Year 2016: Day 13

README.md

@@ -8,7 +8,7 @@ I'm also adding notes that may be useful if you're learning Ruby.
 
 Notes for solving:
 * [2015, complete](year_2015.md)
-* [2016, up to day 12](year_2016.md)
+* [2016, up to day 13](year_2016.md)
 * [2023, up to day 04](year_2023.md)
 
 # How to use

spec/year_2016/day_13_spec.rb

@@ -0,0 +1,23 @@
+require 'year_2016/day_13'
+
+describe Year2016::Day13 do
+  context 'when Part 1' do
+    subject(:sample_one) do
+      described_class.new(10, 7, 4, true)
+    end
+
+    it 'gives a final result' do
+      expect(sample_one.to_i).to eq(11)
+    end
+  end
+
+  context 'when Results' do
+    it 'correctly answers part 1' do
+      expect(described_class.new(1_352, 31, 39, true).to_i).to eq(90)
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(1_352, 31, 39).to_i).to eq(135)
+    end
+  end
+end

year_2016.md

@@ -174,3 +174,16 @@ Year2016::Day12
 ```
 
 After yesterday's mess, building a [virtual machine](https://en.wikipedia.org/wiki/Virtual_machine) is a delight. Nothing specific, refer to [Year 2015, Day 23](year_2015.md#day-23-opening-the-turing-lock) for more info.
+
+## Day 13: A Maze of Twisty Little Cubicles
+
+```
+Year2016::Day13
+  when Part 1
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+Thinking in terms of X and Y can be a source of pain, but this one was straight enough.

year_2016/day_13.rb

@@ -0,0 +1,61 @@
+class Year2016
+  class Day13
+    class << self
+      def next_moves(state)
+        [
+          [state[0] - 1, state[1]],
+          [state[0] + 1, state[1]],
+          [state[0], state[1] - 1],
+          [state[0], state[1] + 1]
+        ].reject do |x, y|
+          x.negative? || y.negative?
+        end
+      end
+    end
+
+    def initialize(input_favorite, input_x, input_y, input_part_one = false)
+      @version = input_part_one ? 1 : 2
+      @favorite = input_favorite
+      @reach_x = input_x
+      @reach_y = input_y
+    end
+
+    def cell_value(pos_x, pos_y)
+      [
+        (pos_x * pos_x),
+        (3 * pos_x),
+        (2 * pos_x * pos_y),
+        pos_y,
+        (pos_y * pos_y)
+      ].sum
+    end
+
+    def cell_is_a_wall?(pos)
+      (cell_value(pos[0], pos[1]) + @favorite).to_s(2).chars.count('1').odd?
+    end
+
+    # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength, Metrics/PerceivedComplexity
+    def search(states, moves_tried, moves)
+      return moves if @version == 1 && states.any?([@reach_x, @reach_y])
+      return moves_tried.count{|pos| !cell_is_a_wall?(pos) } if @version == 2 && moves == 51
+
+      next_states = []
+      states.each do |state|
+        next if moves_tried.include?(state)
+
+        moves_tried.add(state)
+        next if cell_is_a_wall?(state)
+
+        Day13.next_moves(state).each do |next_state|
+          next_states.push(next_state) unless moves_tried.include?(next_state)
+        end
+      end
+      search(next_states, moves_tried, moves + 1)
+    end
+    # rubocop:enable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength, Metrics/PerceivedComplexity
+
+    def to_i
+      search([[1, 1]], Set.new, 0)
+    end
+  end
+end