feat: add split_labelled_edge function

lib/graph.ex

@@ -1070,47 +1070,64 @@ defmodule Graph do
       ...> g = Graph.split_edge(g, :a, :c, :b)
       ...> Graph.edges(g)
       [%Graph.Edge{v1: :a, v2: :b, weight: 2}, %Graph.Edge{v1: :b, v2: :c, weight: 2}]
+
+      iex> g = Graph.new |> Graph.add_vertices([:a, :c]) |> Graph.add_edge(:a, :c, label: :first_label, weight: 2) |> Graph.add_edge(:a, :c, label: :second_label, weight: 2)
+      ...> g = Graph.split_edge(g, :a, :c, :b)
+      ...> Graph.edges(g)
+      [%Graph.Edge{v1: :a, v2: :b, label: :first_label, weight: 2}, %Graph.Edge{v1: :a, v2: :b, label: :second_label, weight: 2}, %Graph.Edge{v1: :b, v2: :c, label: :first_label, weight: 2}, %Graph.Edge{v1: :b, v2: :c, label: :second_label, weight: 2}]
   """
   @spec split_edge(t, vertex, vertex, vertex) :: t | {:error, :no_such_edge}
-  def split_edge(%__MODULE__{type: :undirected} = g, v1, v2, v3) do
+  def split_edge(%__MODULE__{} = g, v1, v2, v3) do
+    split_labelled_edge(g, v1, v2, v3, nil)
+  end
+
+  @doc """
+  Like `split_edge/4`, but requires you to specify the labelled edge to split.
+
+  Th implementation of `splt_edge/4` is actually `split_labelled_edge(g, v1, v2, v3, nil)`.
+
+  ## Example
+
+      iex> g = Graph.new |> Graph.add_vertices([:a, :c]) |> Graph.add_edge(:a, :c, label: :first_label, weight: 2) |> Graph.add_edge(:a, :c, label: :second_label, weight: 2)
+      ...> g = Graph.split_labelled_edge(g, :a, :c, :b, :first_label)
+      ...> Graph.edges(g)
+      [%Graph.Edge{v1: :a, v2: :b, label: :first_label, weight: 2}, %Graph.Edge{v1: :a, v2: :c, label: :second_label, weight: 2}, %Graph.Edge{v1: :b, v2: :c, label: :first_label, weight: 2}]
+  """
+  def split_labelled_edge(%__MODULE__{type: :undirected} = g, v1, v2, v3, label) do
     if v1 > v2 do
-      do_split_edge(g, v2, v1, v3)
+      do_split_labelled_edge(g, v2, v1, v3, label)
     else
-      do_split_edge(g, v1, v2, v3)
+      do_split_labelled_edge(g, v1, v2, v3, label)
     end
   end
 
-  def split_edge(%__MODULE__{} = g, v1, v2, v3) do
-    do_split_edge(g, v1, v2, v3)
+  def split_labelled_edge(%__MODULE__{} = g, v1, v2, v3, label) do
+    do_split_labelled_edge(g, v1, v2, v3, label)
   end
 
-  defp do_split_edge(
-         %__MODULE__{in_edges: ie, out_edges: oe, edges: em, vertex_identifier: vertex_identifier} =
+  defp do_split_labelled_edge(
+         %__MODULE__{out_edges: oe, edges: em, vertex_identifier: vertex_identifier} =
            g,
          v1,
          v2,
-         v3
+         v3,
+         label
        ) do
     with v1_id <- vertex_identifier.(v1),
          v2_id <- vertex_identifier.(v2),
          {:ok, v1_out} <- Map.fetch(oe, v1_id),
-         {:ok, v2_in} <- Map.fetch(ie, v2_id),
          true <- MapSet.member?(v1_out, v2_id),
-         meta <- Map.get(em, {v1_id, v2_id}),
-         v1_out <- MapSet.delete(v1_out, v2_id),
-         v2_in <- MapSet.delete(v2_in, v1_id) do
-      g = %__MODULE__{
-        g
-        | in_edges: Map.put(ie, v2_id, v2_in),
-          out_edges: Map.put(oe, v1_id, v1_out)
-      }
-
+         meta <- Map.get(em, {v1_id, v2_id}) do
       g = add_vertex(g, v3)
-
-      Enum.reduce(meta, g, fn {label, weight}, acc ->
-        acc
-        |> add_edge(v1, v3, label: label, weight: weight)
-        |> add_edge(v3, v2, label: label, weight: weight)
+      Enum.reduce(meta, g, fn {edge_label, weight}, acc ->
+        if edge_label == label or label == nil do
+          acc
+          |> add_edge(v1, v3, label: edge_label, weight: weight)
+          |> add_edge(v3, v2, label: edge_label, weight: weight)
+          |> delete_edge(v1, v2, edge_label)
+        else
+          acc
+        end
       end)
     else
       _ -> {:error, :no_such_edge}