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fluxer_gateway/src/guild/guild_subscriptions.erl

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+%% Copyright (C) 2026 Fluxer Contributors
+%%
+%% This file is part of Fluxer.
+%%
+%% Fluxer is free software: you can redistribute it and/or modify
+%% it under the terms of the GNU Affero General Public License as published by
+%% the Free Software Foundation, either version 3 of the License, or
+%% (at your option) any later version.
+%%
+%% Fluxer is distributed in the hope that it will be useful,
+%% but WITHOUT ANY WARRANTY; without even the implied warranty of
+%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+%% GNU Affero General Public License for more details.
+%%
+%% You should have received a copy of the GNU Affero General Public License
+%% along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+
+-module(guild_subscriptions).
+
+-export([
+    init_state/0,
+    subscribe/3,
+    unsubscribe/3,
+    unsubscribe_session/2,
+    update_subscriptions/3,
+    get_subscribed_sessions/2,
+    is_subscribed/3,
+    get_user_ids_for_session/2
+]).
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+-endif.
+
+-type session_id() :: binary().
+-type user_id() :: integer().
+-type subscription_state() :: #{user_id() => sets:set(session_id())}.
+
+-spec init_state() -> subscription_state().
+init_state() -> #{}.
+
+-spec subscribe(session_id(), user_id(), subscription_state()) -> subscription_state().
+subscribe(SessionId, UserId, State) ->
+    Subscribers = maps:get(UserId, State, sets:new()),
+    NewSubscribers = sets:add_element(SessionId, Subscribers),
+    maps:put(UserId, NewSubscribers, State).
+
+-spec unsubscribe(session_id(), user_id(), subscription_state()) -> subscription_state().
+unsubscribe(SessionId, UserId, State) ->
+    case maps:get(UserId, State, undefined) of
+        undefined ->
+            State;
+        Subscribers ->
+            NewSubscribers = sets:del_element(SessionId, Subscribers),
+            case sets:size(NewSubscribers) of
+                0 -> maps:remove(UserId, State);
+                _ -> maps:put(UserId, NewSubscribers, State)
+            end
+    end.
+
+-spec unsubscribe_session(session_id(), subscription_state()) -> subscription_state().
+unsubscribe_session(SessionId, State) ->
+    maps:fold(
+        fun(UserId, Subscribers, Acc) ->
+            NewSubscribers = sets:del_element(SessionId, Subscribers),
+            case sets:size(NewSubscribers) of
+                0 -> Acc;
+                _ -> maps:put(UserId, NewSubscribers, Acc)
+            end
+        end,
+        #{},
+        State
+    ).
+
+-spec update_subscriptions(session_id(), [user_id()], subscription_state()) ->
+    subscription_state().
+update_subscriptions(SessionId, NewMemberIds, State) ->
+    CurrentlySubscribed = get_user_ids_for_session(SessionId, State),
+    NewMemberIdSet = sets:from_list(NewMemberIds),
+
+    ToRemove = sets:subtract(CurrentlySubscribed, NewMemberIdSet),
+    ToAdd = sets:subtract(NewMemberIdSet, CurrentlySubscribed),
+
+    State1 = sets:fold(
+        fun(UserId, AccState) ->
+            unsubscribe(SessionId, UserId, AccState)
+        end,
+        State,
+        ToRemove
+    ),
+
+    sets:fold(
+        fun(UserId, AccState) ->
+            subscribe(SessionId, UserId, AccState)
+        end,
+        State1,
+        ToAdd
+    ).
+
+-spec get_subscribed_sessions(user_id(), subscription_state()) -> [session_id()].
+get_subscribed_sessions(UserId, State) ->
+    case maps:get(UserId, State, undefined) of
+        undefined -> [];
+        Subscribers -> sets:to_list(Subscribers)
+    end.
+
+-spec is_subscribed(session_id(), user_id(), subscription_state()) -> boolean().
+is_subscribed(SessionId, UserId, State) ->
+    case maps:get(UserId, State, undefined) of
+        undefined -> false;
+        Subscribers -> sets:is_element(SessionId, Subscribers)
+    end.
+
+-spec get_user_ids_for_session(session_id(), subscription_state()) -> sets:set(user_id()).
+get_user_ids_for_session(SessionId, State) ->
+    maps:fold(
+        fun(UserId, Subscribers, Acc) ->
+            case sets:is_element(SessionId, Subscribers) of
+                true -> sets:add_element(UserId, Acc);
+                false -> Acc
+            end
+        end,
+        sets:new(),
+        State
+    ).
+
+-ifdef(TEST).
+
+init_state_test() ->
+    ?assertEqual(#{}, init_state()).
+
+subscribe_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    ?assert(is_subscribed(<<"session1">>, 123, State1)),
+    ?assertNot(is_subscribed(<<"session2">>, 123, State1)).
+
+subscribe_multiple_sessions_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    State2 = subscribe(<<"session2">>, 123, State1),
+    ?assert(is_subscribed(<<"session1">>, 123, State2)),
+    ?assert(is_subscribed(<<"session2">>, 123, State2)).
+
+unsubscribe_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    State2 = unsubscribe(<<"session1">>, 123, State1),
+    ?assertNot(is_subscribed(<<"session1">>, 123, State2)).
+
+unsubscribe_one_of_many_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    State2 = subscribe(<<"session2">>, 123, State1),
+    State3 = unsubscribe(<<"session1">>, 123, State2),
+    ?assertNot(is_subscribed(<<"session1">>, 123, State3)),
+    ?assert(is_subscribed(<<"session2">>, 123, State3)).
+
+unsubscribe_session_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    State2 = subscribe(<<"session1">>, 456, State1),
+    State3 = subscribe(<<"session2">>, 123, State2),
+    State4 = unsubscribe_session(<<"session1">>, State3),
+    ?assertNot(is_subscribed(<<"session1">>, 123, State4)),
+    ?assertNot(is_subscribed(<<"session1">>, 456, State4)),
+    ?assert(is_subscribed(<<"session2">>, 123, State4)).
+
+get_subscribed_sessions_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 123, State0),
+    State2 = subscribe(<<"session2">>, 123, State1),
+    Sessions = lists:sort(get_subscribed_sessions(123, State2)),
+    ?assertEqual([<<"session1">>, <<"session2">>], Sessions).
+
+update_subscriptions_test() ->
+    State0 = init_state(),
+    State1 = subscribe(<<"session1">>, 100, State0),
+    State2 = subscribe(<<"session1">>, 200, State1),
+    State3 = update_subscriptions(<<"session1">>, [200, 300], State2),
+    ?assertNot(is_subscribed(<<"session1">>, 100, State3)),
+    ?assert(is_subscribed(<<"session1">>, 200, State3)),
+    ?assert(is_subscribed(<<"session1">>, 300, State3)).
+
+-endif.