[Shootout-list] nsieve for Ruby

[Glenn Parker]

#!/usr/bin/ruby
#
# The Great Computer Language Shootout
# http://shootout.alioth.debian.org/
#
# Contributed by Glenn Parker, March 2005

def sieve(m)
   flags = "\x1" * m
   count = 0
   pmax = m - 1
   2.step(pmax, 1) do |p|
     if flags[p] == 1
       count += 1
       p.step(pmax, p) do |mult|
	flags[mult] = 0
       end
     end
   end
   count
end

n = (ARGV[0] || 2).to_i
n.step(n - 2, -1) do |exponent|
   break if exponent < 0
   m = 2 ** exponent * 10_000
   count = sieve(m)
   printf "Primes up to %8d %8d\n", m, count
end