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absinthe lib absinthe traversal.ex
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lib/absinthe/traversal.ex 

defmodule Absinthe.Traversal do

  # Graph traversal utilities for dealing with ASTs and schemas using the

  # `Absinthe.Traversal.Node` protocol.



  @moduledoc false



  alias __MODULE__

  alias Absinthe.Traversal.Node



  @type t :: %{context: any, seen: [Node.t], path: [Node.t]}

  defstruct context: nil, seen: [], path: []



  # Instructions defining behavior during traversal

  # * `{:ok, value, traversal}`: The value of the node is `value`, and traversal

  #   should continue to children (using `traversal`)

  # * `{:prune, value, traversal}`: The value of the node is `value` and

  #   traversal should NOT continue to children, but to siblings (using

  #   `traversal`)

  # * `{:error, message}`: Bad stuff happened, explained by `message`

  @type instruction_t :: {:ok, any} | {:prune, any} | {:error, any}



  # Traverse, reducing nodes using a given function to evaluate their value.

  @doc false

  @spec reduce(Node.t, any, any, (Node.t -> instruction_t)) :: any

  def reduce(node, context, initial_value, node_evaluator) do

    {result, _traversal} = do_reduce(node, %Traversal{context: context}, initial_value, node_evaluator)

    result

  end



  # Reduce using a traversal struct

  @spec do_reduce(Node.t, t, any, (Node.t -> instruction_t)) :: {any, t}

  defp do_reduce(node, traversal, initial_value, node_evaluator) do

    if seen?(traversal, node) do

      {initial_value, traversal}

    else

      case node_evaluator.(node, traversal, initial_value) do

        {:ok, value, next_traversal} ->

          reduce_children(node, next_traversal |> put_seen(node), value, node_evaluator)

        {:prune, value, next_traversal} ->

          {value, next_traversal |> put_seen(node)}

      end

    end

  end



  # Traverse a node's children

  @spec reduce(Node.t, t, any, (Node.t -> instruction_t)) :: any

  defp reduce_children(node, traversal, initial, node_evalator) do

    Enum.reduce(Node.children(node, traversal), {initial, traversal}, fn

      child, {this_value, this_traversal} ->

        do_reduce(child, this_traversal, this_value, node_evalator)

    end)

  end



  @spec seen?(t, Node.t) :: boolean

  defp seen?(traversal, node), do: traversal.seen |> Enum.member?(node)



  @spec put_seen(t, Node.t) :: t

  defp put_seen(traversal, node) do

    %{traversal | seen: [node | traversal.seen]}

  end



end