17.rb

space = Hash.new('.')
data.lines.map(&:chomp).each.with_index do |row, x|
  row.each_char.with_index { |c, y| space[[ x, y, 0, 0 ]] = c }
end

6.times do
  copy = space.dup

  # bounds
  coords = space.keys
  mx, nx = coords.map { |c| c[0] }.min - 1, coords.map { |c| c[0] }.max + 1
  my, ny = coords.map { |c| c[1] }.min - 1, coords.map { |c| c[1] }.max + 1
  mz, nz = coords.map { |c| c[2] }.min - 1, coords.map { |c| c[2] }.max + 1
  mw, nw = coords.map { |c| c[3] }.min - 1, coords.map { |c| c[3] }.max + 1

  # advance
  mx.upto(nx).each do |x|
    my.upto(ny).each do |y|
      mz.upto(nz).each do |z|
        mw.upto(nw).each do |w|
          adj = 0
          -1.upto(1).each do |dx|
            -1.upto(1).each do |dy|
              -1.upto(1).each do |dz|
                -1.upto(1).each do |dw|
                  next if dx == 0 && dy == 0 && dz == 0 && dw == 0
                  adj += 1 if space[[ x + dx, y + dy, z + dz, w + dw ]] == '#'
                end
              end
            end
          end

          copy[[ x, y, z, w ]] = case space[[ x, y, z, w ]]
          when '#' then [ 2, 3 ].include?(adj) ? '#' : '.'
          when '.' then adj == 3 ? '#' : '.'
          end
        end
      end
    end
  end
  space = copy
end
p space.values.count { |c| c == '#' }