Advent 2021 - Day 4

advent.2021.4.rb

class Board
  class Cell
    attr_reader :row, :column, :value

    def initialize(row:, column:, value:)
      @row = row
      @column = column
      @value = value
      @marked = false
    end

    def mark
      self.marked = true
    end

    def marked?
      @marked
    end

    private

    attr_writer :marked
  end

  attr_reader :rows

  def initialize(data)
    @rows = data.each_with_index.map do |row, i|
      row.each_with_index.map do |value, j|
        Cell.new(row: i, column: j, value: value)
      end
    end
  end

  def columns
    rows.transpose
  end

  def vectors
    rows + columns
  end

  def cells
    rows.flatten
  end

  def on_draw(value)
    cells.each { |c| c.mark if c.value == value }
  end

  def bingo?
    vectors.any? { |v| v.all?(&:marked?) }
  end

  def unmarked_cells
    cells.reject(&:marked?)
  end
end

def parse_input
  input = File.read('input.txt')
  numbers_to_draw, *boards = *input.split("\n\n")

  numbers_to_draw = numbers_to_draw.split(',').map(&:to_i)
  boards = boards
    .map { |b| b.lines.map { |l| l.strip.split(/ +/).map(&:to_i) } }
    .map { |b| Board.new(b) }

  return [numbers_to_draw, boards]
end

def part_one
  numbers_to_draw, boards = *parse_input

  numbers_to_draw.each do |drawn_number|
    boards.each do |board|
      board.on_draw(drawn_number)
      if board.bingo?
        return board.unmarked_cells.map(&:value).sum * drawn_number
      end
    end
  end
end

def part_two
  part_two_helper(*parse_input)
end

def part_two_helper(numbers_to_draw, boards)
  raise 'impossible' if numbers_to_draw.empty? || boards.empty?

  boards.each { |b| b.on_draw(numbers_to_draw.first) }
  if boards.all?(&:bingo?)
    return boards.last.unmarked_cells.map(&:value).sum * numbers_to_draw.first
  end
  part_two_helper(numbers_to_draw[1..], boards.reject(&:bingo?))
end