fitness.ex

defmodule TspSolver.Fitness do
  import TspSolver.Helpers

  @city_distances HashDict.new(
    [
      { {:Aberdeen,  :Dundee},     67 },
      { {:Aberdeen,  :Edinburgh}, 129 },
      { {:Aberdeen,  :Glasgow},   146 },
      { {:Aberdeen,  :Inverness}, 105 },
      { {:Aberdeen,  :Perth},      84 },

      { {:Dundee,    :Edinburgh},  63 },
      { {:Dundee,    :Glasgow},    83 },
      { {:Dundee,    :Inverness}, 138 },
      { {:Dundee,    :Perth},      22 },

      { {:Edinburgh, :Glasgow},    43 },
      { {:Edinburgh, :Inverness}, 154 },
      { {:Edinburgh, :Perth},      45 },

      { {:Glasgow,   :Inverness}, 175 },
      { {:Glasgow,   :Perth},      62 },

      { {:Inverness, :Perth},     117 }
    ]
  )

  def of_individual(individual) do
    Enum.reduce individual, 0, fn (city, distance) ->
      next_city_index = index_of(individual, city) + 1
      next_city = next_city(individual, next_city_index)
      distance + _distance_between(city, next_city)
    end
  end

  defp next_city(cities, index) when index < Kernel.length(cities) do
    Enum.at(cities, index)
  end
  defp next_city(cities, _), do: List.first(cities)

  defp _distance_between(city, next_city) do
    HashDict.get @city_distances, cities(city, next_city)
  end

  defp cities(city, next_city) do
    [city, next_city] |> Enum.sort |> Kernel.list_to_tuple
  end
end