defmodule GraphQL.Relay.Connection do
  @moduledoc """
  You define a Connection for Relay when you want a paginated data source.

  For example, in a Todo application a User can have many Todos that you want
  to list and paginate through. A Connection is the perfect fit for this.

  For an example see https://github.com/graphql-elixir/graphql_relay/blob/44e6912a08468743f7dfa1343723914bbf6809e1/examples/todo/web/graphql/user.ex

  For the Connection type specification see: https://facebook.github.io/relay/graphql/connections.htm#sec-Connection-Types
  """
  alias GraphQL.Type.Boolean
  alias GraphQL.Type.Int
  alias GraphQL.Type.NonNull
  alias GraphQL.Type.ObjectType

  def new(config) do
    node_type = config[:node_type]

    name = config[:name] || Map.get(node_type, :name)
    edge_fields = config[:edge_fields] || %{}
    connection_fields = config[:connection_fields] || %{}
    resolve_node = config[:resolve_node]
    resolve_cursor = config[:resolve_cursor]

    edge_type = %ObjectType{
      name: "#{name}Edge",
      description: "An edge in a connection.",
      fields: Map.merge(GraphQL.Relay.resolve_maybe_thunk(edge_fields), %{
        node: %{
          type: node_type,
          description: "The item at the end of the edge",
          resolve: resolve_node
        },
        cursor: %{
          type: %NonNull{ofType: %GraphQL.Type.String{}},
          description: "A cursor for use in pagination",
          resolve: resolve_cursor
        }
      })
    }

    connection_type = %ObjectType{
      name: "#{name}Connection",
      description: "A connection to a list of items.",
      fields: Map.merge(GraphQL.Relay.resolve_maybe_thunk(connection_fields), %{
        edges: %{
          type: %GraphQL.Type.List{ofType: edge_type},
          description: "Information to aid in pagination."
        },
        pageInfo: %{
          type: %NonNull{ofType: page_info},
          description: "Information to aid in pagination."
        },
      })
    }

    %{edge_type: edge_type, connection_type: connection_type}
  end

  def args(additional_args \\ %{}) do
    Map.merge(forward_args, backward_args)
      |> Map.merge(additional_args)
  end

  def forward_args do
    %{
      "after": %{type: %GraphQL.Type.String{}},
      first: %{type: %Int{}}
    }
  end

  def backward_args do
    %{
      before: %{type: %GraphQL.Type.String{}},
      last: %{type: %Int{}}
    }
  end

  def page_info do
    %ObjectType{
      name: "PageInfo",
      description: "Information about pagination in a connection.",
      fields: %{
        hasNextPage: %{
          type: %NonNull{ofType: %Boolean{}},
          description: "When paginating forwards, are there more items?"
        },
        hasPreviousPage: %{
          type: %NonNull{ofType: %Boolean{}},
          description: "When paginating backwards, are there more items?"
        },
        startCursor: %{
          type: %GraphQL.Type.String{},
          description: "When paginating backwards, the cursor to continue."
        },
        endCursor: %{
          type: %GraphQL.Type.String{},
          description: "When paginating forwards, the cursor to continue."
        }
      }
    }
  end
end