Extract related behavior to separate class

This creates a Tuner, which is responsible for tuning a guitar. The
guitar still has a `tune` method on its public API, but the complexity
of doing the tuning is delegated to this other class.

This isn't an object that represents a real-life object, but still has
value being separate. Now the class it was extracted from, the guitar,
can focus on doing all of its guitar activities. Before it looked like
the main purpose of a guitar was to tune itself with the percentage of
code that was dedicated to tuning.

lib/blues/guitar.rb

@@ -2,20 +2,9 @@
 
 module Blues
   class Guitar
-    VALID_TUNINGS = [
-      :all_fifths,
-      :all_fourths,
-      :down_half_step,
-      :drop_d,
-      :modal_c,
-      :open_a,
-      :standard,
-    ].freeze
-
     attr_reader :strings
 
     def initialize(amplifier: nil)
-      @tuning = nil
       @strings = Array.new(6) { |i| GuitarString.new(number: i + 1) }
       @amplifier = amplifier
     end
@@ -34,11 +23,7 @@ def pick(string:, fret:)
     end
 
     def tune(tuning = :standard)
-      raise "unknown tuning" unless VALID_TUNINGS.include?(tuning)
-
-      send("#{tuning}_tuning")
-
-      @tuning = tuning
+      Tuner.new(self).tune(tuning)
     end
 
     def restring(gauge_set:)
@@ -49,31 +34,5 @@ def restring(gauge_set:)
         )
       end
     end
-
-    private
-
-    def standard_tuning
-      @strings[5].tune(note: :e, octave: 2)
-      @strings[4].tune(note: :a, octave: 2)
-      @strings[3].tune(note: :d, octave: 3)
-      @strings[2].tune(note: :g, octave: 3)
-      @strings[1].tune(note: :b, octave: 3)
-      @strings[0].tune(note: :e, octave: 4)
-    end
-
-    def down_half_step_tuning
-      @strings[5].tune(note: :e_flat, octave: 2)
-      @strings[4].tune(note: :a_flat, octave: 2)
-      @strings[3].tune(note: :d_flat, octave: 3)
-      @strings[2].tune(note: :g_flat, octave: 3)
-      @strings[1].tune(note: :b_flat, octave: 3)
-      @strings[0].tune(note: :e_flat, octave: 4)
-    end
-
-    def drop_d_tuning = nil
-    def open_a_tuning = nil
-    def modal_c_tuning = nil
-    def all_fourths_tuning = nil
-    def all_fifths_tuning = nil
   end
 end

lib/blues/tuner.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module Blues
+  class Tuner
+    VALID_TUNINGS = [
+      :all_fifths,
+      :all_fourths,
+      :down_half_step,
+      :drop_d,
+      :modal_c,
+      :open_a,
+      :standard,
+    ].freeze
+
+    def initialize(guitar)
+      @guitar = guitar
+    end
+
+    def tune(tuning = :standard)
+      raise "unknown tuning" unless VALID_TUNINGS.include?(tuning)
+
+      send("#{tuning}_tuning")
+    end
+
+    private
+
+    def standard_tuning
+      @guitar.strings[5].tune(note: :e, octave: 2)
+      @guitar.strings[4].tune(note: :a, octave: 2)
+      @guitar.strings[3].tune(note: :d, octave: 3)
+      @guitar.strings[2].tune(note: :g, octave: 3)
+      @guitar.strings[1].tune(note: :b, octave: 3)
+      @guitar.strings[0].tune(note: :e, octave: 4)
+    end
+
+    def down_half_step_tuning
+      @guitar.strings[5].tune(note: :e_flat, octave: 2)
+      @guitar.strings[4].tune(note: :a_flat, octave: 2)
+      @guitar.strings[3].tune(note: :d_flat, octave: 3)
+      @guitar.strings[2].tune(note: :g_flat, octave: 3)
+      @guitar.strings[1].tune(note: :b_flat, octave: 3)
+      @guitar.strings[0].tune(note: :e_flat, octave: 4)
+    end
+
+    def drop_d_tuning = nil
+    def open_a_tuning = nil
+    def modal_c_tuning = nil
+    def all_fourths_tuning = nil
+    def all_fifths_tuning = nil
+  end
+end