Deadfish interpreter written in Ruby (http://esolangs.org/wiki/Deadfish)

deadfish.rb

#!/usr/bin/env ruby

n = 0
while true
  print '>> '
  gets.chomp.each_char do |c|
    n = 0 if [-1, 256].include?(n)
    case c
      when 'd' then n -= 1
      when 'i' then n += 1
      when 'o' then puts n
      when 's' then n *= n
    end
  end
end