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

      iex> alias Graphic, as: G
      ...> g = G.new()
      ...> g |> G.bridge("a", "b")
      ...> g |> G.edges()
      [
        {[:"$e" | 0], "a", "b", []}
      ]
  """
  @moduledoc since: "0.1.1"
  @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)
  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)
  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 "render graph as a d2 diagram."
  def as_d2(g), do: g|>order()|>Enum.map(&
    g|>neighbors(&1)|>Enum.map(fn n -> "#{&1} -> #{n}"
  end) )|>List.flatten()|>Enum.join("\n")

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