chore: solution for day14b

lib/day14.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'base64'
+
 # Day14
 class Day14
   def load(file)
@@ -17,23 +19,72 @@ def parse
   end
 
   def tilt(map, direction)
+    width = map.keys.max_by(&:first).first
+    height = map.keys.max_by(&:last).last
+
     case direction
     when :north
-      width = map.keys.max_by(&:first).first
-      height = map.keys.max_by(&:last).last
       (0..width).each do |x|
         (0..height).each do |y|
-          if map[[x, y]] == 'O'
-            map[[x, y]] = '.'
-            while y >= 0
-              y -= 1
-              if map[[x, y]] == '.'
-                next
-              else
-                map[[x, y + 1]] = 'O'
-                break
-              end
-            end
+          next unless map[[x, y]] == 'O'
+
+          map[[x, y]] = '.'
+          yy = y
+          while yy >= 0
+            yy -= 1
+            next if map[[x, yy]] == '.'
+
+            map[[x, yy + 1]] = 'O'
+            break
+          end
+        end
+      end
+    when :west
+      (0..width).each do |x|
+        (0..height).each do |y|
+          next unless map[[x, y]] == 'O'
+
+          map[[x, y]] = '.'
+          xx = x
+          while xx >= 0
+            xx -= 1
+            next if map[[xx, y]] == '.'
+
+            map[[xx + 1, y]] = 'O'
+            break
+
+          end
+        end
+      end
+    when :south
+      (0..width).each do |x|
+        height.downto(0).each do |y|
+          next unless map[[x, y]] == 'O'
+
+          map[[x, y]] = '.'
+          yy = y
+          while yy <= height
+            yy += 1
+            next if map[[x, yy]] == '.'
+
+            map[[x, yy - 1]] = 'O'
+            break
+          end
+        end
+      end
+    when :east
+      width.downto(0).each do |x|
+        (0..height).each do |y|
+          next unless map[[x, y]] == 'O'
+
+          map[[x, y]] = '.'
+          xx = x
+          while xx <= width
+            xx += 1
+            next if map[[xx, y]] == '.'
+
+            map[[xx - 1, y]] = 'O'
+            break
           end
         end
       end
@@ -55,9 +106,74 @@ def weight(map)
     sum
   end
 
+  def key(map)
+    key = ''
+    width = map.keys.max_by(&:first).first
+    height = map.keys.max_by(&:last).last
+    (0..width).map do |x|
+      (0..height).map do |y|
+        key += map[[x, y]]
+      end
+    end
+    Base64.encode64(key)
+  end
+
+  def draw(map)
+    puts
+    width = map.keys.max_by(&:first).first
+    height = map.keys.max_by(&:last).last
+    (0..height).each do |y|
+      (0..width).each do |x|
+        print map[[x, y]]
+      end
+      puts
+    end
+  end
+
   def part1
     map = parse
-    new_map = tilt(map, direction = :north)
-    weight(new_map)
+    map = tilt(map, direction = :north)
+    weight(map)
+  end
+
+  def part2
+    map = parse
+
+    cycles = 0
+    cycle_key = ''
+    cycle_found = false
+
+    cache = {}
+    keys = []
+
+    break_at = 1_000_000_000
+
+    loop do
+      cycles += 1
+
+      %i[north west south east].each do |direction|
+        map = tilt(map, direction)
+      end
+
+      # look for a cycle
+      key = key(map)
+      if cache[key]
+        cycle_key = key if cycle_key == ''
+        keys.push(cycles) if key == cycle_key
+      end
+      cache[key] = true
+
+      # cycle found - find next cycle
+      if keys.size == 2 && !cycle_found
+        cycle = keys[1] - keys[0]
+        offset = keys[0] - cycle
+        break_at = cycle + offset + ((1_000_000_000 - offset) % cycle) + cycle
+        cycle_found = true
+      end
+
+      break if cycles == break_at
+    end
+
+    weight(map)
   end
 end

test/unit/day14_test.rb

@@ -8,9 +8,21 @@ def test_day14a
     assert_equal(136, sut.part1)
   end
 
+  def test_day14b
+    sut = Day14.new
+    sut.load('data/day14a.txt')
+    assert_equal(64, sut.part2)
+  end
+
   def test_day14_part1
     sut = Day14.new
     sut.load('data/day14.txt')
     assert_equal(108_792, sut.part1)
   end
+
+  def test_day14_part2
+    sut = Day14.new
+    sut.load('data/day14.txt')
+    assert_equal(99_118, sut.part2)
+  end
 end