accept

%% @doc HTTP Accept header parser and content-type negotiation helper
-module(accept_header).

-export([parse/1,
         negotiate/2]).

-include("accept.hrl").

%%====================================================================
%% Public API
%%====================================================================

%% @doc
%% Parses Accept header, returns a list of media_ranges.
%%
%% <pre lang="erlang-repl">
%% accept_header:parse("text/*;q=0.3, text/html;q=0.7, text/html;level=1,"
%%                     "text/html;level=2;q=0.4, */*;q=0.5").
%% [{media_range,"text","*",0.3,[]},
%%  {media_range,"text","html",0.7,[]},
%%  {media_range,"text","html",1,[{"level","1"}]},
%%  {media_range,"text","html",0.4,[{"level","2"}]},
%%  {media_range,"*","*",0.5,[]}]
%% </pre>
%% @end
-spec parse(AcceptString) -> Result when
    AcceptString :: binary() | string (),
    Result :: [media_range()].
parse(AcceptString) ->
  accept_parser:map_options(fun parse_media_range/1, AcceptString).

%% @doc
%% Negotiates the most appropriate content_type given the accept header
%% and a list of alternatives.
%%
%% <pre lang="erlang-repl">
%% accept_header:negotiate("text/*;q=0.3, text/html;q=0.7, text/html;level=1,"
%%                         "text/html;level=2;q=0.4, */*;q=0.5",
%%                         ["text/html;level=2", "text/html;level-3"]).
%% "text/html;level-3"
%% </pre>
%% @end
-spec negotiate(Header, Alternatives) -> Match when
    Header :: BinaryOrString,
    Alternatives :: [Alternative],
    Alternative :: BinaryOrString | {BinaryOrString | Tag},
    BinaryOrString :: binary() | string(),
    Tag :: any(),
    Match :: Tag | nomatch.
negotiate(Header, Alternatives) ->

  MediaRanges = parse(Header),

  Alts = lists:map(fun (Alt) ->
                       {A, Tag} = accept_neg:alt_tag(Alt),

                       PA = parse_media_range(accept_parser:ensure_string(A)),
                       %% list of Alt-MR scores
                       AltMRScores = lists:map(fun (MR) ->
                                                   {score_alt(MR, PA), MR}
                                               end,
                                               MediaRanges),
                       %% best Media Range match for this Alternative
                       [{Score, BMR} | _] = accept_neg:sort_scored(AltMRScores),
                       case Score of
                         0 ->
                           {-1, Tag};
                         _ ->
                           #media_range{q = BMRQ} = BMR,
                           {BMRQ, Tag}
                       end
                   end,
                   Alternatives),

  {Q, Tag} = accept_neg:find_preferred_best(Alts),
  case Q of
    Q when Q =< 0 -> undefined;
    _ -> Tag
  end.

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

parse_media_range(#accept_option{option=Option,
                                 q=Q,
                                 params=Params}) ->

  [Type, Subtype] = lists:map(fun string:strip/1, string:tokens(Option, "/")),

  #media_range{type = Type,
               subtype = Subtype,
               q = Q,
               params = Params};
parse_media_range(String) ->
  parse_media_range(accept_parser:parse_option(String)).

%% Alternative "text/plain; version=4"
%% text/plain; version=4 > text/plan > text/plain; n=v > text/* > */* > image/*
score_alt(#media_range{type = Type,
                       subtype = SubType,
                       params = MRParams},
          #media_range{type = Type,
                       subtype = SubType,
                       params = AltParams}) ->
  8 + 4 + accept_neg:score_params(MRParams, AltParams);
score_alt(#media_range{type = Type,
                       subtype = "*"},
          #media_range{type = Type}) ->
  8 + 3;
score_alt(#media_range{type = "*"},
          _) ->
  8;
score_alt(_, _) ->
  0.