A library for manipulating arbitrary permutations. It is capable of:

• testing if a permutation is valid (checkPermutation)
• testing if two permutations are equal (permutationsEqual)
• converting from cycle to permutation (cycleToPerm)
• composing permutations (composePermutations)
• inverting permutations (inverse)
• checking if a permutation is an identity permutation (isIdentity)
• calculating the orbits of a permutation (orbits)
• decomposing a permutation into transpositions (transpositionDecomposition)

Permutations are represented as objects, so that that this permutation:

/  1  2  3  4  5 \
\  2  3  4  5  1 /

.. is represented as:

var p1 = { "1" : "2", "2": "3", "3": "4", "4": "5", "5": "1" }

Cycles are represented as lists, so that this cycle:

(12345)

... is represented as:

var o1 = ["1", "2", "3", "4", "5"];

Cycle to Permutation

You can convert a cycle to a permutation.

> var o = require('orbits')
> var p1 = { "1" : "2", "2": "3", "3": "4", "4": "5", "5": "1" }
undefined
> var o1 = ["1", "2", "3", "4", "5"];
undefined
> o.permutationsEqual(p1, o.cycleToPerm(o1));
true
>

Inverse

You can also calculate the inverse of a permutation.

> o.inverse(p1);
{ '1': '5', '2': '1', '3': '2', '4': '3', '5': '4' }
> o.inverse(o.cycleToPerm(o1));
{ '1': '5', '2': '1', '3': '2', '4': '3', '5': '4' }

Orbits

Given another permutation,

var p2 = {"a": "b", "b": "c", "c": "a", "d": "e", "e": "d"};

You can decompose p2 into orbits:

> o.orbits(p2);
[ [ 'a', 'b', 'c' ], [ 'd', 'e' ] ]

Composing permutations

You can compose p1 and p2:

> var p3 = o.composePermutations(p1, p2);
undefined
> p3
{ '1': '2',
  '2': '3',
  '3': '4',
  '4': '5',
  '5': '1',
  a: 'b',
  b: 'c',
  c: 'a',
  d: 'e',
  e: 'd' }
>

And then compose p3 with itself:

> o.composePermutations(p3, p3);
{ '1': '3',
  '2': '4',
  '3': '5',
  '4': '1',
  '5': '2',
  a: 'c',
  b: 'a',
  c: 'b',
  d: 'd',
  e: 'e' }

Transposition Decomposition

We can decompose p3 into transpositions.

> o.transpositionDecomposition(p3);
[ [ '1', '2' ],
  [ '2', '3' ],
  [ '3', '4' ],
  [ '4', '5' ],
  [ 'a', 'b' ],
  [ 'b', 'c' ],
  [ 'd', 'e' ] ]

License

orbits is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

orbits is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License along with orbits. If not, see http://www.gnu.org/licenses/.