-module(delivery_state).
-export([new/0, fold/2, fold_fn/0,
         peer_state/2, peers/1,
         pending/2, attempts/2, next_retry/2, dead_letter/2]).

%% Delivery-state projection. Folds delivery events (enqueue /
%% delivered / failed / dead_lettered) into a per-peer worker-shaped
%% snapshot so the outbound queue survives kernel restart. Per design
%% §13.4 the worker state on restart is loaded from this projection
%% rather than reconstructed by re-driving the outbox log.
%%
%% Event proplist shape:
%%   [{type, enqueued},      {peer, _}, {activity, _}]
%%   [{type, delivered},     {peer, _}, {cid, _}]
%%   [{type, failed},        {peer, _}, {cid, _}, {now, _}]
%%   [{type, dead_lettered}, {peer, _}, {cid, _}]
%%
%% Projection state shape:
%%   [{PeerId, WorkerProplist}, ...]
%%
%% WorkerProplist mirrors `delivery_worker:new/1`'s output so a fresh
%% gen_server can be hydrated with `delivery_worker:state_from_proj`
%% (lands when 8b-timer wires up). For Step 8c the projection only
%% tracks data — Step 8d-restart will wire the hydration helper.

new() -> [].

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

fold(Event, State) ->
    case envelope:get_field(type, Event) of
        {ok, enqueued}      -> fold_enqueued(Event, State);
        {ok, delivered}     -> fold_delivered(Event, State);
        {ok, failed}        -> fold_failed(Event, State);
        {ok, dead_lettered} -> fold_dead_lettered(Event, State);
        _                   -> State
    end.

fold_enqueued(Event, State) ->
    case {envelope:get_field(peer, Event),
          envelope:get_field(activity, Event)} of
        {{ok, Peer}, {ok, Act}} ->
            Worker = ensure_peer(Peer, State),
            Pending = field(pending, Worker),
            Worker1 = set_field(pending, Pending ++ [Act], Worker),
            set_peer(Peer, Worker1, State);
        _ -> State
    end.

fold_delivered(Event, State) ->
    case {envelope:get_field(peer, Event),
          envelope:get_field(cid, Event)} of
        {{ok, Peer}, {ok, Cid}} ->
            case find_keyed(Peer, State) of
                {ok, Worker} ->
                    Worker1 = drop_pending_by_cid(Cid, Worker),
                    Worker2 = clear_retry_for(Cid, Worker1),
                    set_peer(Peer, Worker2, State);
                _ -> State
            end;
        _ -> State
    end.

fold_failed(Event, State) ->
    case {envelope:get_field(peer, Event),
          envelope:get_field(cid, Event),
          envelope:get_field(now, Event)} of
        {{ok, Peer}, {ok, Cid}, {ok, Now}} ->
            case find_keyed(Peer, State) of
                {ok, Worker} ->
                    Attempts = field(attempts, Worker),
                    Current = case find_keyed(Cid, Attempts) of
                        {ok, N} -> N;
                        _ -> 0
                    end,
                    New = Current + 1,
                    Attempts1 = set_keyed(Cid, New, Attempts),
                    Worker1 = set_field(attempts, Attempts1, Worker),
                    Worker2 = case delivery_worker:backoff_for(New) of
                        dead_letter ->
                            dead_letter_pending(Cid, Worker1);
                        Seconds ->
                            NR = field(next_retry, Worker1),
                            NextAt = Now + Seconds,
                            set_field(next_retry, set_keyed(Cid, NextAt, NR), Worker1)
                    end,
                    set_peer(Peer, Worker2, State);
                _ -> State
            end;
        _ -> State
    end.

fold_dead_lettered(Event, State) ->
    case {envelope:get_field(peer, Event),
          envelope:get_field(cid, Event)} of
        {{ok, Peer}, {ok, Cid}} ->
            case find_keyed(Peer, State) of
                {ok, Worker} ->
                    set_peer(Peer, dead_letter_pending(Cid, Worker), State);
                _ -> State
            end;
        _ -> State
    end.

%% ── Accessors ─────────────────────────────────────────────────

peer_state(Peer, State) ->
    case find_keyed(Peer, State) of
        {ok, Worker} -> {ok, Worker};
        _            -> not_found
    end.

peers(State) -> [P || {P, _} <- State].

pending(Peer, State) ->
    worker_field(Peer, pending, State, []).

attempts(Peer, State) ->
    worker_field(Peer, attempts, State, []).

next_retry(Peer, State) ->
    worker_field(Peer, next_retry, State, []).

dead_letter(Peer, State) ->
    worker_field(Peer, dead_letter, State, []).

%% ── Internal ──────────────────────────────────────────────────

worker_field(Peer, Field, State, Default) ->
    case find_keyed(Peer, State) of
        {ok, Worker} ->
            case find_keyed(Field, Worker) of
                {ok, V} -> V;
                _       -> Default
            end;
        _ -> Default
    end.

ensure_peer(Peer, State) ->
    case find_keyed(Peer, State) of
        {ok, Worker} -> Worker;
        _            -> empty_worker(Peer)
    end.

empty_worker(Peer) ->
    [{peer, Peer},
     {pending, []},
     {attempts, []},
     {next_retry, []},
     {dead_letter, []}].

set_peer(Peer, Worker, State) ->
    set_keyed(Peer, Worker, State).

drop_pending_by_cid(Cid, Worker) ->
    Pending = field(pending, Worker),
    Kept = [A || A <- Pending, activity_cid(A) =/= Cid],
    set_field(pending, Kept, Worker).

clear_retry_for(Cid, Worker) ->
    A1 = del_keyed(Cid, field(attempts, Worker)),
    NR1 = del_keyed(Cid, field(next_retry, Worker)),
    set_field(attempts, A1, set_field(next_retry, NR1, Worker)).

dead_letter_pending(Cid, Worker) ->
    Pending = field(pending, Worker),
    {Match, Rest} = split_by_cid(Cid, Pending),
    DL = field(dead_letter, Worker),
    Worker1 = set_field(pending, Rest, Worker),
    Worker2 = case Match of
        none -> Worker1;
        Act  -> set_field(dead_letter, DL ++ [Act], Worker1)
    end,
    clear_retry_for(Cid, Worker2).

split_by_cid(Cid, List) -> split_by_cid(Cid, List, []).
split_by_cid(_, [], Acc) -> {none, lists:reverse(Acc)};
split_by_cid(Cid, [A | Rest], Acc) ->
    case activity_cid(A) of
        Cid -> {A, lists:reverse(Acc) ++ Rest};
        _   -> split_by_cid(Cid, Rest, [A | Acc])
    end.

activity_cid(Activity) ->
    case envelope:get_field(id, Activity) of
        {ok, Cid} -> Cid;
        _         -> nil
    end.

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

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

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)].

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