defmodule Facebook do
  @moduledoc """
  Provides API wrappers for the Facebook Graph API

  See: https://developers.facebook.com/docs/graph-api
  """

  use Application

  alias Facebook.Config
  alias Facebook.GraphAPI
  alias Facebook.GraphVideoAPI
  alias Facebook.ResponseFormatter

  def start(_type, _args) do
    children = [Config]
    opts = [strategy: :one_for_one, name: Facebook.Supervisor]

    Supervisor.start_link(children, opts)
  end

  @typedoc """
  A token which is used to authenticate requests to Facebook's Graph API.

  A user access token may be generated with
  [Facebook Login](https://developers.facebook.com/docs/facebook-login/).
  Access tokens for testing purposes may be retrieved from Facebook's
  [Access Token Tool](https://developers.facebook.com/tools/accesstoken/)
  or using the
  [Graph Api Explorer](https://developers.facebook.com/tools/explorer/).
  """
  @type access_token :: String.t()

  @typedoc """
  Also referred to as an App ID, this may be found on your app dashboard.
  """
  @type client_id :: String.t()

  @typedoc """
  Also referred to as an App Secret, this may be found on your app dashboard.
  """
  @type client_secret :: String.t()

  @typedoc """
  Query values used for supplying or requesting edge attributes.
  """
  @type fields :: list

  @typedoc """
  Relative path to media file.
  """
  @type file_path :: String.t()

  @type limit :: number
  @type num_resp :: {:ok, number} | {:error, Map.t()}

  @typedoc """
  An id composed of a page and post ids separated with an underscore.
  """
  @type object_id :: String.t()
  @type page_id :: String.t() | integer

  @typedoc """
  Additional attributes for media file uploads
  """
  @type params :: list

  @typedoc """
  Can be:
    * `:angry`
    * `:haha`
    * `:love`
    * `:none`
    * `:sad`
    * `:thankful`
    * `:wow`
  """
  @type react_type :: atom
  @type reaction :: :reaction
  @type resp :: {:ok, Map.t()} | {:error, Map.t()}

  @typedoc """
  A type of feed or object.

  Feed scopes:
    * `:feed`
    * `:posts`
    * `:promotabled_posts` (Admin permission needed)
    * `:tagged`

  Object scopes:
    * `:likes`
    * `:comments`
  """
  @type scope :: atom | String.t()

  @typedoc """
  A reason for settling a payment dispute.

  Reasons:
    * `:GRANTED_REPLACEMENT_ITEM`
    * `:DENIED_REFUND`
    * `:BANNED_USER`
  """
  @type dispute_reason :: atom | String.t()

  @typedoc """
  A reason for refunding a payment.

  Reasons:
    * `:MALICIOUS_FRAUD`
    * `:FRIENDLY_FRAUD`
    * `:CUSTOMER_SERVICE`
  """
  @type refunds_reason :: atom | String.t()

  @type currency :: String.t()
  @type amount :: Number.t()

  @typedoc """
  A base64-encoded JSON string, concatenated to a signature with a single dot.
  E.g.: "<base64-encoded hmac/sha256 signature>.<base64-encoded JSON payload>"
  """
  @type signed_request :: String.t()

  @type using_app_secret :: boolean

  @doc """
  If you want to use an appsecret proof, pass it into set_app_secret:

  ## Example
      iex> Facebook.set_app_secret("app_secret")

  See: https://developers.facebook.com/docs/graph-api/securing-requests
  """
  def set_app_secret(app_secret) do
    Config.app_secret(app_secret)
  end

  @doc """
  Basic user infos of the logged in user specified by the `t:access_token/0`

  ## Examples
      iex> Facebook.me("id,first_name", "<Access Token>")
      {:ok, %{"first_name" => "...", "id" => "..."}}

      iex> Facebook.me([fields: "id,first_name"], "<Access Token>")
      {:ok, %{"first_name" => "...", "id" => "..."}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/
  """
  @spec me(fields :: String.t(), access_token) :: resp
  def me(fields, access_token) when is_binary(fields) do
    me([fields: fields], access_token)
  end

  @spec me(fields, access_token) :: resp
  def me(fields, access_token) do
    params =
      fields
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/me)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Accounts for the logged in user specified by the `t:access_token/0`

  ## Examples
      iex> Facebook.my_accounts("<Access Token>")
      {:ok, %{"data" => [...]}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/accounts
  """
  @spec my_accounts(access_token) :: resp
  def my_accounts(access_token) do
    params =
      []
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/me/accounts)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Publish to a graph edge using the supplied token.
  Publish to a feed. Author (user or page) is determined from the supplied token.

  The `t:page_id/0` is the id for the user or page feed to publish to.
  Apps need both `manage_pages` and `publish_pages` permissions to be able to publish as a Page.
  The `publish_actions` permission is required to publish as an individual.

  See Facebook's publishing documentation for more info:

  * https://developers.facebook.com/docs/pages/publishing
  * https://developers.facebook.com/docs/pages/publishing#personal_post
  * https://developers.facebook.com/docs/facebook-login/permissions#reference-publish_pages

  ## Examples
      iex> # publish a message
      iex> Facebook.publish(:feed, "<Feed Id>", [message: "<Message Body"], "<Access Token>")
      {:ok, %{"id" => "{page_id}_{post_id}"}}

      iex> # publish a link and message
      iex> Facebook.publish(:feed, "<Feed Id>", [message: "<Message Body", link: "www.example.com"], "<Access Token>")
      {:ok, %{"id" => "{page_id}_{post_id}"}}

      iex> # create a Facebook Campaign
      iex> Facebook.publish(:campaigns, "act_1234546", [objective: "LINK_CLICKS", name: "a campaign"], "<Access Token>")
      {:ok, %{"id" => "{campaign_id}"}}
  """
  @spec publish(edge :: atom(), parent_id :: String.t(), params, access_token) :: resp
  def publish(edge, parent_id, params, access_token) do
    params =
      params
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{parent_id}/#{edge})
    |> GraphAPI.post("", [], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Publish media to a feed. Author (user or page) is determined from the supplied token.

  The `t:page_id/0` is the id for the user or page feed to publish to.
  Same `:feed` publishing permissions apply.

  ## Example
      iex> Facebook.publish(:photo, "<Feed Id>", "<Image Path>", [], "<Access Token>")
      {:ok, %{"id" => photo_id, "post_id" => "{page_id}_{post_id}"}

      iex> Facebook.publish(:video, "<Feed Id>", "<Video Path>", [], "<Access Token>")
      {:ok, %{"id" => video_id}

  See: https://developers.facebook.com/docs/pages/publishing#fotos_videos
  """
  @spec publish(:photo, page_id, file_path, params, access_token) :: resp
  def publish(:photo, page_id, file_path, params, access_token) do
    params =
      params
      |> add_access_token(access_token)

    payload = media_payload(file_path)

    ~s(/#{page_id}/photos)
    |> GraphAPI.post(payload, [], params: params)
    |> ResponseFormatter.format_response()
  end

  @spec publish(
          :video,
          page_id,
          file_path,
          params,
          access_token,
          options :: list
        ) :: resp
  # credo:disable-for-next-line Credo.Check.Refactor.FunctionArity
  def publish(:video, page_id, file_path, params, access_token, options \\ []) do
    params =
      params
      |> add_access_token(access_token)

    options = options ++ [params: params]
    payload = media_payload(file_path)

    ~s(/#{page_id}/videos)
    |> GraphVideoAPI.post(payload, [], options)
    |> ResponseFormatter.format_response()
  end

  defp media_payload(file_path) do
    {
      :multipart,
      [
        {
          :file,
          file_path,
          {"form-data", [filename: Path.basename(file_path)]},
          []
        }
      ]
    }
  end

  @doc """
  A Picture for a Facebook User

  `type` may be:
    * `"small"`
    * `"normal"`
    * `"album"`
    * `"large"`
    * `"square"`

  ## Example
      iex> Facebook.picture("<Some Id>", "small", "<Access Token>")
      {:ok, %{"data": "..."}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/picture/
  """
  @spec picture(page_id, type :: String.t(), access_token) :: resp
  def picture(page_id, type, access_token) do
    params =
      [type: type, redirect: false]
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{page_id}/picture)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  A Picture for a Facebook User with custom dimensions

  ## Example
      iex> Facebook.picture("<Some Id>", 480, 480, "<Access Token>")
      {:ok, %{"data": "..."}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/picture/
  """
  @spec picture(page_id, width :: integer, height :: integer, access_token) :: resp
  def picture(page_id, width, height, access_token) do
    params =
      [width: width, height: height, redirect: false]
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{page_id}/picture)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Likes of the currently logged in user specified by the `t:access_token/0`

  ## Example
      iex> Facebook.my_likes("<Access Token>")
      {:ok, %{"data" => [...]}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/likes
  """
  @spec my_likes(access_token) :: resp
  def my_likes(access_token) do
    params =
      []
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/me/likes)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Retrieves a list of granted permissions

  ## Example
      iex> Facebook.permissions("<Some Id>", "<Access Token>")
      {:ok, %{"data" => [%{"permission" => "...", "status" => "..."}]}}

  See: https://developers.facebook.com/docs/graph-api/reference/user/permissions
  """
  @spec permissions(page_id, access_token) :: resp
  def permissions(page_id, access_token) do
    params =
      []
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{page_id}/permissions)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Get the count of fans for the provided `t:page_id/0`

  ## Example
      iex> Facebook.fan_count("CocaColaMx", "<Access Token>")
      {:ok, %{"fan_count" => fan_count, "id" => id}}

  See: https://developers.facebook.com/docs/graph-api/reference/page/
  """
  @spec fan_count(page_id, access_token) :: integer
  def fan_count(page_id, access_token) do
    page(page_id, access_token, ["fan_count"])
  end

  @doc """
  Basic Graph object information by object ID

  ## Example
      iex> Facebook.get_object("1234567", "<Access Token>")
      {:ok, %{"id" => id}}
  """
  @spec get_object(object_id :: String.t(), access_token) :: resp
  def get_object(object_id, access_token) do
    get_object(object_id, access_token, [])
  end

  @doc """
  Get Graph object information for the specified params for the provided object ID

  ## Example
      iex> Facebook.get_object("1234567", "<Access Token>", [fields: "id,name"])
      {:ok, %{"id" => id, "name" => name}

  See: https://developers.facebook.com/docs/graph-api/reference/page
  """
  @spec get_object(object_id, access_token, params) :: resp
  def get_object(object_id, access_token, params) do
    params =
      params
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{object_id})
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Gets an object edge for a specific object ID

  ## Examples
      iex> Facebook.get_object_edge(:adlabels, "act_12345", "<Access Token>")
      iex> Facebook.page_feed(:leads, "1223344332", "<Access Token>", [fields: "created_time,id,ad_id,form_id,field_data"])
      {:ok, %{"data" => [...]}}

  """
  # credo:disable-for-lines:1 Credo.Check.Readability.MaxLineLength
  @spec get_object_edge(edge :: atom | String.t(), object_id :: String.t(), access_token, params) ::
          resp
  def get_object_edge(edge, object_id, access_token, params \\ []) do
    params =
      params
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{object_id}/#{edge})
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Basic page information for the provided `t:page_id/0`

  ## Example
      iex> Facebook.page("CocaColaMx", "<Access Token>")
      {:ok, %{"id" => id, "name" => name}}

  See: https://developers.facebook.com/docs/graph-api/reference/page
  """
  @spec page(page_id, access_token) :: resp
  def page(page_id, access_token) do
    page(page_id, access_token, [])
  end

  @doc """
  Get page information for the specified fields for the provided `t:page_id/0`

  ## Example
      iex> Facebook.page("CocaColaMx", "<Access Token>", "id")
      {:ok, %{"id" => id}

  See: https://developers.facebook.com/docs/graph-api/reference/page
  """
  @spec page(page_id, access_token, fields) :: resp
  def page(page_id, access_token, fields) do
    params = [fields: Enum.join(fields, ",")]
    get_object(page_id, access_token, params)
  end

  @doc """
  Gets the feed of posts (including status updates) and links published by this
  page, or by others on this page.

  This function can retrieve four `t:scope/0` types:
    * `:feed`
    * `:posts`
    * `:promotable_posts` (*Admin permission needed*)
    * `:tagged`

  A `t:scope/0` must be provided. It is an atom, which represents the type of feed.

  *A limit of posts may be given. The maximum number that must be provided is
  100.*

  ## Examples
      iex> Facebook.page_feed(:posts, "CocaColaMx", "<Access Token>")
      iex> Facebook.page_feed(:tagged, "CocaColaMx", "<Access Token>", 55)
      iex> Facebook.page_feed(:promotable_posts, "CocaColaMx", "<Access Token>")
      iex> Facebook.page_feed(:feed, "CocaColaMx", "<Access Token>", 55, "id,name")
      {:ok, %{"data" => [...]}}

  See: https://developers.facebook.com/docs/graph-api/reference/page/feed

  """
  # credo:disable-for-lines:2 Credo.Check.Readability.MaxLineLength
  @spec page_feed(scope, page_id, access_token, limit, fields :: String.t()) :: resp
  def page_feed(scope, page_id, access_token, limit \\ 25, fields \\ "") when limit <= 100 do
    params = [limit: limit, fields: fields]
    get_object_edge(scope, page_id, access_token, params)
  end

  @doc """
  Gets the number of elements that a scope has in a given object.

  An *object* stands for: post, comment, link, status update, photo.

  If you want to get the likes of a page, please see `fan_count/2`.

  Expected scopes:
    * `:likes`
    * `:comments`

  ## Example
      iex> Facebook.object_count(:likes, "1326382730725053_1326476257382367", "<Access Token>")
      {:ok, 10}
      iex> Facebook.object_count(:comments, "1326382730725053_1326476257382367", "<Access Token>")
      {:ok, 5}

  See:
    * https://developers.facebook.com/docs/graph-api/reference/object/likes
    * https://developers.facebook.com/docs/graph-api/reference/object/comments
  """
  @spec object_count(scope, object_id, access_token) :: num_resp
  def object_count(scope, object_id, access_token) when is_atom(scope) do
    params =
      [summary: true]
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    scp =
      scope
      |> Atom.to_string()
      |> String.downcase()

    ~s(/#{object_id}/#{scp})
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
    |> get_summary
    |> summary_count
  end

  @doc """
  Gets the number of reactions that an object has.

  Expected `t:react_type/0`:
    * `:haha`
    * `:wow`
    * `:thankful`
    * `:sad`
    * `:angry`
    * `:love`
    * `:none`

  ## Examples
      iex> Facebook.object_count(
        :reaction,
        :wow,
        "769860109692136_1173416799336463",
        "<Access Token>"
      )
      {:ok, 100}
  """
  @spec object_count(reaction, react_type, object_id, access_token) :: num_resp
  def object_count(:reaction, react_type, object_id, access_token) when is_atom(react_type) do
    type =
      react_type
      |> Atom.to_string()
      |> String.upcase()

    params =
      [type: type, summary: "total_count"]
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{object_id}/reactions)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
    |> get_summary
    |> summary_count
  end

  @doc """
  Get all the object reactions with single request.

  ## Examples
      iex> Facebook.object_count_all("769860109692136_1173416799336463", "<Access Token>")
      {:ok, %{"angry" => 0, "haha" => 1, "like" => 0, "love" => 0, "sad" => 0, "wow" => 0}}
  """
  @spec object_count_all(object_id, access_token) :: resp
  def object_count_all(object_id, access_token) do
    graph_query = """
    reactions.type(LIKE).summary(total_count).limit(0).as(like),
    reactions.type(LOVE).summary(total_count).limit(0).as(love),
    reactions.type(WOW).summary(total_count).limit(0).as(wow),
    reactions.type(HAHA).summary(total_count).limit(0).as(haha),
    reactions.type(SAD).summary(total_count).limit(0).as(sad),
    reactions.type(ANGRY).summary(total_count).limit(0).as(angry)
    """

    params =
      [fields: graph_query]
      |> add_app_secret(access_token)
      |> add_access_token(access_token)

    ~s(/#{object_id})
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
    |> summary_count_all
  end

  @doc """
  Gets payment info about a single payment.

  ## Examples
      iex> Facebook.payment("769860109692136", "<App Access Token>", "id,request_id,actions")
      {:ok, %{"request_id" => "abc2387238", "id" => "116397053038597", "actions" => [ %{ "type" => "charge", ... } ] } }

  See:
    * https://developers.facebook.com/docs/graph-api/reference/payment
  """
  @spec payment(object_id, access_token, fields) :: resp
  def payment(payment_id, access_token, fields \\ "") do
    params =
      [fields: fields]
      |> add_access_token(access_token)

    ~s(/#{payment_id})
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Settle a payment dispute.

  ## Examples
      iex> Facebook.payment_dispute("769860109692136", "<App Access Token>", :DENIED_REFUND)
      {:ok, %{"success" => true}}

  See:
    * https://developers.facebook.com/docs/graph-api/reference/payment/dispute
  """
  @spec payment_dispute(object_id, access_token, dispute_reason) :: resp
  def payment_dispute(payment_id, access_token, reason) do
    params =
      []
      |> add_access_token(access_token)

    body = URI.encode_query(%{reason: reason})

    ~s(/#{payment_id}/dispute)
    |> GraphAPI.post(body, params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Refund a payment.

  ## Examples
      iex> Facebook.payment_refunds("769860109692136", "<App Access Token>", "EUR", 10.99, :CUSTOMER_SERVICE)
      {:ok, %{"success" => true}}

  See:
    * https://developers.facebook.com/docs/graph-api/reference/payment/refunds
  """
  # credo:disable-for-lines:1 Credo.Check.Readability.MaxLineLength
  @spec payment_refunds(object_id, access_token, currency, amount, refunds_reason) :: resp
  def payment_refunds(payment_id, access_token, currency, amount, reason) do
    params =
      []
      |> add_access_token(access_token)

    body =
      URI.encode_query(%{
        currency: currency,
        amount: amount,
        reason: reason
      })

    ~s(/#{payment_id}/refunds)
    |> GraphAPI.post(body, params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Exchange an authorization code for an access token.

  If you are implementing user authentication, the `code` is generated from a Facebook
  endpoint which is outside of the Graph API. Please see the
  [Manually Build a Login Flow](https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow#confirm)
  documentation for more details.

  ## Examples
      iex> Facebook.access_token("client_id", "client_secret", "redirect_uri", "code")
      {:ok, %{
        "access_token" => access_token,
        "expires_in" => 5183976,
        "token_type" => "bearer"
      }}

  See: https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow#confirm
  """
  @spec access_token(client_id, client_secret, String.t(), String.t()) :: resp
  def access_token(client_id, client_secret, redirect_uri, code) do
    [
      client_id: client_id,
      client_secret: client_secret,
      redirect_uri: redirect_uri,
      code: code
    ]
    |> get_access_token
  end

  @doc """
  Exchange a short lived access token for a long lived one

  ## Examples
      iex> Facebook.long_lived_access_token("client_id", "client_secret", "access_token")
      {:ok, %{
        "access_token" => access_token,
        "expires_in" => 5183976,
        "token_type" => "bearer"
      }}

  See: https://developers.facebook.com/docs/facebook-login/access-tokens/expiration-and-extension
  """
  @spec long_lived_access_token(client_id, client_secret, access_token) :: resp
  def long_lived_access_token(client_id, client_secret, access_token) do
    [
      grant_type: "fb_exchange_token",
      client_id: client_id,
      client_secret: client_secret,
      fb_exchange_token: access_token
    ]
    |> get_access_token
  end

  @doc """
  Get all test users for an app.

  The access token in this case needs to be an app access token.
  See:
    - https://developers.facebook.com/docs/facebook-login/access-tokens#apptokens
    - https://developers.facebook.com/docs/graph-api/reference/v2.8/app/accounts/test-users

  ## Examples
      iex> Facebook.test_users("appId", "appId|appSecret")
      {:ok, %{"data" => [
        %{
          "access_token" => "ACCESS_TOKEN",
          "id" => "USER_ID",
          "login_url" => "https://developers.facebook.com/checkpoint/test-user-login/USER_ID/"
        }
      ]}
  """
  @spec test_users(client_id, access_token) :: resp
  def test_users(client_id, access_token) do
    params =
      []
      |> add_access_token(access_token)

    ~s(/#{client_id}/accounts/test-users)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Returns metadata about a given access token.

  This includes data such as the user for which the token was issued,
  whether the token is still valid, when it expires, and what permissions the
  app has for the given user.

  This may be used to programatically debug issues with large sets of access tokens.

  An app access token or an app developer's user access token for the
  app associated with the input_token is required to acces.

  See:
   - https://developers.facebook.com/docs/graph-api/reference/v2.11/debug_token
   - https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow#checktoken

  ## Examples
      iex> Facebook.debug_token("INPUT_TOKEN", "ACCESS_TOKEN")
      {:ok, %{"data" => [
        %{
          "app_id": "APP_ID",
          "type": "USER",
          "application": "APP_NAME",
          "expires_at": 1352419328,
          "is_valid": true,
          "issued_at": 1347235328,
          "scopes": [
              "email",
              "publish_actions"
          ],
          "user_id": "USER_ID"
        }
      ]}
  """
  @spec debug_token(access_token, access_token) :: resp
  def debug_token(input_token, access_token) do
    params = add_access_token([input_token: input_token], access_token)

    ~s(/debug_token)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  @doc """
  Decodes a signed request from a client SDK (in-app payments), verifies the
  signature and (if it is valid) returns its decoded contents.
  """
  @spec decode_signed_request(signed_request) :: resp
  def decode_signed_request(signed_request) do
    with [signature_str | [payload_str | _]] <-
           String.split(signed_request, "."),
         {:ok, signature} <- Base.url_decode64(signature_str),
         _signature_verification = ^signature <- signature(payload_str),
         {:ok, payload} <- Base.url_decode64(payload_str),
         {:ok, payload} <- JSON.decode(payload) do
      {:ok, payload}
    else
      _ -> {:error, %{}}
    end
  end

  # Builds a signature just like Facebook does for its signed_requests.
  def sign(payload) do
    payload_str = Base.url_encode64(payload)
    "#{signature_base64(payload_str)}.#{payload_str}"
  end

  # Request access token and extract the access token from the access token
  # response
  defp get_access_token(params) do
    ~s(/oauth/access_token)
    |> GraphAPI.get([], params: params)
    |> ResponseFormatter.format_response()
  end

  # Provides the summary of a GET request when the 'summary' query parameter is
  # set to true.
  defp get_summary(summary_response) do
    case summary_response do
      {:error, error} -> {:error, error}
      {:ok, %{"summary" => summary}} -> summary
    end
  end

  # Gets the 'total_count' attribute from a summary request.
  defp summary_count(%{"total_count" => count}), do: {:ok, count}

  # Returns an error if the summary request fails.
  defp summary_count({:error, error}), do: {:error, error}

  defp summary_count_all({:ok, data}), do: summary_count_all(data)

  # Returns an error if the summary request fails.
  defp summary_count_all({:error, error}), do: {:error, error}

  # Calculate the reactions summary
  defp summary_count_all(summary) do
    summary
    |> Map.keys()
    |> Enum.reject(fn x -> x === "id" end)
    |> Enum.map(fn x ->
      [x, get_in(summary[x], ["summary", "total_count"])]
    end)
    |> Enum.reduce(%{}, fn [name, count], acc ->
      Map.put(acc, name, count)
    end)
    |> (&{:ok, &1}).()
  end

  # Hashes the token together with the app secret according to the
  # guidelines of facebook to build an unencoded/raw signature.
  defp signature(str) do
    :crypto.hmac(:sha256, Config.app_secret(), str)
  end

  # Uses signature/1 to build a urlsafe base64-encoded signature
  defp signature_base64(str) do
    str |> signature() |> Base.url_encode64()
  end

  # Uses signature/1 to build a lowercase base16-encoded signature
  defp signature_base16(str) do
    str |> signature() |> Base.encode16(case: :lower)
  end

  # Add the appsecret_proof to the GraphAPI request params if the app secret is
  # defined
  defp add_app_secret(params, access_token) do
    if is_nil(Config.app_secret()) do
      params
    else
      params ++ [appsecret_proof: signature_base16(access_token)]
    end
  end

  ## Add access_token to params
  defp add_access_token(fields, token) do
    fields ++ [access_token: token]
  end
end