make a list of prime numbers.

listprimes_simple.rb

#!/opt/local/bin/ruby1.9 -w
# -*- coding: utf-8 -*-
# listprime.rb: make a list of prime numbers.

require 'prime'

def die(*x)
  STDERR.puts x
  exit 1
end

if ARGV.length < 2
  die <<USAGE
Usage: #{__FILE__} <start> <range>
print prime numbers (start <= primes < start + range)
USAGE
end

start = ARGV.shift.to_i
range = ARGV.shift.to_i

def prime(n)
  return [false, 1] if n.even?
  c = 1
  d = 3
  limit = Math::sqrt(n)
  while d <= limit
    return [false, c] if (n % d == 0)
    c += 1
    d += 2
  end
  [true, c]
end

t0 = Time.now
(start...(start + range)).each do |n|
  result = prime(n)
  print "#{n} (#{result[1]})\n" if result[0]
end
elapsed = Time.now - t0
print "Elapsed: #{elapsed}\n"