Day 16 - Puzzle 2

.rspec

@@ -1,2 +1,2 @@
 --color
---format documentation
+--format progress

lib/solutions/day_12.rb

@@ -19,9 +19,6 @@ def part_two(input)
 
     price_perimeter, price_sides = @grid.calculate_price_area_perimeter
 
-    puts "Price perimeter: #{price_perimeter}"
-    puts "Price sides: #{price_sides}"
-
     price_sides
   end
 end

lib/solutions/day_16.rb

@@ -1,6 +1,167 @@
 require 'set'
 
 class Day16 
+
+  class DayTwo
+    DIRECTIONS = [
+      [0, -1],  # North
+      [1, 0],   # East
+      [0, 1],   # South
+      [-1, 0]   # West
+    ]
+
+    TURN_COST = 1000
+    MOVE_COST = 1
+
+    def initialize(input)
+      @grid = input.split("\n")
+      @rows = @grid.size
+      @cols = @grid[0].size
+    end
+
+    def run!
+      # Run, Dijkstra, Run!
+      start_xy = find_cell('S')
+      end_xy = find_cell('E')
+
+      direction = 1 # East
+
+      # 3D array to keep track of distances from each cell
+      distances = Array.new(@cols) { Array.new(@rows) { Array.new(4, Float::INFINITY) } }
+
+      # Start cell
+      distances[start_xy[0]][start_xy[1]][direction] = 0
+
+      # Priority queue for [cost, x, y, direction]
+      p_queue = []
+      p_queue << [0, start_xy[0], start_xy[1], direction]
+
+      while !p_queue.empty?
+        # Sort priority queueu by cost
+        p_queue.sort_by! { |a| a[0] }
+
+        # Get next items from queue
+        cost, x, y, direction = p_queue.shift
+
+        # Reached end
+        # return cost if x == end_xy[0] && y == end_xy[1]
+
+        # Skip if we already have a lower cost to this cell
+        next if distances[x][y][direction] < cost
+
+        # Try moving forward
+        new_x = x + DIRECTIONS[direction][0]
+        new_y = y + DIRECTIONS[direction][1]
+
+        if can_move?(new_x, new_y)
+          new_cost = cost + MOVE_COST
+          if new_cost < distances[new_x][new_y][direction]
+            distances[new_x][new_y][direction] = new_cost
+            p_queue << [new_cost, new_x, new_y, direction]
+          end
+        end
+
+        # Try turning
+        # Left
+        left_dir = (direction - 1) % 4
+        new_cost = cost + TURN_COST
+        # Turning costs, even if we do not move. But only do it if moving
+        # afterwards makes sense
+        if new_cost < distances[x][y][left_dir]
+          distances[x][y][left_dir] = new_cost
+          p_queue << [new_cost, x, y, left_dir]
+        end
+
+        # Right
+        right_dir = (direction + 1) % 4
+        new_cost = cost + TURN_COST
+
+        if new_cost < distances[x][y][right_dir]
+          distances[x][y][right_dir] = new_cost
+          p_queue << [new_cost, x, y, right_dir]
+        end
+      end
+
+      # Moar work to do, backtracking steps for best cost
+      best_cost = 4.times.map { |d| distances[end_xy[0]][end_xy[1]][d] }.min
+      return Float::INFINITY if best_cost == Float::INFINITY
+
+      on_shortest_path = Array.new(@cols) { Array.new(@rows) { Array.new(4, false) } }    
+      queue = []
+
+      4.times do |d|
+        if distances[end_xy[0]][end_xy[1]][d] == best_cost
+          on_shortest_path[end_xy[0]][end_xy[1]][d] = true
+          queue << [end_xy[0], end_xy[1], d]
+        end
+      end
+
+      shortest_path_cells = Set.new
+      shortest_path_cells << end_xy
+
+
+
+      while !queue.empty?
+        x, y, d = queue.shift
+        cost_here = distances[x][y][d]
+
+        # Check moves
+        new_x = x - DIRECTIONS[d][0]
+        new_y = y - DIRECTIONS[d][1]
+        if distances[new_x][new_y][d] + MOVE_COST == cost_here && can_move?(x, y)
+          if !on_shortest_path[new_x][new_y][d]
+            on_shortest_path[new_x][new_y][d] = true
+            shortest_path_cells << [new_x, new_y]
+            queue << [new_x, new_y, d]
+          end
+        end
+
+        # Check turns
+        left_dir = (d - 1) % 4
+        if distances[x][y][left_dir] + TURN_COST == cost_here
+          unless on_shortest_path[x][y][left_dir]
+            on_shortest_path[x][y][left_dir] = true
+            shortest_path_cells << [x, y]
+            queue << [x, y, left_dir]
+          end
+        end
+
+        right_dir = (d + 1) % 4
+        if distances[x][y][right_dir] + TURN_COST == cost_here
+          unless on_shortest_path[x][y][right_dir]
+            on_shortest_path[x][y][right_dir] = true
+            shortest_path_cells << [x, y]
+            queue << [x, y, right_dir]
+          end
+        end
+      end
+
+      shortest_path_cells.size
+    end
+
+    def can_move?(x, y)
+      return false if x < 0 || x >= @cols
+      return false if y < 0 || y >= @rows
+      return false if @grid[y][x] == '#'
+      true
+    end
+
+    def inside?(x, y) 
+      x >= 0 && x < @cols && y >= 0 && y < @rows
+    end
+
+
+    def find_cell(ch)
+      @grid.each_with_index do |row, y|
+        x = row.index(ch)
+        if x
+          return [x, y]
+        end
+      end
+      nil
+    end
+  end
+
   class DayOne
     DIRECTIONS = [
       [0, -1],  # North
@@ -97,7 +258,6 @@ def find_cell(ch)
       @grid.each_with_index do |row, y|
         x = row.index(ch)
         if x
-          puts "Found #{ch} at #{x}, #{y}"
           return [x, y]
         end
       end
@@ -111,7 +271,8 @@ def part_one(input)
   end
 
   def part_two(input)
-    0
+    sim = DayTwo.new(input)
+    sim.run!
   end
 end
 

spec/solutions/day_16_spec.rb

@@ -53,9 +53,51 @@
   end
 
   describe '#part_two' do
-    it 'calculates the correct solutions for part two' do
-      expect(subject.part_two(input)).to eq(0)
-    end
+  it 'calculates example 1' do
+    expect(subject.part_two(
+<<-INPUT
+###############
+#.......#....E#
+#.#.###.#.###.#
+#.....#.#...#.#
+#.###.#####.#.#
+#.#.#.......#.#
+#.#.#####.###.#
+#...........#.#
+###.#.#####.#.#
+#...#.....#.#.#
+#.#.#.###.#.#.#
+#.....#...#.#.#
+#.###.#.#.#.#.#
+#S..#.....#...#
+###############  
+INPUT
+    )).to eq(45)
+  end
+
+  it 'calculates example 2' do
+    expect(subject.part_two(
+<<-INPUT
+#################
+#...#...#...#..E#
+#.#.#.#.#.#.#.#.#
+#.#.#.#...#...#.#
+#.#.#.#.###.#.#.#
+#...#.#.#.....#.#
+#.#.#.#.#.#####.#
+#.#...#.#.#.....#
+#.#.#####.#.###.#
+#.#.#.......#...#
+#.#.###.#####.###
+#.#.#...#.....#.#
+#.#.#.#####.###.#
+#.#.#.........#.#
+#.#.#.#########.#
+#S#.............#
+#################
+INPUT
+    )).to eq(64)
+  end    
   end
 end