sampler.rb

#!/usr/bin/env ruby

limit = ARGV[0].to_i

result = Array.new(limit)

count = 0
STDIN.each_line do |line|
    if count < limit
        result[count] = line
    else
        j = rand(count + 1)
        result[j] = line if j < limit
    end
    count += 1
end

result.each { |l| print l }