day 19, part 2

no regex

Author: robbkidd

2024/ruby/day19.rb

@@ -2,18 +2,41 @@
 
 class Day19 < Day # >
 
+  def initialize(*args)
+    super
+    towels, designs = input.split("\n\n")
+    @towel_patterns = towels.split(", ")
+    @desired_designs = designs.split("\n")
+    @design_cache = Hash.new(0)
+  end
+
   # @example
   #   day.part1 #=> 6
-def part1
-  towels, designs = input.split("\n\n")
-  towel_matcher = Regexp.new(/\A(#{towels.gsub(", ", "|")})+\z/)
-  desired_designs = designs.split("\n")
-  desired_designs.count { |pattern| pattern.match?(towel_matcher) }
-end
+  def part1
+    towel_matcher = Regexp.new(/\A(#{@towel_patterns.join("|")})+\z/)
+    @desired_designs.count { |pattern| pattern.match?(towel_matcher) }
+  end
 
   # @example
   #   day.part2 #=> 16
   def part2
+    @desired_designs.inject(0) do |count, design|
+      count += ways_to_make_design(design)
+    end
+  end
+
+  # @example
+  #   day.ways_to_make_design("brwrr") #=> 2
+  #   day.ways_to_make_design("bggr") #=> 1
+  #   day.ways_to_make_design("gbbr") #=> 4
+  def ways_to_make_design(design)
+    return @design_cache[design] if @design_cache[design] > 0
+    return 1 if design.empty?
+
+    @design_cache[design] =
+      @towel_patterns.reduce(0) { |ways, towel|
+        ways += design.end_with?(towel) ? ways_to_make_design(design[..(-(towel.length+1))]) : 0
+      }
   end
 
   EXAMPLE_INPUT = File.read("../inputs/day19-example-input.txt")