations, permutation-next! does not modify
v and returns #f.  In this way, one can iterate over permutations
using code like

(let ((v (identity-permutation n)))
(let loop ((p v))
(when p
(do-something-with-permutations p ...)
(loop (permutation-next! p)))))

> (permutation-next v) : Returns the permutation following v in
lexigraphical order without modifying v.  If v is the last
permutation in a lexical sequence, returns #f.

> (permutation-signature v) : Returns either 1 or -1 corresponding to
whether v is even or odd, respectively.  An odd permutation requires
an odd number of interchanges to be turned into the identity
permutation; similarly for an even permutation.

** SRFI-42 Generator

> (:permutations var [(index i)] n) : SRFI-42 generator.  Generates
sequentially the n-element permutations in lexigraphical order,
optionally binding i to the current number in the sequence.  The
generation is lazy, so one need not worry about exhausting available
memory by generating long permutations.

Version 2, June 1991

Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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Yoyodyne, Inc., hereby disclaims all copyright interest in the program
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<signature of Ty Coon>, 1 April 1989
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