-module(shmem).

-on_load(load_nif/0).
-export([open/2, close/1, pread/3, pwrite/3]).


load_nif() ->
  Path = case code:lib_dir(corelib) of
    P when is_list(P) -> filename:join(P, priv);
    _ -> "./priv"
  end,
  % check if there is a file on disk
  NifName = Path ++ "/shmem",
  case file:read_file_info(NifName ++ ".so") of
    {ok, _} ->
      case erlang:load_nif(NifName, LoadInfo) of
        ok ->
          ok;
        {error, {load_failed, NifLoadError}} ->
          case re:run(NifLoadError, "'([^']+)'",[{capture,all_but_first,list}]) of
            {match, [RealNifMsg]} -> {error, {load_failed, RealNifMsg}};
            _ -> {error, {load_failed, NifLoadError}}
          end;
        {error, NifLoadError} ->
          {error, NifLoadError}
      end;
    {error, enoent} -> ok
  end.


-type shmem() :: any().

-type shmem_opts() :: #{
  len := pos_integer(),
  write => true,
  create => true
}.

-type shm_error() ::
  out_of_range |
  readonly |
  mmap_failed |
  shm_open_failed |
  closed.

-spec open(ShmName, Opts) -> {ok, shmem()} | {error, shm_error()}
  when ShmName :: binary(), Opts :: shmem_opts().

open(ShmName, #{len := Length} = Opts) ->
  Write = maps:get(write, Opts, false),
  Create = maps:get(create, Opts, false),
  open0(ShmName, Length, Write, Create).


open0(ShmName, Len, Write, Create) ->
  erlang:nif_error(not_loaded, [ShmName, Len, Write, Create]).


-spec pread(Shm, Offset, Len) -> {ok, binary()} | {error, shm_error()}
  when Shm::shmem(), Offset::non_neg_integer(), Len::pos_integer().

pread(Shm, Offset, Len) ->
  erlang:nif_error(not_loaded, [Shm, Offset, Len]).



-spec pwrite(Shm, Offset, Binary) -> ok | {error, shm_error()}
  when Shm::shmem(), Offset::non_neg_integer(), Binary::binary().

pwrite(Shm, Offset, Binary) ->
  erlang:nif_error(not_loaded, [Shm, Offset, size(Binary)]).




-spec close(Shm) -> ok
  when Shm::shmem().

close(_Shm) ->
  ok.