-module(lager_gelf_formatter).

-include_lib("lager/include/lager.hrl").

-export([format/2, format/3]).

-spec format(lager_msg:lager_msg(),list()) -> any().
format(Msg,[]) ->
    MetaData = lager_msg:metadata(Msg),
    Keys = proplists:get_keys(MetaData),
    format(Msg, lists:append([version, host, message, timestamp, level], Keys));
format(Message,[{metadata, AdditionalMetaData} | Rest]) ->
    Destinations = lager_msg:destinations(Message),
    Metadata = lists:append(lager_msg:metadata(Message), AdditionalMetaData),
    Severity = lager_msg:severity(Message),
    Timestamp = lager_msg:timestamp(Message),
    Content = lager_msg:message(Message),
    NewMessage = lager_msg:new(Content, Timestamp, Severity, Metadata, Destinations),
    format(NewMessage, Rest);
format(Message,Config) ->
    lists:flatten("{" ++ string:join([output(V,Message) || V <- Config], ", ") ++ "}").

-spec format(lager_msg:lager_msg(),list(), list()) -> any().
format(Msg, Config, _Color) ->
    format(Msg, Config).

-spec output(term(),lager_msg:lager_msg()) -> iolist().
output(message,Msg) -> "\"short_message\": \"" ++ re:replace(lager_msg:message(Msg), "\"", "\\\\\\\"", [global, {return, list}]) ++ "\"";
output(version,_Msg) -> "\"version\": \"1.1\"";
output(host,_Msg) ->
    {ok, Host} = inet:gethostname(),
    property("host", Host);
output(timestamp,Msg) ->
    {D, T} = lager_msg:datetime(Msg),
    property("timestamp", D ++ "T" ++ T);
output(level,Msg) ->
    property("level", integer_to_list(lager_msg:severity_as_int(Msg)));
output(Prop,Msg) when is_atom(Prop) ->
    Metadata = lager_msg:metadata(Msg),
    Key = make_printable(Prop),
    Value = make_printable(get_metadata(Prop,Metadata,"Undefined")),
    property(Key, Value);
output({Prop,Default},Msg) when is_atom(Prop) ->
    Metadata = lager_msg:metadata(Msg),
    Key = make_printable(Prop),
    Value = make_printable(get_metadata(Prop,Metadata,output(Default,Msg))),
    property(Key, Value);
output(Other,_) -> make_printable(Other).

%% create string '"key": "value"' from key and value
-spec property(string(), string()) -> string().
property(Key, Value) ->
    "\"" ++ Key ++ "\": \"" ++ Value ++ "\"".

-spec make_printable(any()) -> iolist().
make_printable(A) when is_atom(A) -> atom_to_list(A);
make_printable(P) when is_pid(P) -> pid_to_list(P);
make_printable(L) when is_list(L) orelse is_binary(L) -> L;
make_printable(Other) -> io_lib:format("~p",[Other]).

get_metadata(Key, Metadata, Default) ->
    case lists:keyfind(Key, 1, Metadata) of
        false ->
            Default;
        {Key, Value} ->
            Value
    end.