advent-of-code/2023/ruby/day_13.rb

def reflection_points(pattern)
  (0.5...pattern.length-0.5).step(1).filter_map {|fold|
    a = pattern[0..fold.floor]
    b = pattern[fold.ceil..]
    if a.reverse.zip(b).reject { _1.any?(&:nil?) }.all? { _1 == _2 }
      fold
    else
      nil
    end
  }.map(&:ceil)
end

Pattern = Data.define(:pattern) do
  def reflection_axes
    ary = to_a
    reflection_points(ary).map { [:y, _1] } + reflection_points(ary.transpose).map { [:x, _1] }
  end

  def smudges
    return enum_for(__method__) unless block_given?

    max_y, max_x = extents
    (0..max_y).each do |y|
      (0..max_x).each do |x|
        smudged = case pattern[[y,x]]
                  when ?# then ?.
                  when ?. then ?#
                  else fail
                  end
        yield Pattern.new(pattern.merge({[y,x] => smudged}))
      end
    end
  end

  def to_a
    max_y, max_x = extents
    (0..max_y).map {|y|
      (0..max_x).map {|x|
        pattern.fetch([y,x])
      }
    }
  end

  def to_s
    to_a.map(&:join).join("\n")
  end

  def extents
    max_y = pattern.keys.map(&:first).max
    max_x = pattern.keys.map(&:last).max
    [max_y, max_x]
  end
end

patterns = ARGF.read.strip.split("\n\n")
  .map {|pattern|
    pattern.split("\n").map(&:chars)
      .each.with_index.inject({}) {|h, (row, y)|
        h.merge(row.each.with_index.to_h {|elem,x| [[y,x], elem]})
      }
  }.map { Pattern.new(_1) }

# part one
x, y = patterns.map { _1.reflection_axes.fetch(0) }
  .partition {|axis,_| axis == :x }
  .map { _1.map(&:last) } # keep index (discard axis)
p x.sum + y.sum { _1 * 100 }

# part two

x, y = patterns.map {|pattern|
  original_reflection_axis = pattern.reflection_axes.fetch(0)
  pattern.smudges.lazy
    .flat_map { _1.reflection_axes}
    .reject { _1 == original_reflection_axis }
    .first
}.partition {|axis,_| axis == :x }
  .map { _1.map(&:last) } # keep index (discard axis)
p x.sum + y.sum { _1 * 100 }