example_bson_id.rb

require 'digest/md5'

def bson_id
  [
    '%08x' % Time.now.utc.to_i,                    # 4 bytes
    Digest::MD5.hexdigest(`hostname`.strip)[0..5], # 3 bytes
    '%04x' % $$,                                   # 2 bytes
    '%06x' % rand(0..('f' * 6).to_i(16))           # 3 bytes
  ]                                                # 12 bytes total
end

puts bson_id.inspect
puts bson_id.join