day 22, part 2

Lotsa Parallel

Author: robbkidd

2024/ruby/day22.rb

@@ -2,24 +2,115 @@
 require 'parallel'
 
 class Day22 < Day # >
+  attr_reader :buyers
+
+  def initialize(*args)
+    super
+    @buyers =
+      Parallel
+        .map(input.scan(/\d+/)) { |initial_secret_number|
+          Buyer.new(initial_secret_number.to_i)
+        }
+    puts "initialized" if ENV['DEBUG']
+  end
 
   # @example
+  #   day = Day22.new('1 10 100 2024')
   #   day.part1 #=> 37327623
   def part1
-    initial_secret_numbers = input.split("\n").map(&:to_i)
-    Parallel.map(initial_secret_numbers) { |initial_secret_number|
-      2000.times.inject(initial_secret_number) { |secret_number, _|
-        secret_number = ((secret_number * 64) ^ secret_number) % 16777216
-        secret_number = ((secret_number / 32).floor ^ secret_number) % 16777216
-        secret_number = ((secret_number * 2048) ^ secret_number) % 16777216
-      }
-    }.reduce(&:+)
+    puts "starting part1" if ENV['DEBUG']
+
+    Parallel
+      .map(buyers) { |buyer| buyer.secret_numbers.last }
+      .reduce(&:+)
   end
 
   # @example
-  #   day.part2 #=> 'how are you'
+  #   day = Day22.new('1 2 3 2024')
+  #   day.part2 #=> 23
   def part2
+    puts "starting part2" if ENV['DEBUG']
+
+    unique_sequences =
+      buyers
+        .inject(Set.new) { |unique_sequences, buyer|
+          unique_sequences.merge(buyer.price_sequence_table.keys)
+        }
+
+    puts "unique_sequences: #{unique_sequences.count}" if ENV['DEBUG']
+
+    Parallel
+      .map(unique_sequences) { |sequence|
+        buyers
+          .inject(0) { |bananas, buyer|
+            bananas += (buyer.price_sequence_table[sequence] || 0)
+          }
+      }
+      .max
   end
 
   EXAMPLE_INPUT = File.read("../inputs/day22-example-input.txt")
 end
+
+class Buyer
+  attr_reader :secret_numbers, :prices, :price_sequence_table
+
+  # @example
+  #  buyer = Buyer.new(1)
+  #  buyer.secret_numbers.first #=> 1
+  #  buyer.secret_numbers.last #=> 8685429
+  #  buyer.prices.first #=> 1
+  #  buyer.prices.last #=> 9
+  def initialize(initial_secret_number)
+    @secret_numbers = [initial_secret_number]
+    @prices = [initial_secret_number % 10]
+    @price_sequence_table = Hash.new
+    generate_secret_numbers
+    generate_sequence_lookup
+  end
+
+  # @example 1
+  #   buyer = Buyer.new(1)
+  #   buyer.secret_numbers.last #=> 8685429
+  # @example 10
+  #   buyer = Buyer.new(10)
+  #   buyer.secret_numbers.last #=> 4700978
+  # @example 100
+  #   buyer = Buyer.new(100)
+  #   buyer.secret_numbers.last #=> 15273692
+  # @example 2024
+  #   buyer = Buyer.new(2024)
+  #   buyer.secret_numbers.last #=> 8667524
+  def generate_secret_numbers(evolutions=2000)
+    evolutions.times do
+      secret_number = @secret_numbers.last
+      secret_number = ((secret_number * 64) ^ secret_number) % 16777216
+      secret_number = ((secret_number / 32).floor ^ secret_number) % 16777216
+      secret_number = ((secret_number * 2048) ^ secret_number) % 16777216
+      @secret_numbers << secret_number
+      @prices << secret_number % 10
+    end
+  end
+
+  # @example
+  #  buyer = Buyer.new(123)
+  #  buyer.price_changes.take(9) #=> [-3, 6, -1, -1, 0, 2, -2, 0, -2]
+  #  buyer.price_changes.count #=> 2000
+  def price_changes
+    @prices.each_cons(2).map { |a, b| b - a }
+  end
+
+  # @example
+  #   buyer = Buyer.new(1)
+  #   buyer.price_sequence_table[ [-2,1,-1,3] ] #=> 7
+  def generate_sequence_lookup
+    price_changes.each_cons(4).with_index do |four_deltas, i|
+      @price_sequence_table[four_deltas] ||= @prices[i + 4]
+    end
+  end
+
+  def price_sequence_table
+    generate_sequence_lookup if @price_sequence_table.empty?
+    @price_sequence_table
+  end
+end