# Input: 
# choices = {
#   'color' => %w(blue red green),
#   'sizes' => %w(small medium large extra-large),
#   'style' => %w(tshirt longsleeve)
# }
# 
# Output:
# [{"sizes"=>"small", "color"=>"blue", "style"=>"tshirt"},
#  {"sizes"=>"small", "color"=>"blue", "style"=>"longsleeve"},
#  {"sizes"=>"medium", "color"=>"blue", "style"=>"tshirt"},
#  {"sizes"=>"medium", "color"=>"blue", "style"=>"longsleeve"},
#  {"sizes"=>"large", "color"=>"blue", "style"=>"tshirt"},
#  {"sizes"=>"large", "color"=>"blue", "style"=>"longsleeve"},
#  {"sizes"=>"extra-large", "color"=>"blue", "style"=>"tshirt"},
#  {"sizes"=>"extra-large", "color"=>"blue", "style"=>"longsleeve"},
#  {"sizes"=>"small", "color"=>"red", "style"=>"tshirt"},
#  {"sizes"=>"small", "color"=>"red", "style"=>"longsleeve"},
#  {"sizes"=>"medium", "color"=>"red", "style"=>"tshirt"},
#  {"sizes"=>"medium", "color"=>"red", "style"=>"longsleeve"},
#  {"sizes"=>"large", "color"=>"red", "style"=>"tshirt"},
#  {"sizes"=>"large", "color"=>"red", "style"=>"longsleeve"},
#  {"sizes"=>"extra-large", "color"=>"red", "style"=>"tshirt"},
#  {"sizes"=>"extra-large", "color"=>"red", "style"=>"longsleeve"},
#  {"sizes"=>"small", "color"=>"green", "style"=>"tshirt"},
#  {"sizes"=>"small", "color"=>"green", "style"=>"longsleeve"},
#  {"sizes"=>"medium", "color"=>"green", "style"=>"tshirt"},
#  {"sizes"=>"medium", "color"=>"green", "style"=>"longsleeve"},
#  {"sizes"=>"large", "color"=>"green", "style"=>"tshirt"},
#  {"sizes"=>"large", "color"=>"green", "style"=>"longsleeve"},
#  {"sizes"=>"extra-large", "color"=>"green", "style"=>"tshirt"},
#  {"sizes"=>"extra-large", "color"=>"green", "style"=>"longsleeve"}]

class Hash
  def permutations_of_arrays
    permute(map {|key, vals|
      vals.map {|val| [key, val]}
    }).map {|pairs| Hash[pairs]}
  end

private
  def permute(arrs)
    fst, rst = arrs[0], arrs[1..-1]
    init = fst.map {|x| [x]}
    rst.inject(init) {|acc, opts| permute_merge(acc, opts)}
  end

  def permute_merge(acc, to_merge)
    acc.map {|acc_arr|
      to_merge.map do |to_merge_el|
        acc_arr + [to_merge_el]
      end
    }.flatten(1)
  end
end