Optimized Ruby perfect_number generator (See http://www.dotkam.com/2012/01/24/clojure-perfect-language-for-perfect-numbers/)

perfect_number2.rb

#!/usr/bin/env ruby -wKU

if __FILE__ == $PROGRAM_NAME
 
  range = ARGV.first.to_i
  raise 'Usage: perfect_number.rb [range]' unless range != 0

  def divisors(number)
    sqrt = Math.sqrt(number) + 1  # divsors come in pair.. so we can optimize
    lower_factors = (1..sqrt).select { |value| number % value == 0 }
    upper_factors = lower_factors.map { |value| number / value }  # i.e. 
    (lower_factors + upper_factors).uniq
  end

  range.times do |value|
    sum_of_divisors = divisors(value).inject { |sum,n| sum + n }
    puts value if sum_of_divisors == 2 * value
  end

end

Results

$ time ./perfect_number.rb 8028                                                                                                                                
6
28
496
./perfect_number.rb 8028  0.53s user 0.01s system 99% cpu 0.544 total

This optimized version is *much* better than the original at https://gist.github.com/3961422

The original unoptimized version => https://gist.github.com/3961422