Place lines in separate voices.

lib/roborabb.rb

@@ -15,14 +15,36 @@ def to_lilypond
         format_bar(bar)
       end
 
-      score.map {|x| x[:upper] }.join(' ') +
-      score.map {|x| x[:lower] }.join(' ')
+      lilypond do
+        voice(:up)   { score.map {|x| x[:upper] }.join(' ') } +
+        voice(:down) { score.map {|x| x[:lower] }.join(' ') }
+      end
     end
 
     protected
 
     attr_accessor :generator, :opts
 
+    def lilypond
+      <<-LP
+      \\version "2.14.2"
+      \\new DrumStaff <<
+        #{yield}
+      >>
+      LP
+    end
+
+    def voice(direction)
+      result = <<-LP
+      \\new DrumVoice {
+        \\override Rest #'direction = ##{direction}
+        \\stemUp   \\drummode {
+          #{yield}
+        }
+      }
+      LP
+    end
+
     def format_bar(bar)
       {
         upper: format_notes(bar, expand(hashslice(bar.notes, :hihat))),

spec/unit_spec.rb

@@ -144,5 +144,28 @@ def construct(attributes)
       formatter = described_class.new(generator, bars: 1)
       formatter.to_lilypond.should include("r8 hh8")
     end
+
+    it 'includes lilypond preamble' do
+      generator = [stub(unit: 8, notes: {})].each
+      formatter = described_class.new(generator, bars: 1)
+      output = formatter.to_lilypond
+      output.should include("\\version")
+      output.should include("\\new DrumStaff")
+    end
+
+    it 'places hihats and kick/snare in different voices' do
+      generator = [stub(unit: 8, notes: {
+        hihat: [true, true],
+        kick:  [true, false],
+        snare: [false, true]
+      })].each
+      formatter = described_class.new(generator, bars: 1)
+      output = formatter.to_lilypond
+      voices = output.split("\\new DrumVoice")[1..-1]
+      voices[0].should include("hh8 hh8")
+      voices[0].should include("\\override Rest #'direction = #up")
+      voices[1].should include("bd8 sn8")
+      voices[1].should include("\\override Rest #'direction = #down")
+    end
   end
 end