jukebox.rb

def skip_tracks(arr, i)
  start = inc = i % arr.size()
  count = 1
  
  while inc > 0 && count < arr.size() do
    c = start
    
    begin
      before = (c - inc) % arr.size()
      arr[before], arr[c] = arr[c], arr[before]
      count = count + 1
      c = (c + inc) % arr.size()
    end while (c + inc) % arr.size() != start
    
    count = count + 1
    start = start - 1
  end
end

def skip_tracks_new_version(playlist, tracks)
  return if tracks.abs >= playlist.size or tracks.zero?
  
  if tracks > 0
    playlist.push(playlist.shift(tracks)).flatten!
  else
    playlist.unshift(playlist.pop(tracks.abs)).flatten!
  end
end

jukebox_spec.rb

require 'jukebox'

describe "jukebox" do 
  before do
    @playlist = %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10]
    @variations = [
      %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10],
      %w[song2 song3 song4 song5 song6 song7 song8 song9 song10 song1],
      %w[song3 song4 song5 song6 song7 song8 song9 song10 song1 song2],
      %w[song4 song5 song6 song7 song8 song9 song10 song1 song2 song3],
      %w[song5 song6 song7 song8 song9 song10 song1 song2 song3 song4],
      %w[song6 song7 song8 song9 song10 song1 song2 song3 song4 song5],
      %w[song7 song8 song9 song10 song1 song2 song3 song4 song5 song6],
      %w[song8 song9 song10 song1 song2 song3 song4 song5 song6 song7],
      %w[song9 song10 song1 song2 song3 song4 song5 song6 song7 song8],
      %w[song10 song1 song2 song3 song4 song5 song6 song7 song8 song9]
    ]
  end
  
  describe "initial implementation" do
    it "should not skip 0" do
      skip_tracks(@playlist, 0)
      @playlist.should == @variations[0]
    end
  
    it "should skip forward" do
      (1..9).each do |n|
        skip_tracks(@playlist, n)
        @playlist.should == @variations[n]
        @playlist = %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10]
      end
    end
    
    it "should not skip 'playlist' length forward" do
      skip_tracks(@playlist, @playlist.length)
      @playlist.should == @variations[0]
    end
    
    it "should not skip pass end forward" do
      # this fails in the initial impl
    end
    
    it "should skip backward" do
      (1..9).each do |n|
        skip_tracks(@playlist, -n)
        @playlist.should == @variations[-n]
        @playlist = %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10]
      end
    end
    
    it "should not skip 'playlist' length backward" do
      skip_tracks(@playlist, -@playlist.length)
      @playlist.should == @variations[0]
    end

    it "should not skip pass end backward" do
      # this fails in the initial impl
    end
  end
  
  describe "new implementation" do
    it "should not skip 0" do
      skip_tracks_new_version(@playlist, 0)
      @playlist.should == @variations[0]
    end
    
    it "should skip forward" do
      (1..9).each do |n|
        skip_tracks_new_version(@playlist, n)
        @playlist.should == @variations[n]
        @playlist = %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10]
      end
    end
    
    it "should not skip 'playlist' length forward" do
      skip_tracks_new_version(@playlist, -@playlist.length)
      @playlist.should == @variations[0]
    end
    
    it "should not skip pass end forward" do
      skip_tracks_new_version(@playlist, 11)
      @playlist.should == @variations[0]
    end
    
    it "should skip backward" do
      (1..9).each do |n|
        skip_tracks_new_version(@playlist, -n)
        @playlist.should == @variations[-n]
        @playlist = %w[song1 song2 song3 song4 song5 song6 song7 song8 song9 song10]
      end
    end
    
    it "should not skip 'playlist' length backward" do
      skip_tracks_new_version(@playlist, -@playlist.length)
      @playlist.should == @variations[0]
    end
    
    it "should not skip pass end backward" do
      skip_tracks_new_version(@playlist, -11)
      @playlist.should == @variations[0]
    end
  end
end