Solution to challenge 8 from "El País"

challenge8.rb

# http://www.elpais.com/videos/sociedad/cubo/suma/cero/elpepusoc/20110505elpepusoc_2/Ves/

faces_vertices = [[0,1,2,3], [0,1,4,5], [4,5,6,7], [2,3,6,7], [0,3,4,7], [1,2,5,6]]

vertices_solution = ([+1, -1].repeated_permutation(8)).select do |vertices_values|
  faces_values = faces_vertices.map do |vertices|
    vertices_values.values_at(*vertices).inject(:*)
  end
  (vertices_values + faces_values).inject(:+) == 0
end

p vertices_solution #=> []