permutations-count

Calculate the number of possible permutations given the size of a set and the size of the ordered sub-sets (permutations).

Install

$ npm install --save permutations-count

Usage

var permutationsCount = require('permutations-count');

var numPermutations = permutationsCount(12, 3);
console.log(numPermutations); // => 1320

console.log(permutationsCount(1000, 30));
// ... this will take a few seconds
// => 6.444605211972133e+89

permutationsCount(n, r)

• Returns number
• Requires n >= r

The returned number is the number of ways of obtaining an ordered subset of r elements from a set of n elements.

Large numbers

Works for large numbers thanks to big-integer via big-factorial. But when the numbers get large the calculations are slow, and you loose precision when the result is > 9007199254740992.

From the big-integer readme:

bigInt("18446744073709551616").toJSNumber() => 18446744073709552000