%%% @doc DCB conditional-append primitive.
%%%
%%% Implements append_if_no_tag_matches/4 as a khepri:transaction/2
%%% body. The transaction:
%%%
%%%   1. Verifies the tag-filter context ({context_changed, _} on
%%%      conflict).
%%%   2. For integrity-enabled stores: verifies the seq counter and
%%%      chain-tip still match what the caller observed when
%%%      pre-stamping MACs. On mismatch aborts with
%%%      {dcb_state_changed, _} and the outer loop retries with a
%%%      fresh snapshot.
%%%   3. Writes events under ?DCB_STREAM_PATH ++ [SeqKey] plus one
%%%      tag-index entry per tag at ?BY_TAG_PATH ++ [Tag, SeqKey].
%%%   4. Updates the seq counter (and chain-tip if integrity is
%%%      enabled).
%%%
%%% The whole sequence happens inside one Ra log entry; atomic across
%%% the cluster. Either everything commits, or an abort comes back
%%% and nothing changed.
%%%
%%% == Integrity (reckon-db 3.2.0+) ==
%%%
%%% On integrity-enabled stores, DCB events carry prev_event_hash +
%%% mac like any other integrity-bearing event. Because Khepri's
%%% Horus extractor rejects code containing crypto:* calls (even on
%%% unreachable branches; extraction sees the whole function), MAC
%%% chains are pre-computed OUTSIDE the transaction. Inside the
%%% transaction we verify the chain-tip + counter are still what we
%%% saw, then write the pre-stamped records.
%%%
%%% Two retry vectors:
%%%   - {context_changed, _}: tag-filter conflict. Callers (e.g.,
%%%     evoq_decision_runtime) retry with fresh context.
%%%   - {dcb_state_changed, _}: counter or chain-tip moved between
%%%     our pre-stamp read and the transaction. The retry loop in this
%%%     module handles it transparently, bounded by
%%%     ?INTEGRITY_RETRY_BUDGET.
%%% @end
-module(reckon_db_dcb).

-include("reckon_db.hrl").
-include_lib("khepri/include/khepri.hrl").

-export([append_if_no_tag_matches/4]).

%% Bounded retry budget for the pre-stamp / transaction race in
%% integrity-on stores. Heavy contention escalates rather than spins.
-define(INTEGRITY_RETRY_BUDGET, 5).

%%====================================================================
%% Public API
%%====================================================================

-spec append_if_no_tag_matches(
    StoreId   :: atom() | binary(),
    TagFilter :: reckon_gater_types:tag_filter(),
    SeqCutoff :: reckon_gater_types:seq_cutoff(),
    Events    :: [map()]
) ->
      {ok, LastSeq :: non_neg_integer()}
    | {error, {context_changed, non_neg_integer()}}
    | {error, no_events}
    | {error, dcb_concurrent_writer_exhausted}
    | {error, term()}.
append_if_no_tag_matches(_StoreId, _TagFilter, _SeqCutoff, []) ->
    {error, no_events};
append_if_no_tag_matches(StoreId, TagFilter, SeqCutoff, Events)
  when is_list(Events), is_integer(SeqCutoff) ->
    IntegrityCtx = setup_integrity_ctx(StoreId),
    try_append(StoreId, TagFilter, SeqCutoff, Events, IntegrityCtx,
               ?INTEGRITY_RETRY_BUDGET).

%%====================================================================
%% Outer retry loop (chain-tip / counter races on integrity-on stores)
%%====================================================================

try_append(_StoreId, _TF, _SC, _Events, _Ctx, 0) ->
    {error, dcb_concurrent_writer_exhausted};
try_append(StoreId, TagFilter, SeqCutoff, Events, IntegrityCtx, Retries) ->
    Now = erlang:system_time(millisecond),
    EpochUs = erlang:system_time(microsecond),
    Snapshot = take_snapshot(StoreId, IntegrityCtx),
    {Stamped, FinalTip} = stamp_events(Events, Now, EpochUs, Snapshot),
    case khepri:transaction(
           StoreId,
           fun() -> tx_body(TagFilter, SeqCutoff, Stamped, Snapshot, FinalTip) end) of
        {ok, LastSeq} ->
            {ok, LastSeq};
        {error, {context_changed, _} = Reason} ->
            {error, Reason};
        {error, {dcb_state_changed, _}} ->
            try_append(StoreId, TagFilter, SeqCutoff, Events, IntegrityCtx,
                       Retries - 1);
        {error, _} = Error ->
            Error
    end.

%%====================================================================
%% Outside transaction: integrity context + state snapshot
%%====================================================================

setup_integrity_ctx(StoreId) ->
    case reckon_db_integrity_key:is_enabled(StoreId) of
        false -> disabled;
        true  -> {enabled, reckon_db_integrity_key:get(StoreId)}
    end.

%% disabled snapshot:
%%   For integrity-off stores. The transaction picks seqs from the
%%   live counter; no pre-stamp verification needed beyond tag-filter.
%%
%% #{integrity := {enabled, Key}, ...} snapshot:
%%   For integrity-on stores. Carries the seq-counter value and
%%   chain-tip value that the transaction will verify haven't moved.
take_snapshot(_StoreId, disabled) ->
    disabled;
take_snapshot(StoreId, {enabled, Key}) ->
    ExpectedCounter =
        case khepri:get(StoreId, ?DCB_SEQ_COUNTER_PATH) of
            {ok, N} when is_integer(N) -> N;
            _                          -> undefined
        end,
    ExpectedTip =
        case khepri:get(StoreId, ?DCB_CHAIN_TIP_PATH) of
            {ok, T} when is_binary(T) -> T;
            _                         -> reckon_gater_integrity:genesis_prev_hash()
        end,
    NextSeq = case ExpectedCounter of
                  undefined -> 0;
                  N1        -> N1 + 1
              end,
    #{integrity         => {enabled, Key},
      expected_counter  => ExpectedCounter,
      expected_tip      => ExpectedTip,
      next_seq          => NextSeq}.

%%====================================================================
%% Outside transaction: pre-stamp event records
%%====================================================================

%% Returns {[{Seq, #event{}}], FinalTip}.
%%   - For disabled snapshots, Seq is undefined (the transaction
%%     picks the seq from the live counter) and FinalTip is
%%     undefined (no chain to track).
%%   - For integrity-on snapshots, Seq is pre-assigned, the record
%%     carries prev_event_hash + mac, and FinalTip is the chain-hash
%%     of the last event.
stamp_events(EventMaps, Now, EpochUs, disabled) ->
    Stamped = [{undefined, build_event_record(EM, undefined, Now, EpochUs)}
               || EM <- EventMaps],
    {Stamped, undefined};
stamp_events(EventMaps, Now, EpochUs,
             #{integrity := {enabled, Key},
               expected_tip := Tip,
               next_seq := StartSeq}) ->
    stamp_chain(EventMaps, StartSeq, Now, EpochUs, Key, Tip, []).

stamp_chain([], _Seq, _Now, _EpochUs, _Key, Tip, Acc) ->
    {lists:reverse(Acc), Tip};
stamp_chain([EM | Rest], Seq, Now, EpochUs, Key, Tip, Acc) ->
    Record0 = build_event_record(EM, Seq, Now, EpochUs),
    Record1 = Record0#event{prev_event_hash = Tip},
    Mac = reckon_gater_integrity:compute_event_mac(Record1, Key),
    Record2 = Record1#event{mac = Mac},
    NextTip = reckon_gater_integrity:compute_chain_hash(Record2, Tip),
    stamp_chain(Rest, Seq + 1, Now, EpochUs, Key, NextTip,
                [{Seq, Record2} | Acc]).

%%====================================================================
%% Transaction body — strictly Horus-extractable
%%====================================================================
%% NO crypto, NO try/catch, NO message passing. Pre-stamped records
%% arrive ready to write; we verify state pre-conditions and commit.

%% Integrity-OFF path. The transaction picks seqs from the live counter.
tx_body(TagFilter, SeqCutoff, Stamped, disabled, _FinalTip) ->
    case reckon_db_dcb_filter:match_any_above_cutoff(TagFilter, SeqCutoff) of
        {true, MaxSeq} ->
            khepri_tx:abort({context_changed, MaxSeq});
        false ->
            BaseSeq = next_base_seq_in_tx(),
            LastSeq = write_off(Stamped, BaseSeq),
            ok = khepri_tx:put(?DCB_SEQ_COUNTER_PATH, LastSeq),
            LastSeq
    end;
%% Integrity-ON path. Verify counter + chain-tip match snapshot, then
%% write pre-stamped events at their pre-assigned seqs.
tx_body(TagFilter, SeqCutoff, Stamped,
        #{expected_counter := ExpectedCounter,
          expected_tip     := ExpectedTip,
          next_seq         := StartSeq},
        FinalTip) ->
    case verify_counter(ExpectedCounter) of
        {error, Actual} ->
            khepri_tx:abort({dcb_state_changed, {counter, Actual}});
        ok ->
            case verify_tip(ExpectedTip) of
                {error, Actual} ->
                    khepri_tx:abort({dcb_state_changed, {tip, Actual}});
                ok ->
                    case reckon_db_dcb_filter:match_any_above_cutoff(
                           TagFilter, SeqCutoff) of
                        {true, MaxSeq} ->
                            khepri_tx:abort({context_changed, MaxSeq});
                        false ->
                            LastSeq = write_on(Stamped, StartSeq),
                            ok = khepri_tx:put(?DCB_SEQ_COUNTER_PATH, LastSeq),
                            ok = khepri_tx:put(?DCB_CHAIN_TIP_PATH, FinalTip),
                            LastSeq
                    end
            end
    end.

%% Integrity-off write: live counter assigns seqs, stamp the seq into
%% the record and write at the right path.
write_off([], LastSeq) ->
    LastSeq;
write_off([{undefined, Record0} | Rest], Seq) ->
    Record = Record0#event{version = Seq},
    write_one_record(Record, Seq),
    case Rest of
        [] -> Seq;
        _  -> write_off(Rest, Seq + 1)
    end.

%% Integrity-on write: records already carry the right version (Seq).
write_on([], LastSeq) ->
    LastSeq;
write_on([{Seq, Record} | Rest], ExpectedSeq) ->
    %% Pre-assigned seq must equal the expected position in the chain.
    %% If these diverge, our snapshot was inconsistent — but the
    %% counter+tip verification already prevents that.
    case Seq =:= ExpectedSeq of
        true ->
            write_one_record(Record, Seq),
            case Rest of
                [] -> Seq;
                _  -> write_on(Rest, ExpectedSeq + 1)
            end;
        false ->
            khepri_tx:abort({dcb_state_changed, {seq_skew, Seq, ExpectedSeq}})
    end.

write_one_record(#event{tags = Tags} = Record, Seq) ->
    ok = khepri_tx:put(reckon_db_dcb_paths:event_path(Seq), Record),
    TagList = case Tags of
                  undefined -> [];
                  L when is_list(L) -> L
              end,
    lists:foreach(
        fun(Tag) when is_binary(Tag) ->
            ok = khepri_tx:put(reckon_db_dcb_paths:by_tag_path(Tag, Seq), #{})
        end,
        TagList),
    ok.

verify_counter(ExpectedCounter) ->
    Actual = case khepri_tx:get(?DCB_SEQ_COUNTER_PATH) of
                 {ok, N} when is_integer(N) -> N;
                 _                          -> undefined
             end,
    case Actual =:= ExpectedCounter of
        true  -> ok;
        false -> {error, Actual}
    end.

verify_tip(ExpectedTip) ->
    Actual = case khepri_tx:get(?DCB_CHAIN_TIP_PATH) of
                 {ok, T} when is_binary(T) -> T;
                 %% Absent path means "no integrity-bearing DCB events
                 %% yet"; the genesis hash IS what we expected if the
                 %% snapshot also saw none.
                 _                         -> reckon_gater_integrity:genesis_prev_hash()
             end,
    case Actual =:= ExpectedTip of
        true  -> ok;
        false -> {error, Actual}
    end.

next_base_seq_in_tx() ->
    case khepri_tx:get(?DCB_SEQ_COUNTER_PATH) of
        {ok, LastAssigned} when is_integer(LastAssigned), LastAssigned >= 0 ->
            LastAssigned + 1;
        _ ->
            0
    end.

%%====================================================================
%% Event record builder
%%====================================================================

%% Builds a #event{} record. Seq may be undefined for the
%% integrity-off path (the transaction stamps version at write time);
%% otherwise it's the pre-assigned global DCB seq.
build_event_record(EventMap, Seq, Now, EpochUs) ->
    EventId = maps:get(event_id, EventMap, generate_event_id(Seq, EpochUs)),
    EventType = maps:get(event_type, EventMap),
    Data = maps:get(data, EventMap),
    Metadata = maps:get(metadata, EventMap, #{}),
    Tags = case maps:get(tags, EventMap, undefined) of
               undefined         -> undefined;
               []                -> undefined;
               L when is_list(L) -> L
           end,
    #event{
        event_id   = EventId,
        event_type = EventType,
        stream_id  = ?DCB_STREAM,
        version    = case Seq of undefined -> 0; _ -> Seq end,
        data       = Data,
        metadata   = Metadata,
        tags       = Tags,
        timestamp  = Now,
        epoch_us   = EpochUs
        %% prev_event_hash + mac stamped later by stamp_chain/7 on
        %% integrity-on stores; default undefined otherwise.
    }.

generate_event_id(undefined, EpochUs) ->
    iolist_to_binary(io_lib:format("dcb-~p-pre", [EpochUs]));
generate_event_id(Seq, EpochUs) ->
    iolist_to_binary(io_lib:format("dcb-~p-~p", [EpochUs, Seq])).