cout with test

#!/usr/bin/env ruby

def cout(*args)
  raise 'no args' if args.empty?
  format = args.shift
  out = ''
  until format.empty?
    if m = format.match(/%(.)/)
      out << m.pre_match
      case m[1]
      when '%'
        out << '%'
      when 'd'
        raise 'few args' if args.empty?
        out << args.shift.to_i.to_s
      else
        raise "illegal input: \"#{m[1]}\""
      end
      format = m.post_match
    else
      break
    end
  end
  raise 'unmatch %' if format.include? '%'
  raise "too many args: #{args}" unless args.empty?
  puts out << format
end

require 'test/unit'
class TestCout < Test::Unit::TestCase
  def capture_cout(*args)
    if args.empty?
      capture_output do
        cout
      end
    else
      capture_output do
        cout *args
      end
    end
  end
  def test_cout
    assert_raise_message('unmatch %') { capture_cout '%' }
    assert_raise_message('too many args: [2]') { capture_cout 'abc%d', 1, 2 }
    assert_raise_message('few args') { capture_cout 'abc%d' }
    assert_raise_message('illegal input: "i"') { capture_cout '%d %i', 1, 2 }
    assert_raise_message('no args') { capture_cout }
    assert_equal ["1 etc.\n", ''], capture_cout('%d etc.', 1)
  end
end