-module(shelf_ffi).
-export([
    open_no_load/3,
    close/3, cleanup/3,
    insert/3, insert_list/3, insert_new/3,
    lookup_set/2, lookup_bag/2, member/2,
    delete_key/2, delete_object/3, delete_all/1,
    to_list/1, fold/3, size/1,
    save/2, sync_dets/1, sync_dets/2,
    update_counter/3,
    dets_fold_into_ets_strict/3,
    dets_insert/2, dets_insert_list/2,
    dets_delete_key/2, dets_delete_object/3, dets_delete_all/1,
    reload_atomic/3,
    validate_path/2,
    normalize_path/1
]).

%% ── DETS atom registry ──────────────────────────────────────────────────
%% DETS requires atom names. To avoid unbounded atom creation from
%% user-provided paths, we maintain a registry ETS table that maps
%% path binaries to deterministic atoms from a bounded pool.
%%
%% The registry ETS table is owned by a dedicated long-lived process
%% (shelf_dets_registry_owner) so it survives the death of any individual
%% caller process.

-define(REGISTRY, shelf_dets_registry).
-define(POOL_SIZE, 65536).
-define(MAX_COLLISION_ATTEMPTS, 100).

ensure_registry() ->
    case ets:whereis(?REGISTRY) of
        undefined -> start_registry_owner();
        _ -> ok
    end.

start_registry_owner() ->
    Pid = spawn(fun registry_loop/0),
    try register(shelf_dets_registry_owner, Pid) of
        true ->
            Pid ! {create_registry, self()},
            receive
                {registry_created, Pid} -> ok
            after 5000 ->
                error(registry_timeout)
            end
    catch
        error:badarg ->
            %% Another process won the race to register. Stop the orphan.
            Pid ! stop,
            wait_for_registry(20)
    end.

registry_loop() ->
    receive
        {create_registry, From} ->
            try
                ets:new(?REGISTRY, [set, public, named_table,
                                    {keypos, 1}, {read_concurrency, true}]),
                From ! {registry_created, self()}
            catch
                _:badarg ->
                    %% Table already exists (edge case), still ack.
                    From ! {registry_created, self()}
            end,
            registry_loop();
        stop ->
            ok;
        _ ->
            registry_loop()
    end.

wait_for_registry(0) ->
    error(registry_not_available);
wait_for_registry(N) ->
    case ets:whereis(?REGISTRY) of
        undefined ->
            timer:sleep(50),
            wait_for_registry(N - 1);
        _ ->
            ok
    end.

%% Map a path binary to a bounded atom. Uses erlang:phash2 to hash
%% the path into a fixed pool of atoms (shelf_dets_0 .. shelf_dets_N).
%% Collisions are capped at MAX_COLLISION_ATTEMPTS to keep atom creation bounded.
path_to_dets_name(Path) ->
    ensure_registry(),
    case ets:lookup(?REGISTRY, Path) of
        [{Path, Name}] ->
            Name;
        [] ->
            Hash = erlang:phash2(Path, ?POOL_SIZE),
            Name = find_available_name(Path, Hash, 0),
            %% Use insert_new to handle races — if another process
            %% registered this path first, use their name.
            case ets:insert_new(?REGISTRY, {Path, Name}) of
                true -> Name;
                false ->
                    case ets:lookup(?REGISTRY, Path) of
                        [{Path, ExistingName}] -> ExistingName;
                        [] -> path_to_dets_name(Path)
                    end
            end
    end.

find_available_name(_Path, _Hash, Attempt) when Attempt >= ?MAX_COLLISION_ATTEMPTS ->
    error(atom_pool_exhausted);
find_available_name(Path, Hash, Attempt) ->
    Candidate = list_to_atom("shelf_dets_" ++ integer_to_list(Hash) ++ "_" ++ integer_to_list(Attempt)),
    %% Check if this atom is already used by a different path
    case ets:match_object(?REGISTRY, {'_', Candidate}) of
        [] -> Candidate;
        [{Path, Candidate}] -> Candidate;  %% Same path, reuse
        _ -> find_available_name(Path, Hash, Attempt + 1)  %% Collision, try next
    end.

%% Remove a path from the registry (called on close).
unregister_dets_name(Path) ->
    ensure_registry(),
    ets:delete(?REGISTRY, Path),
    ok.

%% ── Open (no load) ──────────────────────────────────────────────────────
%% Creates an ETS table + opens a DETS file but does NOT load DETS into ETS.
%% Used by the validated loading path where Gleam decodes entries before insertion.

open_no_load(_Name, Path, TypeBin) ->
    Type = binary_to_atom(TypeBin, utf8),
    DetsName = path_to_dets_name(Path),
    try
        {ok, Dets} = dets:open_file(DetsName, [
            {file, binary_to_list(Path)},
            {type, Type},
            {repair, true}
        ]),
        try
            Ets = ets:new(shelf_ets, [Type, protected, {keypos, 1}, {read_concurrency, true}]),
            %% Spawn a guardian to close DETS if the owning process dies.
            %% Safe to call erlang:monitor inside the spawned process: if
            %% OwnerPid is already dead when monitor/2 runs, it delivers
            %% an immediate 'DOWN' rather than silently dropping it.
            OwnerPid = self(),
            Guardian = spawn(fun() ->
                erlang:monitor(process, OwnerPid),
                receive
                    {'DOWN', _, process, OwnerPid, _} ->
                        _ = dets:close(Dets),
                        unregister_dets_name(Path);
                    stop ->
                        ok
                end
            end),
            {ok, {Ets, Dets, Guardian}}
        catch
            _:badarg ->
                _ = dets:close(Dets),
                unregister_dets_name(Path),
                {error, {erlang_error, <<"Failed to create table">>}}
        end
    catch
        _:{badmatch, {error, Reason}} ->
            unregister_dets_name(Path),
            {error, translate_error(Reason)};
        _:Reason ->
            unregister_dets_name(Path),
            {error, translate_error(Reason)}
    end.

%% ── Streaming DETS → ETS loaders ────────────────────────────────────────
%% Validate and insert entries one at a time using dets:foldl, avoiding
%% materializing the entire DETS contents into a Gleam list.
%% To avoid row-by-row ETS boundary crossing, we batch entries.

-define(LOAD_BATCH_SIZE, 5000).

flush_batch(_Ets, []) -> ok;
%% Reverse restores DETS traversal order before bulk insert.
%% ETS bag tables preserve insertion order, so this matters for
%% callers that expect values under a key to stay in DETS order.
flush_batch(Ets, Batch) -> ets:insert(Ets, lists:reverse(Batch)).

%% Abort on first decode failure using throw.
%% DecoderFun takes a raw entry and returns {ok, Pair} or {error, Errors}.
dets_fold_into_ets_strict(Dets, Ets, DecoderFun) ->
    try
        Result = dets:foldl(
            fun(Entry, {Count, Batch}) ->
                case DecoderFun(Entry) of
                    {ok, Pair} ->
                        NewBatch = [Pair | Batch],
                        case Count + 1 of
                            ?LOAD_BATCH_SIZE ->
                                flush_batch(Ets, NewBatch),
                                {0, []};
                            NewCount ->
                                {NewCount, NewBatch}
                        end;
                    {error, Errors} ->
                        throw({type_mismatch, Errors})
                end
            end,
            {0, []},
            Dets
        ),
        case Result of
            {error, Reason} -> {error, translate_error(Reason)};
            {_, FinalBatch} ->
                flush_batch(Ets, FinalBatch),
                {ok, nil}
        end
    catch
        throw:{type_mismatch, Errors} -> {error, {type_mismatch, Errors}};
        _:CatchReason -> {error, translate_error(CatchReason)}
    end.

%% ── Atomic reload ───────────────────────────────────────────────────────
%% Load DETS entries into a scratch ETS table first.  Only replace the
%% live table contents after the full validation succeeds; on failure the
%% live ETS is untouched.

reload_atomic(Ets, Dets, DecoderFun) ->
    case check_owner(Ets) of
        {error, _} = Err -> Err;
        ok -> reload_atomic_validated(Ets, Dets, DecoderFun)
    end.

reload_atomic_validated(Ets, Dets, DecoderFun) ->
    Type = ets:info(Ets, type),
    Scratch = ets:new(shelf_reload_scratch, [Type, protected]),
    try
        case dets_fold_into_ets_strict(Dets, Scratch, DecoderFun) of
            {ok, nil} ->
                %% Validation passed — swap contents.
                ets:delete_all_objects(Ets),
                ets:insert(Ets, ets:tab2list(Scratch)),
                ets:delete(Scratch),
                {ok, nil};
            {error, _} = Err ->
                ets:delete(Scratch),
                Err
        end
    catch
        _:Reason ->
            (catch ets:delete(Scratch)),
            {error, translate_error(Reason)}
    end.

%% ── Guardian ────────────────────────────────────────────────────────────

stop_guardian(Guardian) ->
    Guardian ! stop,
    ok.

%% ── Cleanup ─────────────────────────────────────────────────────────────
%% Delete ETS table and close DETS without saving. Used on validation failure.

cleanup(Ets, Dets, Guardian) ->
    stop_guardian(Guardian),
    Path = try dets_to_path(Dets) catch _:_ -> undefined end,
    DetsResult = (catch dets:close(Dets)),
    _ = (catch ets:delete(Ets)),
    case Path of
        undefined -> ok;
        _ -> unregister_dets_name(Path)
    end,
    case DetsResult of
        ok -> {ok, nil};
        {error, Reason} -> {error, translate_error(Reason)};
        _ -> {ok, nil}
    end.

%% ── Close ───────────────────────────────────────────────────────────────
%% Atomic save ETS→DETS via temp file, close DETS, delete ETS.
%% On save failure, everything is left intact so the caller can retry.

close(Ets, Dets, Guardian) ->
    case check_owner(Ets) of
        {error, _} = Err -> Err;
        ok ->
            Path = dets_to_path(Dets),
            case attempt_close_save(Ets, Dets) of
                ok ->
                    finalize_close(Ets, Dets, Guardian, Path);
                {error, Reason} = Err ->
                    case preserve_table_after_close_error(Path, Reason) of
                        true ->
                            Err;
                        false ->
                            teardown_resources(Ets, Dets, Guardian, Path),
                            Err
                    end
            end
    end.

attempt_close_save(Ets, Dets) ->
    try save(Ets, Dets) of
        {ok, nil} -> ok;
        {error, Reason} -> {error, Reason}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

preserve_table_after_close_error(_Path, table_closed) -> false;
preserve_table_after_close_error(undefined, _Reason) -> false;
preserve_table_after_close_error(_Path, _Reason) -> true.

finalize_close(Ets, Dets, Guardian, Path) ->
    stop_guardian(Guardian),
    CloseResult = close_dets_handle(Dets),
    _ = (catch ets:delete(Ets)),
    case Path of
        undefined -> ok;
        _ -> unregister_dets_name(Path)
    end,
    case CloseResult of
        ok -> {ok, nil};
        {error, Reason} -> {error, Reason}
    end.

teardown_resources(Ets, Dets, Guardian, Path) ->
    stop_guardian(Guardian),
    _ = close_dets_handle(Dets),
    _ = (catch ets:delete(Ets)),
    case Path of
        undefined -> ok;
        _ -> unregister_dets_name(Path)
    end,
    ok.

close_dets_handle(Dets) ->
    try dets:close(Dets) of
        ok -> ok;
        {error, Reason} -> {error, translate_error(Reason)}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

%% Get the file path from a DETS reference as a binary.
dets_to_path(Dets) ->
    case dets:info(Dets, filename) of
        undefined -> undefined;
        Filename -> list_to_binary(Filename)
    end.

%% ── Insert ──────────────────────────────────────────────────────────────

insert(Ets, _Dets, Object) ->
    try ets:insert(Ets, Object) of
        true -> {ok, nil}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

insert_list(Ets, _Dets, Objects) ->
    try ets:insert(Ets, Objects) of
        true -> {ok, nil}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

insert_new(Ets, _Dets, Object) ->
    %% _Dets is unused here — ETS insert_new only checks ETS.
    %% The Gleam caller handles DETS persistence for WriteThrough mode.
    try ets:insert_new(Ets, Object) of
        true -> {ok, nil};
        false -> {error, key_already_present}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

%% ── Lookup ──────────────────────────────────────────────────────────────
%% Always reads from ETS (fast path).

lookup_set(Ets, Key) ->
    try ets:lookup(Ets, Key) of
        [] -> {error, not_found};
        [{_, Value} | _] -> {ok, Value}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

lookup_bag(Ets, Key) ->
    try ets:lookup(Ets, Key) of
        Results when is_list(Results) ->
            Values = [V || {_, V} <- Results],
            case Values of
                [] -> {error, not_found};
                _ -> {ok, Values}
            end
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

member(Ets, Key) ->
    try {ok, ets:member(Ets, Key)}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

%% ── Delete ──────────────────────────────────────────────────────────────

delete_key(Ets, Key) ->
    try ets:delete(Ets, Key) of
        true -> {ok, nil}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

delete_object(Ets, Key, Value) ->
    try ets:delete_object(Ets, {Key, Value}) of
        true -> {ok, nil}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

delete_all(Ets) ->
    try ets:delete_all_objects(Ets) of
        true -> {ok, nil}
    catch
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

%% ── Query ───────────────────────────────────────────────────────────────

to_list(Ets) ->
    try {ok, ets:tab2list(Ets)}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

fold(Ets, Fun, Acc0) ->
    try ets:foldl(Fun, Acc0, Ets) of
        Result -> {ok, Result}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

size(Ets) ->
    try
        case ets:info(Ets, size) of
            undefined -> {error, table_closed};
            Size -> {ok, Size}
        end
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

%% ── Persistence ─────────────────────────────────────────────────────────

%% Check that the caller is the ETS table owner.
%% Returns ok | {error, not_owner | table_closed}.
check_owner(Ets) ->
    case ets:info(Ets, owner) of
        undefined -> {error, table_closed};
        Pid when Pid =:= self() -> ok;
        _ -> {error, not_owner}
    end.

%% Atomic save: snapshot ETS to a temp DETS file, then rename over the original.
%% This prevents data loss if the process is killed mid-save.
save(Ets, Dets) ->
    case check_owner(Ets) of
        {error, _} = Err -> Err;
        ok ->
            try
                OrigPath = dets_to_path(Dets),
                case OrigPath of
                    undefined ->
                        {error, table_closed};
                    _ ->
                        TmpPath = <<OrigPath/binary, ".tmp">>,
                        Type = dets:info(Dets, type),
                        safe_save_impl(Ets, Dets, OrigPath, TmpPath, Type)
                end
            catch
                _:Reason -> {error, translate_error(Reason)}
            end
    end.

safe_save_impl(Ets, Dets, OrigPath, TmpPath, Type) ->
    TmpPathList = binary_to_list(TmpPath),
    OrigPathList = binary_to_list(OrigPath),
    TmpName = {shelf_tmp, make_ref()},
    try
        %% 1. Open temp DETS
        {ok, TmpDets} = dets:open_file(TmpName, [
            {file, TmpPathList},
            {type, Type},
            {repair, false}
        ]),
        %% 2. Snapshot ETS into temp DETS
        TmpDets = ets:to_dets(Ets, TmpDets),
        %% 3. Close temp (flushes to disk)
        ok = dets:close(TmpDets),
        %% 4. Close original DETS
        ok = dets:close(Dets),
        %% 5. Atomic rename (POSIX guarantees this is atomic)
        ok = file:rename(TmpPathList, OrigPathList),
        %% 6. Reopen DETS at original path with the same atom name
        DetsName = Dets,
        {ok, _} = dets:open_file(DetsName, [
            {file, OrigPathList},
            {type, Type},
            {repair, true}
        ]),
        {ok, nil}
    catch
        _:Error ->
            %% Clean up temp file on failure
            _ = (catch dets:close(TmpName)),
            _ = (catch file:delete(TmpPathList)),
            %% Try to reopen original if it was closed
            _ = (catch dets:open_file(Dets, [
                {file, OrigPathList},
                {type, Type},
                {repair, true}
            ])),
            {error, translate_error(Error)}
    end.

%% Flush DETS write buffer to OS.
sync_dets(Dets) ->
    try dets:sync(Dets) of
        ok -> {ok, nil};
        {error, Reason} -> {error, translate_error(Reason)}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

%% Owner-guarded sync: only the ETS owner may flush DETS.
sync_dets(Ets, Dets) ->
    case check_owner(Ets) of
        {error, _} = Err -> Err;
        ok -> sync_dets(Dets)
    end.

%% ── Counters ────────────────────────────────────────────────────────────

update_counter(Ets, Key, Increment) ->
    try {ok, ets:update_counter(Ets, Key, Increment)}
    catch
        error:badarg ->
            case ets:info(Ets, owner) of
                undefined ->
                    {error, table_closed};
                OwnerPid when OwnerPid =:= self() ->
                    %% Owner, so badarg is a data problem
                    case ets:lookup(Ets, Key) of
                        [] -> {error, not_found};
                        _ -> {error, {erlang_error, <<"update_counter failed: value is not an integer">>}}
                    end;
                _OtherPid ->
                    {error, not_owner}
            end;
        _:Reason -> {error, classify_ets_error(Ets, Reason)}
    end.

%% ── Targeted DETS operations (for WriteThrough mode) ────────────────────

dets_insert(Dets, Object) ->
    try dets:insert(Dets, Object) of
        ok -> {ok, nil}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

dets_insert_list(Dets, Objects) ->
    try dets:insert(Dets, Objects) of
        ok -> {ok, nil}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

dets_delete_key(Dets, Key) ->
    try dets:delete(Dets, Key) of
        ok -> {ok, nil}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

dets_delete_object(Dets, Key, Value) ->
    try dets:delete_object(Dets, {Key, Value}) of
        ok -> {ok, nil}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

dets_delete_all(Dets) ->
    try dets:delete_all_objects(Dets) of
        ok -> {ok, nil}
    catch
        _:Reason -> {error, translate_error(Reason)}
    end.

%% ── Error translation ──────────────────────────────────────────────────

translate_error(not_found) -> not_found;
translate_error(key_already_present) -> key_already_present;
translate_error(name_conflict) -> name_conflict;
translate_error(not_owner) -> not_owner;
translate_error(type_mismatch) -> type_mismatch;
translate_error({type_mismatch, Errors}) -> {type_mismatch, Errors};
translate_error({invalid_path, Msg}) -> {invalid_path, Msg};
translate_error(table_closed) -> table_closed;
translate_error(badarg) -> table_closed;
translate_error({file_error, _, enoent}) -> {file_error, <<"File not found">>};
translate_error({file_error, _, eacces}) -> {file_error, <<"Permission denied">>};
translate_error({file_error, _, enospc}) -> file_size_limit_exceeded;
translate_error({file_error, _, Reason}) ->
    {file_error, list_to_binary(io_lib:format("~p", [Reason]))};
translate_error({error, Reason}) -> translate_error(Reason);
translate_error(Reason) ->
    {erlang_error, list_to_binary(io_lib:format("~p", [Reason]))}.

%% Classify an ETS badarg error: is the table closed, or is the caller
%% not the owner?  For `protected` tables, writes from non-owners raise
%% badarg just like a deleted table does.
classify_ets_error(Ets, badarg) ->
    case ets:info(Ets, owner) of
        undefined ->
            %% Table doesn't exist — genuinely closed / deleted
            table_closed;
        OwnerPid when OwnerPid =:= self() ->
            %% We are the owner but still got badarg — bad arguments
            {erlang_error, <<"ETS badarg: invalid arguments">>};
        _OtherPid ->
            %% Table exists but we aren't the owner
            not_owner
    end;
classify_ets_error(_Ets, Reason) ->
    translate_error(Reason).

%% ── Path validation ────────────────────────────────────────────────────

validate_path(Path, BaseDirectory) ->
    BaseAbs = filename:absname(binary_to_list(BaseDirectory)),
    Resolved = filename:absname(binary_to_list(Path), BaseAbs),
    %% Normalize by splitting and rejoining (resolves . and ..)
    Normalized = normalize_path(Resolved),
    NormalizedBase = normalize_path(BaseAbs),
    %% Ensure the resolved path is inside the base directory.
    %% We must check for a directory boundary to prevent sibling directory
    %% bypass (e.g. base="/app/data" must not match "/app/data_sibling/x").
    case Normalized =:= NormalizedBase orelse
         lists:prefix(NormalizedBase ++ "/", Normalized) of
        true ->
            {ok, list_to_binary(Normalized)};
        false ->
            {error, {invalid_path, <<"Path escapes base directory">>}}
    end.

normalize_path(Path) ->
    Parts = filename:split(Path),
    NormalizedParts = normalize_parts(Parts, []),
    filename:join(NormalizedParts).

normalize_parts([], Acc) ->
    lists:reverse(Acc);
normalize_parts(["." | Rest], Acc) ->
    normalize_parts(Rest, Acc);
normalize_parts([".." | Rest], [_ | Acc]) ->
    normalize_parts(Rest, Acc);
normalize_parts([".." | Rest], []) ->
    %% Already at root, ignore
    normalize_parts(Rest, []);
normalize_parts([Part | Rest], Acc) ->
    normalize_parts(Rest, [Part | Acc]).