-module(peer_actors).
-export([new/0, lookup/2, store/3, evict/2, peers/1,
         lookup_or_fetch/3,
         start_link/0, start_link/1, stop/0,
         lookup_srv/1, store_srv/2, lookup_or_fetch_srv/2,
         peers_srv/0, evict_srv/1]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2]).
-behaviour(gen_server).

%% Peer-actors cache. On first inbound from a new peer, the
%% federation layer needs the peer's `:public_keys` (and eventually
%% other actor-doc fields) to verify the inbound signature. Fetching
%% the peer's actor doc on every inbound would be wasteful, so we
%% cache the peer-AS keyed by ActorId atom. Per design §13.6 stale-
%% key invalidation defers to v3 — for v2 entries are TTL-free.
%%
%% State shape (pure-functional):
%%   [{PeerActorId, PeerActorState}, ...]
%%
%% PeerActorState is the same shape that envelope:verify_signature/2
%% reads — a proplist with :public_keys (a list of key proplists).
%%
%% lookup_or_fetch/3 is the load-bearing entry point: a miss invokes
%% the caller-supplied FetchFn (1-arity, takes PeerActorId, returns
%% {ok, PeerAS} | {error, Reason}). The cache stores successful
%% fetches; errors do NOT poison the cache so the caller can retry.
%%
%% gen_server wrapper exposes the same API for the http inbox
%% handler. Tests inline start_link with operations (same port quirks
%% as registry / projection / nx_kernel).

%% ── Pure-functional API ─────────────────────────────────────────

new() -> [].

lookup(PeerId, State) ->
    case find_keyed(PeerId, State) of
        {ok, PeerAS} -> {ok, PeerAS};
        {error, _}   -> not_found
    end.

store(PeerId, PeerAS, State) ->
    set_keyed(PeerId, PeerAS, State).

evict(PeerId, State) ->
    delete_keyed(PeerId, State).

peers(State) -> [Id || {Id, _AS} <- State].

%% lookup_or_fetch/3 — cache hit returns {ok, PeerAS, State}
%% unchanged. Cache miss calls FetchFn; success path stores and
%% returns {ok, PeerAS, NewState}; failure returns {error, Reason,
%% State} so the caller knows the cache state and can retry on
%% transient errors.

lookup_or_fetch(PeerId, FetchFn, State) ->
    case find_keyed(PeerId, State) of
        {ok, PeerAS} -> {ok, PeerAS, State};
        {error, _}   ->
            case FetchFn(PeerId) of
                {ok, PeerAS}    -> {ok, PeerAS, store(PeerId, PeerAS, State)};
                {error, Reason} -> {error, Reason, State};
                Other           -> {error, {bad_fetch_return, Other}, State}
            end
    end.

%% ── gen_server wrapper ──────────────────────────────────────────
%%
%% Mirrors registry / projection / nx_kernel patterns. Registered
%% name `peer_actors` so callers (http_server inbox handler) can
%% find it without threading the Pid through Cfg.

start_link() ->
    start_link([]).

start_link(InitialState) ->
    Pid = gen_server:start_link(peer_actors, [InitialState]),
    erlang:register(peer_actors, Pid),
    Pid.

stop() ->
    R = gen_server:call(peer_actors, '$gen_stop'),
    erlang:unregister(peer_actors),
    R.

lookup_srv(PeerId) ->
    gen_server:call(peer_actors, {lookup, PeerId}).

store_srv(PeerId, PeerAS) ->
    gen_server:call(peer_actors, {store, PeerId, PeerAS}).

%% lookup_or_fetch_srv/2 — same shape as the pure form. FetchFn must
%% be a 1-arity fun. Reply is {ok, PeerAS} on hit-or-fetched,
%% {error, Reason} on fetch failure.

lookup_or_fetch_srv(PeerId, FetchFn) ->
    gen_server:call(peer_actors, {lookup_or_fetch, PeerId, FetchFn}).

peers_srv() ->
    gen_server:call(peer_actors, get_peers).

evict_srv(PeerId) ->
    gen_server:call(peer_actors, {evict, PeerId}).

%% gen_server callbacks

init([InitialState]) ->
    {ok, InitialState}.

handle_call({lookup, PeerId}, _From, State) ->
    {reply, lookup(PeerId, State), State};
handle_call({store, PeerId, PeerAS}, _From, State) ->
    {reply, ok, store(PeerId, PeerAS, State)};
handle_call({lookup_or_fetch, PeerId, FetchFn}, _From, State) ->
    case lookup_or_fetch(PeerId, FetchFn, State) of
        {ok, PeerAS, NewState}      -> {reply, {ok, PeerAS}, NewState};
        {error, Reason, SameState}  -> {reply, {error, Reason}, SameState}
    end;
handle_call(get_peers, _From, State) ->
    {reply, peers(State), State};
handle_call({evict, PeerId}, _From, State) ->
    {reply, ok, evict(PeerId, State)}.

handle_cast(_, S) -> {noreply, S}.

handle_info(_, S) -> {noreply, S}.

%% ── Internal 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)].

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