euler21.rb

   1 load 'primes.rb'
   2 load 'divisors.rb'
   3
   4 require 'logger'
   5 $log = Logger.new(STDERR)
   6 $log.level = Logger::WARN
   7
   8 def amicable_pairs(limit)
   9   cache = [0,0]
  10   2.upto(limit) do |n|
  11     cache << n.sum_of_proper_divisors
  12     $log.debug { "Sum of proper divisors of #{n} is #{n.sum_of_proper_divisors}" }
  13   end
  14   amicable_pairs = []
  15   2.upto(limit) do |n|
  16     if cache[cache[n]] == n and cache[n] > n
  17       amicable_pairs << [n, cache[n]]
  18       $log.info { "Found amicable pairs #{n} and #{cache[n]}" }
  19     end
  20   end
  21   amicable_pairs
  22 end
  23
  24 def sum_of_amicable_pairs(limit)
  25   amicable_pairs(limit).reduce(0) {|a, p| a += p.reduce(:+)}
  26 end
  27
  28 if __FILE__==$0
  29   puts sum_of_amicable_pairs 10000
  30 end
  31
  32 $log.close