-module(prometheus_http).

-export([reply/1,
         setup/0]).

-define(SCRAPE_DURATION, telemetry_scrape_duration_seconds).
-define(SCRAPE_SIZE, telemetry_scrape_size_bytes).
-define(SCRAPE_ENCODED_SIZE, telemetry_scrape_encoded_size_bytes).

%% @doc
%% Render metrics
%% @end
reply(#{path := Path,
        headers := Headers,
        registry := Registry,
        standalone := Standalone}) ->

  case prometheus_http_config:valid_path_and_registry(Path, Registry) of
    {true, RealRegistry} ->
      if_authorized(Path, Headers,
                    fun () ->
                        format_metrics(Headers, RealRegistry)
                    end);
    {registry_conflict, _ReqR, _ConfR} ->
      {409, [], <<>>};
    {registry_not_found, _ReqR} ->
      {404, [], <<>>};
    false ->
      maybe_render_index(Standalone, Path, Headers)
  end.

%% @doc
%% Initializes telemetry metrics.<br/>
%% *NOTE:* If you plug `prometheus_httpd' in your existing httpd instance,
%% you have to call this function manually.
%% @end
setup() ->
  TelemetryRegistry = prometheus_http_config:telemetry_registry(),

  ScrapeDuration = [{name, ?SCRAPE_DURATION},
                    {help, "Scrape duration"},
                    {labels, ["registry", "content_type"]},
                    {registry, TelemetryRegistry}],
  ScrapeSize = [{name, ?SCRAPE_SIZE},
                {help, "Scrape size, not encoded"},
                {labels, ["registry", "content_type"]},
                {registry, TelemetryRegistry}],
  ScrapeEncodedSize = [{name, ?SCRAPE_ENCODED_SIZE},
                       {help, "Scrape size, encoded"},
                       {labels, ["registry", "content_type", "encoding"]},
                       {registry, TelemetryRegistry}],

  prometheus_summary:declare(ScrapeDuration),
  prometheus_summary:declare(ScrapeSize),
  prometheus_summary:declare(ScrapeEncodedSize).

%% ===================================================================
%% Private Parts
%% ===================================================================

format_metrics(Headers, Registry) ->
  Accept = Headers("accept", "text/plain"),
  AcceptEncoding = Headers("accept-encoding", undefined),
  format_metrics(Accept, AcceptEncoding, Registry).

maybe_render_index(Standalone, Path, Headers) ->
  case Standalone of
    true ->
      case Path of
        "/favicon.ico" ->
          %% https://www.iconfinder.com/icons/85652/fire_torch_icon#size=30
          %% http://www.iconbeast.com/
          {200, [{"content-type", "image/png"}],
           base64:decode("iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAABaklEQ"
                         "VRIS+2UsU0EQQxF3+UElAASBUAHUAACKgACcqjg6AByAqACOFEAJU"
                         "ABBHRAQg760liaG+2u7QF0CROd9sb/+dsez+g7G8AzcAE89kjMeoK"
                         "AbeClxF6XBFJSveAH4LAipXXSAcAVcN7YS+tkA26Bk4GaSmcXOIiW"
                         "PQMegyoP6Vj5d4BXr+FR8CUwnxC7qyqh36e/Bf4A1j2xzLBFHX8NQ"
                         "N/LN73p9ri67oWi2IIFVS/VVw3Vn4G1pWqAemh91dDVR0ltem2JOl"
                         "Y5tamsz3VcO+1HkTUaBcuA1qScC17HqRL6rmOV8AwvCbiXC1QOF6X"
                         "UitFCOS6Lw32/Bsk4jiQWvhMFWymjwnvexSjY00n/nwFHXbtulWUG"
                         "/ASsOdY+gf2I/QzYpndK976a9kl+BiyhG2BrRPENOIu4zZbaNIech"
                         "53+9B23gxYaqLoa2VJb7MrASsDgabe9PW5d/4NDL6p3uFba45CzsU"
                         "vfrgozHx5iraMAAAAASUVORK5CYII=")};
        _ ->
          MetricsPath = prometheus_http_config:path(),
          if_authorized(Path, Headers,
                        fun () ->
                            {200, [], prepare_index(MetricsPath)}
                        end)
      end;
    false ->
      false
  end.

if_authorized(URI, Headers, Fun) ->
  case prometheus_http_config:authorization() of
    {invalid_authorize, _} ->
      {500, [], <<>>};
    Auth ->
      case Auth(#{uri => URI,
                  headers => Headers}) of
        true ->
          Fun();
        false ->
          {403, [], <<>>}
      end
  end.

prepare_index(Path) ->
  FileName = filename:join([code:priv_dir(prometheus_httpd), "index.html"]),
  {ok, Content} = file:read_file(FileName),
  re:replace(Content, "M_E_T_R_I_C_S", Path, [global, {return, list}]).

format_metrics(Accept, AcceptEncoding, Registry) ->
  case negotiate_format(Accept) of
    undefined ->
      {406, [], <<>>};
    Format ->
      {ContentType, Scrape} = render_format(Format, Registry),
      case negotiate_encoding(AcceptEncoding) of
        undefined ->
          {406, [], <<>>};
        Encoding ->
          encode_format(ContentType, binary_to_list(Encoding), Scrape, Registry)
      end
  end.

negotiate_format(Accept) ->
  case prometheus_http_config:format() of
    auto ->
      Alternatives = prometheus_http_config:allowed_formats(),
      accept_header:negotiate(Accept, Alternatives);
    Format0 -> Format0
  end.

negotiate_encoding(AcceptEncoding) ->
  %% curl and other tools do not send Accept-Encoding field.
  %% RFC says it means we can render any encoding.
  %% But people want to see something meaningful in console!
  accept_encoding_header:negotiate(AcceptEncoding, [<<"identity">>,
                                                    <<"gzip">>,
                                                    <<"deflate">>]).

render_format(Format, Registry) ->
  ContentType = Format:content_type(),
  TelemetryRegistry = prometheus_http_config:telemetry_registry(),

  Scrape = prometheus_summary:observe_duration(
             TelemetryRegistry,
             ?SCRAPE_DURATION,
             [Registry, ContentType],
             fun () -> Format:format(Registry) end),
  prometheus_summary:observe(TelemetryRegistry,
                             ?SCRAPE_SIZE,
                             [Registry, ContentType],
                             iolist_size(Scrape)),
  {ContentType, Scrape}.

encode_format(ContentType, Encoding, Scrape, Registry) ->
  Encoded = encode_format_(Encoding, Scrape),
  TelemetryRegistry = prometheus_http_config:telemetry_registry(),

  prometheus_summary:observe(TelemetryRegistry,
                             ?SCRAPE_ENCODED_SIZE,
                             [Registry, ContentType, Encoding],
                             iolist_size(Encoded)),
  {200, [{content_type, binary_to_list(ContentType)},
         {content_encoding, Encoding}], Encoded}.

encode_format_("gzip", Scrape) ->
  zlib:gzip(Scrape);
encode_format_("deflate", Scrape) ->
  ZStream = zlib:open(),
  zlib:deflateInit(ZStream),
  try
    zlib:deflate(ZStream, Scrape, finish)
  after
    zlib:deflateEnd(ZStream)
  end;
encode_format_("identity", Scrape) ->
  Scrape.