-module(follower_graph).
-export([fold/2, fold_fn/0, new/0, lookup/2, actors/1,
         following/2, followers/2,
         pending_outbound/2, pending_inbound/2,
         is_following/3, has_follower/3,
         is_pending_outbound/3, is_pending_inbound/3]).

%% Follower-graph projection — Erlang-fun stand-in for the genesis
%% `follower-graph.sx` body. Tracks per-actor follow relationships
%% per design §13.2:
%%
%%   Follow {actor: A, object: B}      A asks to follow B
%%   Accept {actor: B, object: F}      B accepts A's Follow F (= F.actor → F.object)
%%   Reject {actor: B, object: F}      B rejects A's Follow F
%%   Undo   {actor: A, object: F}      A retracts F or unfollows
%%
%% Where F = Follow{A→B} is embedded as the activity's :object
%% proplist for Accept / Reject / Undo.
%%
%% State shape:
%%   [{ActorId, ActorEntry}, ...]
%%
%% ActorEntry = [{following,        [PeerId, ...]},
%%               {followers,        [PeerId, ...]},
%%               {pending_outbound, [PeerId, ...]},  %% I asked, no answer yet
%%               {pending_inbound,  [PeerId, ...]}]  %% asked me, I haven't answered
%%
%% Sets keep insertion order; duplicates aren't added. lists:keyfind/
%% keymember aren't in this substrate, so local find_keyed/has_keyed/
%% set_keyed helpers (same convention as actor_state, define_registry,
%% nx_kernel).

%% ── Public API ──────────────────────────────────────────────────

new() -> [].

actors(State) -> [Id || {Id, _Entry} <- State].

lookup(ActorId, State) ->
    case find_keyed(ActorId, State) of
        {ok, Entry} -> {ok, Entry};
        _           -> not_found
    end.

following(ActorId, State)        -> entry_field(ActorId, following, State).
followers(ActorId, State)        -> entry_field(ActorId, followers, State).
pending_outbound(ActorId, State) -> entry_field(ActorId, pending_outbound, State).
pending_inbound(ActorId, State)  -> entry_field(ActorId, pending_inbound, State).

is_following(ActorId, PeerId, State) ->
    contains(PeerId, following(ActorId, State)).

has_follower(ActorId, PeerId, State) ->
    contains(PeerId, followers(ActorId, State)).

is_pending_outbound(ActorId, PeerId, State) ->
    contains(PeerId, pending_outbound(ActorId, State)).

is_pending_inbound(ActorId, PeerId, State) ->
    contains(PeerId, pending_inbound(ActorId, State)).

%% ── Fold dispatch ───────────────────────────────────────────────

fold(Activity, State) ->
    case envelope:get_field(type, Activity) of
        {ok, follow} -> fold_follow(Activity, State);
        {ok, accept} -> fold_accept(Activity, State);
        {ok, reject} -> fold_reject(Activity, State);
        {ok, undo}   -> fold_undo(Activity, State);
        _            -> State
    end.

fold_fn() ->
    fun (Activity, State) -> fold(Activity, State) end.

%% Follow {actor: A, object: B}:
%%   add B to A's pending_outbound
%%   add A to B's pending_inbound
fold_follow(Activity, State) ->
    case follow_actor_object(Activity) of
        {ok, A, B} when A =/= B ->
            S1 = add_to_field(A, pending_outbound, B, State),
            add_to_field(B, pending_inbound, A, S1);
        _ -> State
    end.

%% Accept {actor: B, object: Follow{A→B}}:
%%   move A from B's pending_inbound to B's followers
%%   move B from A's pending_outbound to A's following
fold_accept(Activity, State) ->
    case nested_follow_actor_object(Activity) of
        {ok, B, A, OrigA, OrigB} when B =:= OrigB, A =:= OrigA, A =/= B ->
            S1 = move_field(B, pending_inbound, followers, A, State),
            move_field(A, pending_outbound, following, B, S1);
        _ -> State
    end.

%% Reject {actor: B, object: Follow{A→B}}:
%%   drop A from B's pending_inbound
%%   drop B from A's pending_outbound
fold_reject(Activity, State) ->
    case nested_follow_actor_object(Activity) of
        {ok, B, A, OrigA, OrigB} when B =:= OrigB, A =:= OrigA, A =/= B ->
            S1 = drop_from_field(B, pending_inbound, A, State),
            drop_from_field(A, pending_outbound, B, S1);
        _ -> State
    end.

%% Undo {actor: X, object: Follow{A→B}}:
%%   Only the original Follow's actor (A) can Undo it.
%%   Drops A↔B from every list on either side.
fold_undo(Activity, State) ->
    case nested_follow_actor_object(Activity) of
        {ok, X, OrigA, OrigA, OrigB} when X =:= OrigA, OrigA =/= OrigB ->
            S1 = drop_from_field(OrigA, following,        OrigB, State),
            S2 = drop_from_field(OrigA, pending_outbound, OrigB, S1),
            S3 = drop_from_field(OrigB, followers,        OrigA, S2),
            drop_from_field(OrigB, pending_inbound, OrigA, S3);
        _ -> State
    end.

%% ── Extraction helpers ─────────────────────────────────────────

follow_actor_object(Activity) ->
    case envelope:get_field(actor, Activity) of
        {ok, A} ->
            case envelope:get_field(object, Activity) of
                {ok, B} when is_atom(B) -> {ok, A, B};
                _ -> not_follow
            end;
        _ -> not_follow
    end.

%% nested_follow_actor_object/1 — pull (Actor, FollowActor, FollowObject)
%% out of an envelope whose :object is itself a Follow proplist.
%% Returns {ok, OuterActor, InferredPeer, InnerActor, InnerObject}.
nested_follow_actor_object(Activity) ->
    case envelope:get_field(actor, Activity) of
        {ok, Outer} ->
            case envelope:get_field(object, Activity) of
                {ok, Inner} when is_list(Inner) ->
                    case nested_is_follow(Inner) of
                        true  ->
                            case {envelope:get_field(actor, Inner),
                                  envelope:get_field(object, Inner)} of
                                {{ok, IA}, {ok, IO}} when is_atom(IO) ->
                                    {ok, Outer, peer_from_inner(Outer, IA, IO), IA, IO};
                                _ -> not_a_follow_wrapper
                            end;
                        false -> not_a_follow_wrapper
                    end;
                _ -> not_a_follow_wrapper
            end;
        _ -> not_a_follow_wrapper
    end.

nested_is_follow(Inner) ->
    case envelope:get_field(type, Inner) of
        {ok, follow} -> true;
        _ -> false
    end.

%% peer_from_inner — for an Accept/Reject by B of Follow{A→B},
%% Outer = B; the "peer" we move state for is A. For an Undo by A,
%% Outer = A; the peer is B. Picking the inner actor/object that
%% isn't Outer gives us the right pair-mate.
peer_from_inner(Outer, IA, _IO) when Outer =:= IA -> IA;
peer_from_inner(_Outer, IA, _IO) -> IA.

%% ── Entry / field accessors ────────────────────────────────────

entry_field(ActorId, Field, State) ->
    case find_keyed(ActorId, State) of
        {ok, Entry} ->
            case find_keyed(Field, Entry) of
                {ok, Val} -> Val;
                _ -> []
            end;
        _ -> []
    end.

empty_entry() ->
    [{following, []},
     {followers, []},
     {pending_outbound, []},
     {pending_inbound, []}].

ensure_entry(ActorId, State) ->
    case find_keyed(ActorId, State) of
        {ok, _} -> State;
        _       -> State ++ [{ActorId, empty_entry()}]
    end.

add_to_field(ActorId, Field, PeerId, State) ->
    S1 = ensure_entry(ActorId, State),
    {ok, Entry} = find_keyed(ActorId, S1),
    Current = entry_field(ActorId, Field, S1),
    NewList = case contains(PeerId, Current) of
        true  -> Current;
        false -> Current ++ [PeerId]
    end,
    NewEntry = set_keyed(Field, NewList, Entry),
    set_keyed(ActorId, NewEntry, S1).

drop_from_field(ActorId, Field, PeerId, State) ->
    case find_keyed(ActorId, State) of
        {ok, Entry} ->
            Current = entry_field(ActorId, Field, State),
            NewList = remove_member(PeerId, Current),
            NewEntry = set_keyed(Field, NewList, Entry),
            set_keyed(ActorId, NewEntry, State);
        _ -> State
    end.

move_field(ActorId, FromField, ToField, PeerId, State) ->
    S1 = drop_from_field(ActorId, FromField, PeerId, State),
    add_to_field(ActorId, ToField, PeerId, S1).

%% ── List helpers ───────────────────────────────────────────────

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

remove_member(_, []) -> [];
remove_member(X, [X | Rest]) -> remove_member(X, Rest);
remove_member(X, [Y | Rest]) -> [Y | remove_member(X, Rest)].

%% ── Keyed-list helpers ─────────────────────────────────────────

find_keyed(_, []) -> {error, not_found};
find_keyed(K, [{K, V} | _]) -> {ok, V};
find_keyed(K, [_ | Rest]) -> find_keyed(K, Rest).

set_keyed(K, V, []) -> [{K, V}];
set_keyed(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
set_keyed(K, V, [P | Rest]) -> [P | set_keyed(K, V, Rest)].