DAY 12 PUZZLE 12

Key was calculating corners instead of edges.

Author: ariejan

lib/solutions/day_12.rb

@@ -15,6 +15,24 @@ def part_two(input)
     @cols = @map[0].length
     @visited = Array.new(@rows) { Array.new(@cols, false) }
 
+    @grid = Grid.new(@map)
+
+    price_perimeter, price_sides = @grid.calculate_price_area_perimeter
+
+    puts "Price perimeter: #{price_perimeter}"
+    puts "Price sides: #{price_sides}"
+
+    price_sides
+  end
+end
+
+class Day12Part1
+  def part_one(input)
+    @map = input.split("\n")
+    @rows = @map.length
+    @cols = @map[0].length
+    @visited = Array.new(@rows) { Array.new(@cols, false) }
+
     total_price
   end
 
@@ -71,65 +89,108 @@ def out_of_bounds?(row, col)
   end
 end
 
-class Day12Part1
-  def part_one(input)
-    @map = input.split("\n")
-    @rows = @map.length
-    @cols = @map[0].length
-    @visited = Array.new(@rows) { Array.new(@cols, false) }
+class Grid
+  attr_reader :width, :height
 
-    total_price
-  end
+  def initialize(grid)
+    @grid = grid
+    @height = @grid.size
+    @width = @grid[0].size
+    @visited = Array.new(@height) { Array.new(@width, false) }
 
-  def total_price
-    regions = find_regions
-    regions.sum { |region| region[:area] * region[:perimeter] }
+    # directions: up, down, left, right
+    @directions = [[-1, 0], [1, 0], [0, -1], [0, 1]]
   end
 
-  def find_regions
-    regions = []
-    (0...@rows).each do |row|
-      (0...@cols).each do |col|
-        next if @visited[row][col]
-
-        char = @map[row][col]
-        area, perimeter = explore_region(row, col, char)
-        regions << { area: area, perimeter: perimeter }
-      end
-    end
-    regions
+  def on_map?(x, y)
+    # check if x, y coord exist
+    x.between?(0, @height - 1) && y.between?(0, @width - 1)
   end
 
-  def explore_region(row, col, char)
-    queue = [[row, col]]
+  def find_plot_BFS(start_x, start_y)
+    plant_type = @grid[start_x][start_y]
+    queue = [[start_x, start_y]]
+    @visited[start_x][start_y] = true
+
     area = 0
     perimeter = 0
+    cells = Set.new([[start_x, start_y]])
 
     until queue.empty?
-      current_row, current_col = queue.shift
-      next if out_of_bounds?(current_row, current_col) || @visited[current_row][current_col]
-
-      next unless @map[current_row][current_col] == char
-
-      @visited[current_row][current_col] = true
+      x, y = queue.shift
       area += 1
 
-      # Check all four directions
-      [[0, 1], [1, 0], [0, -1], [-1, 0]].each do |dr, dc|
-        new_row = current_row + dr
-        new_col = current_col + dc
-        if out_of_bounds?(new_row, new_col) || @map[new_row][new_col] != char
-          perimeter += 1
+      @directions.each do |dx, dy|
+        nx = x + dx
+        ny = y + dy
+
+        if on_map?(nx, ny)
+          if @grid[nx][ny] == plant_type && !@visited[nx][ny]
+            @visited[nx][ny] = true
+            queue << [nx, ny]
+            cells.add([nx, ny])
+          elsif @grid[nx][ny] != plant_type
+            perimeter += 1
+          end
         else
-          queue << [new_row, new_col] unless @visited[new_row][new_col]
+          # Out of bounds contributes to perimeter
+          perimeter += 1
         end
       end
     end
 
-    [area, perimeter]
+    # Calculate the number of sides (== corners) for the plot
+    sides = calculate_corners(cells)
+    [area, perimeter, sides]
   end
 
-  def out_of_bounds?(row, col)
-    row < 0 || row >= @rows || col < 0 || col >= @cols
+  def calculate_corners(cells)
+    corners = 0
+
+    cells.each do |cell|
+      x, y = cell
+      u = [x - 1, y]     # Up
+      d = [x + 1, y]     # Down
+      r = [x, y + 1]     # Right
+      l = [x, y - 1]     # Left
+
+      # External corners
+      corners += 1 if !cells.include?(u) && !cells.include?(r)
+      corners += 1 if !cells.include?(r) && !cells.include?(d)
+      corners += 1 if !cells.include?(d) && !cells.include?(l)
+      corners += 1 if !cells.include?(l) && !cells.include?(u)
+
+      # Internal (diagonal) corners
+      ur = [x - 1, y + 1] # Up-Right
+      ul = [x - 1, y - 1] # Up-Left
+      dr = [x + 1, y + 1] # Down-Right
+      dl = [x + 1, y - 1] # Down-Left
+
+      corners += 1 if cells.include?(u) && cells.include?(r) && !cells.include?(ur)
+      corners += 1 if cells.include?(r) && cells.include?(d) && !cells.include?(dr)
+      corners += 1 if cells.include?(d) && cells.include?(l) && !cells.include?(dl)
+      corners += 1 if cells.include?(l) && cells.include?(u) && !cells.include?(ul)
+    end
+
+    corners
+  end
+
+  def calculate_price_area_perimeter
+    total_price_perimeter = 0
+    total_price_sides = 0
+    @visited = Array.new(@height) { Array.new(@width, false) }
+
+    @height.times do |x|
+      @width.times do |y|
+        next if @visited[x][y]
+
+        area, perimeter, sides = find_plot_BFS(x, y)
+        total_price_perimeter += area * perimeter
+        total_price_sides += area * sides
+      end
+    end
+
+    [total_price_perimeter, total_price_sides]
   end
 end
+

spec/solutions/day_12_spec.rb

@@ -11,7 +11,7 @@
   end
 
   describe '#part_two' do
-    xit 'calculates the correct solutions for part two' do
+    it 'calculates the correct solutions for part two' do
       expect(subject.part_two(input)).to eq(1206)
     end
   end