Day 13, part 1

Author: Robb Kidd

2023/ruby/day13.rb

@@ -0,0 +1,128 @@
+require_relative 'day'
+
+class Day13 < Day # >
+
+  # @example
+  #   day.part1 #=> 405
+  def part1
+    @patterns ||=
+      input
+        .split("\n\n")
+        .map { |stanza| Pattern.new(stanza) }
+
+    @patterns
+      .map { |pattern| pattern.summarize }
+      .reduce(&:+)
+  end
+
+  # example
+  #   day.part2 #=> 'how are you'
+  def part2
+  end
+
+  EXAMPLE_INPUT = <<~INPUT
+    #.##..##.
+    ..#.##.#.
+    ##......#
+    ##......#
+    ..#.##.#.
+    ..##..##.
+    #.#.##.#.
+
+    #...##..#
+    #....#..#
+    ..##..###
+    #####.##.
+    #####.##.
+    ..##..###
+    #....#..#
+  INPUT
+
+  FIRST_PATTERN = EXAMPLE_INPUT.split("\n\n").first
+  SECOND_PATTERN = EXAMPLE_INPUT.split("\n\n").last
+end
+
+class Pattern
+  attr_reader :rows, :columns
+
+  # @example
+  #   p = new(Day13::FIRST_PATTERN)
+  #   p.rows.first #=> "#.##..##."
+  #   p.columns.first #=> '#.##..#'
+  def initialize(input)
+    @input = input
+    @rows = input.split("\n")
+    @columns = @rows.map(&:chars).transpose.map(&:join)
+  end
+
+  # @example 1st
+  #   p = new(Day13::FIRST_PATTERN)
+  #   p.summarize #=> 5
+  # @example 2nd
+  #   p = new(Day13::SECOND_PATTERN)
+  #   p.summarize #=> 400
+  def summarize
+    if row_reflection = line_of_reflection(rows)
+      return row_reflection * 100
+    end
+
+    line_of_reflection(columns)
+  end
+
+  def row_reflection
+    line_of_reflection(rows) *
+    case reflection.length
+    when 0
+      return nil
+    when 1
+      return reflection.first[1] # the index of the second row in the line of reflection accounts for the 1-index of the puzzle
+    else
+      raise("more than one line of reflection for rows. wat?")
+    end
+  end
+
+  # @example 1st_rows
+  #   p = new(Day13::FIRST_PATTERN)
+  #   p.line_of_reflection(p.rows) #=> nil
+  # @example 1st_columns
+  #   p = new(Day13::FIRST_PATTERN)
+  #   p.line_of_reflection(p.columns) #=> 5
+  # @example 2nd_rows
+  #   p = new(Day13::SECOND_PATTERN)
+  #   p.line_of_reflection(p.rows) #=> 4
+  # @example 2nd_columns
+  #   p = new(Day13::SECOND_PATTERN)
+  #   p.line_of_reflection(p.columns) #=> nil
+  def line_of_reflection(dimension)
+    reflections ||=
+      reflection_candidates(dimension)
+        .select { |candidate|
+          candidate[0].downto(0)
+            .zip(candidate[1].upto(dimension.length-1))
+            .select { |pair| pair.all? }
+            .all? { |left, right| dimension[left] == dimension[right] }
+        }
+    case reflections.length
+    when 0
+      return nil
+    when 1
+      # account for the 1-index of the puzzle by returning the second index of a line of reflection
+      return reflections.first[1]
+    else
+      raise("more than one line of reflection. wat?")
+    end
+  end
+
+  # @example
+  #   p = new(Day13::FIRST_PATTERN)
+  #   p.reflection_candidates(p.rows) #=> [[2,3]]
+  #   p.reflection_candidates(p.columns) #=> [[4,5]]
+  def reflection_candidates(dimension)
+      dimension
+        .map.with_index
+        .each_cons(2)
+        .select { |left, right| left[0] == right[0] }
+        .map { |left, right| [left[1], right[1]] }
+  end
+
+end