Ranks and unranks permutations. Based on the following paper:

W. Myrvold, F. Ruskey. (2000) "Ranking and Unranking Permutations in Linear Time". Information Processing Letters.

First install using npm:

npm install permutation-rank

Then you can use it like this:

var prank = require("permutation-rank")
var perm = [0,3,1,2]
var r = prank.rank(perm)
console.log("r = ", r)
var u = prank.unrank(perm.length, r)
console.log("u = ", u)
//Prints:
// r = 15
// u = [ 0, 3, 1, 2 ]

Computes an integer representing the colexicographic rank of the permutation

• permutation is an array encoding some permutation

Returns An integer representing the ranked encoding of the permutation

Computes a permutation from a rank order with the given length

• length is the length of the permuation
• rank is the index of the permutation
• result is an optional argument which stores the result of the inversion

Returns The permutation at the given rank

(c) 2013 Mikola Lysenko. MIT License