A comparison between the mathn library in Ruby and a self-built solution for prime generation.

primes.rb

require 'mathn'

# Generates a list of primes below a given value
def primelist(n)
  k = 1
  a = [2,3]
  while 6*k < n
    a.push(6*k-1)
    a.push(6*k+1)
    k += 1
  end
  count = 0
  y = a[count]
  while y < (n**(0.5))
    a.collect!{|x| x if x % y != 0 or x == y}.compact!
    count += 1
    y = a[count]
  end
  a
end

#using the mathn library
def primeslib(n)
  Prime.each(n).to_a
end

def runner(type)
  puts "#{type}:\n"
  ti = Time.now
  puts "Start time: #{ti}\n"
  yield
  tf = Time.now
  puts "End time: #{tf}\n"
  puts "Difference in time: #{tf-ti}\n"
end

runner("Hand-rolled") {puts primelist(2000000).inject{|sum,a| sum += a}}
runner("mathn") {puts primeslib(2000000).inject{|sum,a| sum += a}}