garden = DATA.readlines(chomp: true).flat_map.with_index {|row, y|
  row.chars.map.with_index {|id, x| [[y,x], id] }
}.to_h

DELTAS = [[-1, 0], [1, 0], [0, -1], [0, 1]]

queue = Set[*garden.keys]
plots = [Set[queue.first]]
until queue.empty?
  yx = plots.last.select { queue.include?(_1) }.first
  if yx.nil?
    plots << Set[queue.first] if queue.first
    next
  end

  queue.delete(yx)

  id = garden.fetch(yx)
  neighbors = DELTAS
    .map {|d| yx.zip(d).map { _1 + _2 }}
    .select { garden.fetch(_1, nil) == id }
  if neighbors.empty?
    plots << Set[queue.first] if queue.first
  else
    plots.last.merge(neighbors)
  end
end

perimeter = ->(plot) {
  plot.flat_map {|yx|
    DELTAS
      .map {|delta| yx.zip(delta).map { _1 + _2 }}
      .reject { garden.fetch(_1, nil) == garden[plot.first] }
  }
}

pp plots
  .map { [garden.fetch(_1.first), _1.size, perimeter[_1].size] }
  .sum { _2 * _3 }

__END__
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