-module(delivery).
-export([delivery_set/2, delivery_set/3,
         collect_recipients/1, suppress_self/2, dedup/1,
         expand_audience/3]).

%% Audience-resolving delivery set computation per design §13.4.
%%
%% delivery_set/2(Activity, KernelState) returns a sorted, deduped
%% list of ActorId atoms — every actor the outgoing Activity needs
%% to be POSTed to. Sources:
%%   - Activity's `:to` field   (single ActorId or list)
%%   - Activity's `:cc` field   (single ActorId or list)
%%   - audience-symbol expansion of `public` and `followers`
%%
%% Self-delivery (the publishing actor reading their own activity
%% on a peer's behalf) is suppressed.
%%
%% Output for Step 7a is the bare ActorId list; Step 8 will resolve
%% each entry to `{PeerInstanceUrl, ActorId}` via the peer-actors
%% cache.

delivery_set(Activity, KernelState) ->
    delivery_set(Activity, KernelState, follower_graph:new()).

delivery_set(Activity, KernelState, FollowerGraph) ->
    Self = sender(Activity),
    Raw = collect_recipients(Activity),
    Expanded = expand_all(Raw, Self, KernelState, FollowerGraph),
    Suppressed = suppress_self(Expanded, Self),
    dedup(Suppressed).

%% collect_recipients/1 — flat list from :to + :cc, normalised so
%% each element is either an ActorId atom or an audience symbol
%% (`public` / `followers`).

collect_recipients(Activity) ->
    To = envelope_field_list(to, Activity),
    Cc = envelope_field_list(cc, Activity),
    To ++ Cc.

envelope_field_list(Field, Activity) ->
    case envelope:get_field(Field, Activity) of
        not_found -> [];
        {ok, V} when is_list(V) -> V;
        {ok, V} -> [V]
    end.

%% expand_audience/3 — `followers` -> the sender's followers
%% proplist entry from a follower_graph state. `public` for v2
%% expands to the same list (per design §13.4: practical Public
%% fan-out is "every follower of the publishing actor"). The
%% explicit shared-inbox peer-instance model defers to v3.
%% Other symbols / explicit ActorIds pass through unchanged.

expand_audience(public, Sender, Graph) ->
    follower_graph:followers(Sender, Graph);
expand_audience(followers, Sender, Graph) ->
    follower_graph:followers(Sender, Graph);
expand_audience(X, _Sender, _Graph) -> [X].

expand_all([], _Self, _State, _Graph) -> [];
expand_all([X | Rest], Self, State, Graph) ->
    expand_audience(X, Self, Graph) ++ expand_all(Rest, Self, State, Graph).

suppress_self([], _Self) -> [];
suppress_self([Self | Rest], Self) -> suppress_self(Rest, Self);
suppress_self([X | Rest], Self) -> [X | suppress_self(Rest, Self)].

dedup(L) -> dedup_acc(L, []).

dedup_acc([], Acc) -> Acc;
dedup_acc([X | Rest], Acc) ->
    case contains(X, Acc) of
        true  -> dedup_acc(Rest, Acc);
        false -> dedup_acc(Rest, Acc ++ [X])
    end.

contains(_, []) -> false;
contains(X, [X | _]) -> true;
contains(X, [_ | Rest]) -> contains(X, Rest).

sender(Activity) ->
    case envelope:get_field(actor, Activity) of
        {ok, A} -> A;
        _ -> nil
    end.