gistfile1.rb

# investigate: #lazy #select

require 'prime'

class Integer
	def factors # returns an array of all factors of self
		(1..Math.sqrt(self) + 1).lazy.select { |n| (self % n).zero? }
	end
end

puts "Started at #{Time.now}."
max = 100_000_000
counter = 0

primes = Array.new(max+1) { |i| false }
Prime.each do |prime|
    break if prime > max
    primes[prime] = true

    n = prime = 1
    factors = n.factors
    counter += n if factors.all? { |d| primes[d + n/d] }
end

p counter
puts "Ended at   #{Time.now}."

http://codereview.stackexchange.com/questions/94577/euler-357-solution-efficiency/94629?noredirect=1#comment172543_94629