F=
# 80
->s,l{s+=' '*(l-s.size%l)
(s.size/l).times{|i|puts [x=s[i*l,l],x.reverse][i%2]}}

require 'minitest/autorun'

describe F do
  def test_case_1
    expected = <<-EOS
Prog
mmar
ing 
zzuP
les 
oC &
de G
 flo
EOS

    actual = capture_stdout(){ F['Programming Puzzles & Code Golf', 4] }

    assert_equal expected, actual
  end

  def test_case_2
    expected = <<-EOS
Hello
roW ,
ld!  
EOS

    actual = capture_stdout(){ F['Hello, World!', 5] }

    assert_equal expected, actual
  end
end



require 'stringio'

module Kernel
  def capture_stdout(console_input = '')
    $stdin = StringIO.new(console_input)
    out = StringIO.new
    $stdout = out
    yield
    return out.string
  ensure
    $stdout = STDOUT
    $stdin = STDIN
  end
end