Day 5 - Puzzle 2

lib/solutions/day_05.rb

@@ -9,7 +9,13 @@ def part_one(input)
   end
 
   def part_two(input)
-    0
+    parse_input(input)
+
+    @updates
+      .reject { |update| correct_order?(update) }
+      .map { |update| reorder(update) }
+      .map { |result| result[result.size / 2] }
+      .sum
   end
 
   def correct_order?(update)
@@ -18,6 +24,14 @@ def correct_order?(update)
     end
   end
 
+  def reorder(update)
+    @relevant_order = @order
+                      .select { |order| update.include?(order[0]) && update.include?(order[1]) }
+
+    re = Reorder.new(@relevant_order)
+    re.reorder_update(update)
+  end
+
   def parse_input(input)
     lines = input.split("\n")
     split_idx = lines.index('')
@@ -26,3 +40,44 @@ def parse_input(input)
     @updates = lines[split_idx + 1..-1].map { |line| line.split(',').map(&:to_i) }
   end
 end
+
+class Reorder
+  def initialize(relevant_order)
+    @relevant_order = relevant_order
+  end
+
+  def reorder_update(update)
+    graph = build_graph(@relevant_order)
+    sorted_order = topological_sort(graph)
+    update.sort_by { |num| sorted_order.index(num) }
+  end
+
+  private
+
+  def build_graph(relevant_order)
+    graph = Hash.new { |hash, key| hash[key] = [] }
+    relevant_order.each do |before, after|
+      graph[before] << after
+    end
+    graph
+  end
+
+  def topological_sort(graph)
+    visited = {}
+    stack = []
+
+    graph.keys.each do |node|
+      topological_sort_util(node, visited, stack, graph) unless visited[node]
+    end
+
+    stack.reverse
+  end
+
+  def topological_sort_util(node, visited, stack, graph)
+    visited[node] = true
+    graph[node].each do |neighbor|
+      topological_sort_util(neighbor, visited, stack, graph) unless visited[neighbor]
+    end
+    stack << node
+  end
+end

spec/solutions/day_05_spec.rb

@@ -12,7 +12,7 @@
 
   describe '#part_two' do
     it 'calculates the correct solutions for part two' do
-      expect(subject.part_two(input)).to eq(0)
+      expect(subject.part_two(input)).to eq(123)
     end
   end
 end