Combinatorics in Ruby

Some simple additions to Integer for combinatorics purposes. Will calculate the number of possibilities in an “n choose m” scenario (no repetitions). Adds factorial because that is required for the calculation.

Usage:

irb(main):001:0> require 'combinatorics'
=> true
irb(main):002:0> 3.fact
=> 6
irb(main):003:0> 10.fact
=> 3628800
irb(main):004:0> 100.fact
=> 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000
irb(main):005:0> 3.choose(2)
=> 3
irb(main):006:0> 6.choose(2)
=> 15
irb(main):007:0> 100.choose(7)
=> 16007560800
irb(main):008:0> 10000.choose(27)
=> 88666586562493644592361321224866562980818106140387273533229708768701222758880000

Code:

class Integer
  def choose(m)
    return (self.fact / (m.fact * (self - m).fact))
  end

  def fact
    (2..self).inject(1) { |f, n| f * n }
  end
end