-module(backfill).
-export([slice/2, slice/3,
         wrap_backfill/1, parse_mode/1,
         all_entries/1, last_n_entries/2, last_t_entries/3,
         since_cid_entries/2, none_entries/0]).

%% Backfill mode slicing per design §13.3 / Step 9. When A follows B
%% with a backfill spec, B's kernel slices the outbox log into the
%% appropriate window and delivers each entry as
%% `{backfilled, true}`-marked envelopes alongside forward-going
%% activity.
%%
%% Mode shapes (per the Follow activity's `:backfill` field):
%%   none                   — newer follower sees only forward content
%%   {last_n, N}            — backfill last N activities (FIFO order)
%%   {last_t, T, NowFn}     — backfill activities with :published in
%%                            (Now - T .. Now]. NowFn is a 0-arity fun
%%                            so tests can fake-time it.
%%   full                   — backfill the entire outbox
%%
%% slice/2 returns the activity list. slice/3 also wraps each entry
%% with `{backfilled, true}` so projections can decide whether to
%% re-fold or skip (the §13.3 Backfilled bodies preserve the
%% original `:id` so replay defence still works on the receiver).
%%
%% parse_mode/1 lifts the Follow activity's `:backfill` proplist
%% (or atom) into the internal mode tuple. Unknown shapes fall back
%% to `none` — the default open-world policy.

slice(Mode, LogState) ->
    slice(Mode, LogState, false).

slice(Mode, LogState, Wrap) ->
    Entries = log:entries(LogState),
    Slice = case Mode of
        none                  -> none_entries();
        full                  -> all_entries(Entries);
        {last_n, N}           -> last_n_entries(N, Entries);
        {last_t, T, NowFn}    -> last_t_entries(T, NowFn, Entries);
        {since_cid, Cid}      -> since_cid_entries(Cid, Entries);
        _                     -> none_entries()
    end,
    case Wrap of
        true  -> wrap_backfill(Slice);
        _     -> Slice
    end.

%% ── Mode-specific entry selection ─────────────────────────────

all_entries(Entries) -> Entries.

none_entries() -> [].

%% last_n_entries/2 — tail N entries in FIFO order.

last_n_entries(N, _) when N =< 0 -> [];
last_n_entries(N, Entries) ->
    Len = length(Entries),
    case Len =< N of
        true  -> Entries;
        false -> drop_n(Len - N, Entries)
    end.

drop_n(0, L) -> L;
drop_n(_, []) -> [];
drop_n(N, [_ | Rest]) -> drop_n(N - 1, Rest).

%% last_t_entries/3 — entries whose :published is within the last
%% T units of (NowFn() - T .. NowFn()]. T and :published are
%% integers (seconds-since-epoch in production; opaque ints in tests).

last_t_entries(T, NowFn, Entries) when is_integer(T), T >= 0 ->
    Now = NowFn(),
    Cutoff = Now - T,
    [E || E <- Entries, in_window(E, Cutoff, Now)];
last_t_entries(_, _, _) -> [].

in_window(Activity, Cutoff, Now) ->
    case envelope:get_field(published, Activity) of
        {ok, P} when is_integer(P), P > Cutoff, P =< Now -> true;
        _ -> false
    end.

%% since_cid_entries/2 — every entry after the one with :id = Cid.
%% If Cid isn't in the log, returns [] (caller's pointer is stale).
%% Used by `GET /actors/<id>/outbox?since=Cid` pagination.

since_cid_entries(_Cid, []) -> [];
since_cid_entries(Cid, [E | Rest]) ->
    case envelope:get_field(id, E) of
        {ok, Cid} -> Rest;
        _         -> since_cid_entries(Cid, Rest)
    end.

%% wrap_backfill/1 — append `{backfilled, true}` to each entry.
%% The receiving projection scheduler reads this field and chooses
%% whether to fold (re-emit) or skip (already known via replay
%% defence on `:id`).

wrap_backfill([]) -> [];
wrap_backfill([E | Rest]) ->
    [E ++ [{backfilled, true}] | wrap_backfill(Rest)].

%% parse_mode/1 — Lift a Follow activity's `:backfill` value into the
%% internal mode tuple. Accepts:
%%   nil / not_found       -> none
%%   none                  -> none
%%   full                  -> full
%%   {last_n, N}           -> {last_n, N}     (already-parsed shape)
%%   {last_t, T, NowFn}    -> pass-through
%%   Proplist with :mode + :limit / :duration -> parsed
%% Unknown shape -> none (open-world default).

parse_mode(nil)                  -> none;
parse_mode(none)                 -> none;
parse_mode(full)                 -> full;
parse_mode({last_n, N})          -> {last_n, N};
parse_mode({last_t, T, NowFn})   -> {last_t, T, NowFn};
parse_mode({since_cid, Cid})     -> {since_cid, Cid};
parse_mode(List) when is_list(List) ->
    case envelope:get_field(mode, List) of
        {ok, last_n} ->
            case envelope:get_field(limit, List) of
                {ok, N} when is_integer(N) -> {last_n, N};
                _ -> none
            end;
        {ok, last_t} ->
            case envelope:get_field(duration, List) of
                {ok, T} when is_integer(T) -> {last_t, T, fun () -> 0 end};
                _ -> none
            end;
        {ok, full} -> full;
        {ok, none} -> none;
        _ -> none
    end;
parse_mode(_) -> none.