Calculate the first factorial where the digits sum to 8001

gistfile1.rb

#!/usr/bin/env ruby

def factorial(n)
  n == 0 ? 1 : n * factorial(n-1)
end

def R(n)
  fact = factorial(n)
  fact.to_s.split("").inject(0) { |sum, n| sum + n.to_i }
end

def find_smallest_factorial_sum(sum, n = 0)
  R(n) == sum ? n : find_smallest_factorial_sum(sum, n+1)
end

puts find_smallest_factorial_sum(8001)