http://stackoverflow.com/questions/6943169/convert-cartesian-product-to-nested-hash-in-ruby

indexed-hash.rb

myhash = {
  {"var1"=>"a", "var2"=>"a"} => 1,
  {"var1"=>"a", "var2"=>"b"} => 2,
  {"var1"=>"b", "var2"=>"a"} => 3,
  {"var1"=>"b", "var2"=>"b"} => 4,
}

nested = {
  "a"=> { 
    "a"=> 1,
    "b"=> 2
  },
  "b"=> { 
    "a"=> 3,
    "b"=> 4
  }
}
  
def index(hash, fields)
  # store the last index of the fields
  last_field = fields.length - 1

  # our indexed version
  indexed = {}

  hash.each do |key, value|
    # our current point in the indexed hash
    point = indexed
    fields.each_with_index do |field, i|
      key_field = key[field]
      if i == last_field
        point[key_field] = value
      else
        # ensure the next point is a hash
        point[key_field] ||= {}
        # move our point up
        point = point[key_field]
      end
    end
  end
  # return our indexed hash
  indexed
end

raise "doesnt work" unless index(myhash, ["var1", "var2"]) == nested
puts "looks like it works"