cycle_tracks.rb

def skip_tracks_magnets(arr, i)
  # Sorry, got no idea how to do with the magnets
  # I probably lost the ability to code that way in Ruby ;)
end

def skip_tracks_ruby playlist, tracks
  return if tracks.zero?
  size = playlist.size
  if tracks >= 0
    tracks %= size
    playlist[0...size-tracks], playlist[size-tracks..-1] = playlist[tracks..-1], playlist[0...tracks]
  else
    tracks = tracks.abs % size
    playlist[0...tracks], playlist[tracks..-1] = playlist[size-tracks..-1], playlist[0...size-tracks]
  end
end

alias :skip_tracks :skip_tracks_ruby

cycle_tracks_spec.rb

require "./cycle_tracks"

describe "Cycle Tracks" do
  let(:playlist) {
    Array.new(10) { |i| "song #{i+1}" }
  }
  let(:size) { playlist.size }
  let(:original_playlist) { playlist.dup.freeze }

  it "should rest with zero" do
    playlist.freeze
    lambda { skip_tracks(playlist, 0) }.should_not raise_error(RuntimeError, "can't modify frozen array")
    playlist.should == original_playlist
  end

  it "should be able to skip tracks ahead" do
    (1..size-1).each { |i|
      playlist = original_playlist.dup
      skip_tracks(playlist, i)
      playlist.should == original_playlist[i..-1] + original_playlist[0...i]
    }
  end

  it "should be able to take negative numbers" do
    (1-size..-1).each { |i|
      playlist = original_playlist.dup
      skip_tracks(playlist, i)
      playlist[0...-i].should == original_playlist[size+i..-1]
      playlist[-i..-1].should == original_playlist[0...size+i]
    }
  end

  it "should be able to do this with any number" do
    (-2*size..2*size).each { |i|
      playlist = original_playlist.dup
      skip_tracks(playlist, i)
      if i < 0
        i = i % size - size
        playlist[0...-i].should == original_playlist[size+i..-1]
        playlist[-i..-1].should == original_playlist[0...size+i]
      elsif i == 0
        playlist.should == original_playlist
      else
        i %= size
        playlist[0...size-i].should == original_playlist[i..-1]
        playlist[size-i..-1].should == original_playlist[0...i]
      end
    }
  end
end