r.rb

require 'prime'

class Integer
    # Make a lazy enumerator of the lower half of factors of self
    def half_factors
        (1..Math.sqrt(self) + 1).lazy.select { |n| (self % n).zero? }
    end
end

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

max = 100_000_000

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

    n = prime - 1
    half_factors = n.half_factors    
    counter += n if half_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/