defmodule Graphic do
  @moduledoc """
  Build directed graphs, query them,
  and compile into common diagramming languages.

      iex> alias Graphic, as: G
      ...> G.new()
      ...> |> G.bridge("a", "b")
      ...> |> G.edges()
      [
        {[:"$e" | 0], "a", "b", []}
      ]

      iex> alias Graphic, as: G
      ...> G.new()
      ...> |> G.bridge("a", "b", "broken!")
      ...> |> G.edges()
      [
        {[:"$e" | 0], "a", "b", "broken!"}
      ]
  """
  @moduledoc since: "0.1.2"
  @dg :digraph

  def new(), do: :digraph.new()
  
  @doc "add a plain edge, and any missing nodes."
  def bridge(g, a, b) do
    if !(g|>@dg.vertex(a)), do: g|>@dg.add_vertex(a)
    if !(g|>@dg.vertex(b)), do: g|>@dg.add_vertex(b)
    g|>@dg.add_edge(a, b)
    g
  end

  @doc "add a labeled edge, and any missing nodes."
  def bridge(g, a, b, label) do
    if !(g|>@dg.vertex(a)), do: g|>@dg.add_vertex(a)
    if !(g|>@dg.vertex(b)), do: g|>@dg.add_vertex(b)
    g|>@dg.add_edge(a, b, label)
    g
  end

  @doc "query all edges in graph."
  def edges(g), do: g|>@dg.edges()|>Enum.map(& g|>@dg.edge(&1))

  # choose a logical order, used in rendering.
  defp order(g), do: g|>:digraph_utils.topsort()

  @doc ~S"""
  render graph as a d2 diagram.

      iex>alias Graphic, as: G
      iex> G.new()
      ...> |>G.bridge("a", "b")
      ...> |>G.bridge("a", "c")
      ...> |>G.bridge("c", "d", "broken!")
      ...> |>G.bridge("b", "d")
      ...> |>G.as_d2()
      \"""
      a -> b
      a -> c
      c -> d: 'broken!'
      b -> d
      \"""
  """
  @doc since: "0.1.2"
  def as_d2(g) do
    g|>order()|>Enum.map(fn n ->
      g
      |>@dg.out_edges(n)
      |>Enum.map(& g|>@dg.edge(&1))
      |>Enum.map(fn {_, a, b, label} ->
        "#{a} -> #{b}#{if (label != []), do: ": '#{label}'"}"
      end)
      |> Enum.sort()
      # g|>neighbors(&1)|>Enum.map(fn n ->
      #   {_, ^&1, ^n, edge} = g|>edge(&1, n)
      #   "#{&1} -> #{n}#{if edge, do: ": '#{edge}'"}"
      # end)
    end)
    |>List.flatten()
    |>Enum.concat([""])
    |>Enum.join("\n")
  end

  @doc "describe neighboring nodes."
  def neighbors(g, n), do: g|>@dg.out_neighbours(n)
end