add sticker state tests and error checks.

lib/rubiks_cube/sticker_state_transform.rb

@@ -16,18 +16,37 @@ class StickerStateTransform
       46, 7 , 50, 16, 52, 25, 48, 34
     ]
 
+    LOCATION_COUNT = 54
+
     def initialize(state)
       @state = state.gsub(/\s/, '').split('')
+      if @state.length < LOCATION_COUNT
+        raise 'too few stickers'
+      elsif @state.length > LOCATION_COUNT
+        raise 'too many stickers'
+      end
     end
 
     def to_cube
-      (edges + corners).join(' ').gsub(/./) { |c| color_mapping.fetch(c, c) }
+      (edges + corners).join(' ').gsub(/./) { |c| map_color(c) }
     end
 
     private
 
+    def map_color(color)
+      return ' ' if color == ' '
+      @color_mapping ||= color_mapping
+      @color_mapping[color] or
+        raise "#{color} is not listed in the center anywhere"
+    end
+
     def color_mapping
-      Hash[state.values_at(*CENTER_LOCATIONS).zip %w(F R B L U D)]
+      center_values = state.values_at(*CENTER_LOCATIONS)
+      dup = center_values.detect do |e|
+        center_values.rindex(e) != center_values.index(e)
+      end
+      raise "#{dup} is listed in the center twice" if dup
+      Hash[center_values.zip %w(F R B L U D)]
     end
 
     def edges

spec/rubiks_cube/cube_spec.rb

@@ -24,8 +24,15 @@
   end
 
   describe '#state=' do
-    it 'changes the state of the cube' do
-      expect { subject.state = 'a b c' }.to change { subject.state }
+    it 'expects a sticker-style state' do
+      expect { subject.state = <<-STATE }.to change { subject.state }
+          R G O R G O O W G
+          G W W B Y W Y Y Y
+          G B B G B W R Y B
+          Y G Y B W G R Y W
+          O R R O R O G Y W
+          B R O R O O W B B
+      STATE
     end
   end
 

spec/rubiks_cube/sticker_state_transform_spec.rb

@@ -20,5 +20,77 @@
         )
       end
     end
+
+    context "for a solved cube" do
+      let(:state) { <<-STATE }
+        BBB BBB BBB WWW WWW WWW RRR RRR RRR YYY YYY YYY OOO OOO OOO GGG GGG GGG
+      STATE
+
+      it 'transforms to cube state' do
+        expect(subject.to_cube).to eq(
+          "UF UR UB UL FL FR BR BL DF DR DB DL UFL URF UBR ULB DLF DFR DRB DBL"
+        )
+      end
+    end
+
+    context 'for an example with a few scrambled faces' do
+      let(:state) { <<-STATE }
+        GGY GGR GGG RRY YYY YYY WWW WWW WWB BBB BBB OBB RRR RRG RYG OOO OOO WOO
+      STATE
+
+      it 'transforms to cube state' do
+        expect(subject.to_cube).to eq(
+          "RF FU UB UL FL UR BR BL DF DR DB DL UFL FUR UBR ULB DLF DFR DRB BLD"
+        )
+      end
+    end
+
+    context 'with too few letters' do
+      let(:state) {
+        'GGY GGR GGG RRY YYY YYY WW WWW WWB BBB BBB OBB RRR RRG RYG OOO OOO WOO'
+      }
+
+      it 'throws an exception' do
+        expect {
+          subject.to_cube
+        }.to raise_error('too few stickers')
+      end
+    end
+
+    context 'with too many letters' do
+      let(:state) { <<-STATE }
+        GGY GGR GGG RRY YYY YYY WWWW WWW WWB BBB BBB OBB RRR RRG RYG OOO OOO WOO
+      STATE
+
+      it 'throws an exception' do
+        expect {
+          subject.to_cube
+        }.to raise_error('too many stickers')
+      end
+    end
+
+    context 'two center faces are identical' do
+      let(:state) { <<-STATE }
+        BBB BBB BBB WWW WBW WWW RRR RRR RRR YYY YYY YYY OOO OOO OOO GGG GGG GGG
+      STATE
+
+      it 'throws an exception' do
+        expect {
+          subject.to_cube
+        }.to raise_error('B is listed in the center twice')
+      end
+    end
+
+    context 'with a color not in a center face' do
+      let(:state) { <<-STATE }
+        PBB BBB BBB WWW WWW WWW RRR RRR RRR YYY YYY YYY OOO OOO OOO GGG GGG GGG
+      STATE
+
+      it 'throws an exception' do
+        expect {
+          subject.to_cube
+        }.to raise_error('P is not listed in the center anywhere')
+      end
+    end
   end
 end