Einstein's Riddle Solver

solver.rb

# EINSTEIN'S RIDDLE
#
# There are five houses in five different colours in a row. In each house lives
# a person with a different nationality. The five owners drink a certain type of
# beverage, smoke a certain brand of cigar and keep a certain pet.
#
# No owners have the same pet, smoke the same brand of cigar, or drink the same
# beverage.
#
# Other facts:
# 1.  The Brit lives in the red house.
# 2.  The Swede keeps dogs as pets.
# 3.  The Dane drinks tea.
# 4.  The green house is on the immediate left of the white house.
# 5.  The green house’s owner drinks coffee.
# 6.  The owner who smokes Pall Mall rears birds.
# 7.  The owner of the yellow house smokes Dunhill.
# 8.  The owner living in the center house drinks milk.
# 9.  The Norwegian lives in the first house.
# 10. The owner who smokes Blends lives next to the one who keeps cats.
# 11. The owner who keeps the horse lives next to the one who smokes Dunhill.
# 12. The owner who smokes Bluemasters drinks beer.
# 13. The German smokes Prince.
# 14. The Norwegian lives next to the blue house.
# 15. The owner who smokes Blends lives next to the one who drinks water.
#
# The question is: who owns the fish?

Block = Struct.new(:colors, :nationalities, :pets, :beverages, :cigars) do
  def to_s
    data = [ ["NUMBER", *(1..colors.length).map(&:to_s)],
             ["COLOR", *colors],
             ["NATIONALITY", *nationalities],
             ["PET", *pets],
             ["BEVERAGE", *beverages],
             ["CIGAR", *cigars] ]

    col_width = data.flatten.map(&:length).max
    
    rows = []
    rows << vertical_bar(col_width, colors.count)
    data.each { |r| rows << as_row(r, col_width) }
    rows << vertical_bar(col_width, colors.count)
    rows.join("\n")
  end

  private
  
  def vertical_bar(width, count)
    "|#{ Array.new(count + 1) { "-" * (width + 2) }.join('+') }|"
  end

  def as_row(vals, width)
    "|#{ vals.map { |v| " #{ v.to_s.ljust(width) } " }.join("|") }|"
  end
end

%i(Brit Norwegian German Swede Dane).permutation.each do |nationalities|

  # 9. The Norwegian lives in the first house.
  next unless nationalities.first == :Norwegian

  %i(red white blue yellow green).permutation.each do |colors|

    # 1. The Brit lives in the red house.
    next unless nationalities.index(:Brit) == colors.index(:red)

    # 4. The green house is on the immediate left of the white house.
    next unless colors.index(:white) - colors.index(:green) == 1

    # 14. The Norwegian lives next to the blue house.
    next unless (nationalities.index(:Norwegian) - colors.index(:blue)).abs == 1

    %i(dog bird cat horse fish).permutation.each do |pets|

      # 2. The Swede keeps dogs as pets.
      next unless nationalities.index(:Swede) == pets.index(:dog)

      %i(milk water beer coffee tea).permutation.each do |beverages|

        # 5. The green house’s owner drinks coffee.
        next unless colors.index(:green) == beverages.index(:coffee)

        # 8. The owner living in the center house drinks milk.
        next unless beverages.index(:milk) == (beverages.length - 1)/2

        # 3. The Dane drinks tea.
        next unless nationalities.index(:Dane) == beverages.index(:tea)

        %i(Prince Blends Bluemasters Dunhills PallMalls).permutation.each do |cigars|

          # 6. The owner who smokes Pall Mall rears birds.
          next unless cigars.index(:PallMalls) == pets.index(:bird)

          # 7. The owner of the yellow house smokes Dunhill.
          next unless cigars.index(:Dunhills) == colors.index(:yellow)

          # 10. The owner who smokes Blends lives next to the one who keeps cats.
          next unless (cigars.index(:Blends) - pets.index(:cat)).abs == 1

          # 11. The owner who keeps the horse lives next to the one who smokes Dunhill.
          next unless (cigars.index(:Dunhills) - pets.index(:horse)).abs == 1

          # 12. The owner who smokes Bluemasters drinks beer.
          next unless cigars.index(:Bluemasters) == beverages.index(:beer)

          # 13. The German smokes Prince.
          next unless cigars.index(:Prince) == nationalities.index(:German)

          # 15. The owner who smokes Blends lives next to the one who drinks water
          next unless (cigars.index(:Blends) - beverages.index(:water)).abs == 1

          puts Block.new(colors, nationalities, pets, beverages, cigars)
        end
      end
    end
  end
end