day 8, part 2

Author: Robb Kidd

2023/ruby/day08.rb

@@ -4,41 +4,73 @@ class Day08 < Day # >
 
   # @example
   #   day.part1 #=> 2
+  # @example with cycles
+  #   day = Day08.new(SIX_STEPS_INPUT)
+  #   day.part1 #=> 6
   def part1
-    parse(input)
+    instructions, network = parse(input)
     steps = 0
-    node = "AAA"
-    @instructions
+    current_node = "AAA"
+    instructions
       .cycle do |instruction|
-        break if node == "ZZZ"
+        break if current_node == "ZZZ"
         steps += 1
-        node = @network[node][instruction]
+        current_node = network[current_node][instruction]
       end
     steps
   end
 
   # @example
-  #   day.part2 #=> 'how are you'
+  #   day = Day08.new(PART2_EXAMPLE_INPUT)
+  #   day.part2 #=> 6
   def part2
-  end
+    instructions, network = parse(input)
+    steps = 0
+    start_nodes = network.keys.select { |node| node[-1] == "A" }
 
-  # @example
-  #   inst, net = day.parse(EXAMPLE_INPUT)
-  #   inst #=> ["R", "L"]
-  def parse(input)
-    stanzas = input.split("\n\n")
+    start_nodes
+      .map { |current_node|
+        Thread.new { # for whatever multitasking Ruby will give me
+          steps = 0
+          a_to_z = nil
+
+          instructions
+            .cycle do |instruction|
+              if current_node[-1] == "Z"         # on a Z node
+                !a_to_z ? a_to_z = steps : break # note how many steps to the first Z node, stop on second Z node
+              end
 
-    @instructions = stanzas[0].chars
+              steps += 1
+              current_node = network[current_node][instruction]
+            end
 
-    @network = {}
-    stanzas[1]
-      .each_line
-      .map { |line| line.scan(/\w+/) }
-      .each { |node, left, right|
-        @network[node] = { "L" => left, "R" => right }
+          if (steps - a_to_z) != a_to_z
+            raise("steps from Z-to-Z must match A-to-Z to use least common multiple")
+          end
+          a_to_z
+        }
       }
+      .map(&:value)
+      .reduce(1, :lcm)
+  end
 
-    [@instructions, @network]
+  # @example
+  #   instructions, network = day.parse(EXAMPLE_INPUT)
+  #   instructions    #=> ["R", "L"]
+  #   network["AAA"]  #=> {"L" => "BBB", "R" => "CCC"}
+  def parse(input)
+    @stanzas ||= input.split("\n\n")
+
+    @parsed ||=
+      [
+        @stanzas[0].chars,
+        @stanzas[1]
+          .each_line
+          .map { |line| line.scan(/\w+/) }
+          .each_with_object({}) { |(node, left, right), network|
+            network[node] = { "L" => left, "R" => right }
+          }
+      ]
   end
 
   EXAMPLE_INPUT = <<~INPUT
@@ -60,4 +92,17 @@ def parse(input)
     BBB = (AAA, ZZZ)
     ZZZ = (ZZZ, ZZZ)
   INPUT
+
+  PART2_EXAMPLE_INPUT = <<~INPUT
+    LR
+
+    11A = (11B, XXX)
+    11B = (XXX, 11Z)
+    11Z = (11B, XXX)
+    22A = (22B, XXX)
+    22B = (22C, 22C)
+    22C = (22Z, 22Z)
+    22Z = (22B, 22B)
+    XXX = (XXX, XXX)
+  INPUT
 end