-module(machi_csum_table).

-export([open/2,
         find/3, write/4, trim/3,
         sync/1,
         calc_unwritten_bytes/1,
         close/1, delete/1]).

-export([encode_csum_file_entry/3, decode_csum_file_entry/1]).

-include("machi.hrl").

-ifdef(TEST).
-export([split_checksum_list_blob_decode/1]).
-endif.

-record(machi_csum_table,
        {file :: filename:filename(),
         fd   :: file:descriptor(),
         table :: ets:tid()}).

-type table() :: #machi_csum_table{}.
-type byte_sequence() :: { Offset :: non_neg_integer(),
                           Size   :: pos_integer()|infinity }.

-export_type([table/0]).

-spec open(filename:filename(), proplists:proplists()) ->
                  {ok, table()} | {error, file:posix()}.
open(CSumFilename, _Opts) ->
    T = ets:new(?MODULE, [private, ordered_set]),
    C0 = #machi_csum_table{
           file=CSumFilename,
           table=T},
    case file:read_file(CSumFilename) of
        %% , [read, raw, binary]) of
        {ok, Bin} ->
            List = case split_checksum_list_blob_decode(Bin) of
                       {List0, <<>>} ->
                           List0;
                       {List0, _Junk} ->
                           %% Partially written, needs repair TODO
                           %% [write(CSumFilename, List),
                           List0
                   end,
            %% assuming all entries are strictly ordered by offset,
            %% trim command should always come after checksum entry.
            %% *if* by any chance that order cound not be kept, we
            %% still can do ordering check and monotonic merge here.
            ets:insert(T, List);
        {error, enoent} ->
            ok;
        Error ->
            throw(Error)
    end,
    {ok, Fd} = file:open(CSumFilename, [raw, binary, append]),
    {ok, C0#machi_csum_table{fd=Fd}}.

-spec find(table(), machi_dt:chunk_pos(), machi_dt:chunk_size()) ->
                  {ok, machi_dt:chunk_csum()} | {error, trimmed|notfound}.
find(#machi_csum_table{table=T}, Offset, Size) ->
    %% TODO: Check whether all bytes here are written or not
    case ets:lookup(T, Offset) of
        [{Offset, Size, trimmed}] -> {error, trimmed};
        [{Offset, Size, Checksum}] -> {ok, Checksum};
        [{Offset, _, _}] -> {error, unknown_chunk};
        [] -> {error, unknown_chunk}
    end.

-spec write(table(), machi_dt:chunk_pos(), machi_dt:chunk_size(),
            machi_dt:chunk_csum()) ->
                   ok | {error, used|file:posix()}.
write(#machi_csum_table{fd=Fd, table=T}, Offset, Size, CSum) ->
    Binary = encode_csum_file_entry_bin(Offset, Size, CSum),
    case file:write(Fd, Binary) of
        ok ->
            case ets:insert_new(T, {Offset, Size, CSum}) of
                true ->
                    ok;
                false ->
                    {error, written}
            end;
        Error ->
            Error
    end.

-spec trim(table(), machi_dt:chunk_pos(), machi_dt:chunk_size()) ->
                  ok | {error, file:posix()}.
trim(#machi_csum_table{fd=Fd, table=T}, Offset, Size) ->
    Binary = encode_csum_file_entry_bin(Offset, Size, trimmed),
    case file:write(Fd, Binary) of
        ok ->
            true = ets:insert(T, {Offset, Size, trimmed}),
            ok;
        Error ->
            Error
    end.

-spec sync(table()) -> ok | {error, file:posix()}.
sync(#machi_csum_table{fd=Fd}) ->
    file:sync(Fd).

-spec calc_unwritten_bytes(table()) -> [byte_sequence()].
calc_unwritten_bytes(#machi_csum_table{table=T}) ->
    case lists:sort(ets:tab2list(T)) of
        [] ->
            [{?MINIMUM_OFFSET, infinity}];
        Sorted ->
            {LastOffset, _, _} = hd(Sorted),
            build_unwritten_bytes_list(Sorted, LastOffset, [])
    end.

-spec close(table()) -> ok.
close(#machi_csum_table{table=T, fd=Fd}) ->
    true = ets:delete(T),
    ok = file:close(Fd).

-spec delete(table()) -> ok.
delete(#machi_csum_table{file=F} = C) ->
    catch close(C),
    case file:delete(F) of
        ok -> ok;
        {error, enoent} -> ok;
        E -> E
    end.

%% @doc Encode `Offset + Size + TaggedCSum' into an `iolist()' type for
%% internal storage by the FLU.

-spec encode_csum_file_entry(
        machi_dt:file_offset(), machi_dt:chunk_size(), machi_dt:chunk_s()) ->
        iolist().
encode_csum_file_entry(Offset, Size, TaggedCSum) ->
    Len = 8 + 4 + byte_size(TaggedCSum),
    [<<$w, Len:8/unsigned-big, Offset:64/unsigned-big, Size:32/unsigned-big>>,
     TaggedCSum].

%% @doc Encode `Offset + Size + TaggedCSum' into an `binary()' type for
%% internal storage by the FLU.

-spec encode_csum_file_entry_bin(
        machi_dt:file_offset(), machi_dt:chunk_size(), machi_dt:chunk_s()) ->
        binary().
encode_csum_file_entry_bin(Offset, Size, trimmed) ->
    <<$t, Offset:64/unsigned-big, Size:32/unsigned-big>>;
encode_csum_file_entry_bin(Offset, Size, TaggedCSum) ->
    Len = 8 + 4 + byte_size(TaggedCSum),
    <<$w, Len:8/unsigned-big, Offset:64/unsigned-big, Size:32/unsigned-big,
      TaggedCSum/binary>>.

%% @doc Decode a single `binary()' blob into an
%%      `{Offset,Size,TaggedCSum}' tuple.
%%
%% The internal encoding (which is currently exposed to the outside world
%% via this function and related ones) is:
%%
%% <ul>
%% <li> 1 byte: record length
%% </li>
%% <li> 8 bytes (unsigned big-endian): byte offset
%% </li>
%% <li> 4 bytes (unsigned big-endian): chunk size
%% </li>
%% <li> all remaining bytes: tagged checksum (1st byte = type tag)
%% </li>
%% </ul>
%%
%% See `machi.hrl' for the tagged checksum types, e.g.,
%% `?CSUM_TAG_NONE'.

-spec decode_csum_file_entry(binary()) ->
        error |
        {machi_dt:file_offset(), machi_dt:chunk_size(), machi_dt:chunk_s()}.
decode_csum_file_entry(<<_:8/unsigned-big, Offset:64/unsigned-big, Size:32/unsigned-big, TaggedCSum/binary>>) ->
    {Offset, Size, TaggedCSum};
decode_csum_file_entry(_Else) ->
    error.

%% @doc Split a `binary()' blob of `checksum_list' data into a list of
%% `{Offset,Size,TaggedCSum}' tuples.

-spec split_checksum_list_blob_decode(binary()) ->
  {list({machi_dt:file_offset(), machi_dt:chunk_size(), machi_dt:chunk_s()}),
   TrailingJunk::binary()}.
split_checksum_list_blob_decode(Bin) ->
    split_checksum_list_blob_decode(Bin, []).

split_checksum_list_blob_decode(<<$w, Len:8/unsigned-big, Part:Len/binary, Rest/binary>>, Acc)->
    One = <<Len:8/unsigned-big, Part/binary>>,
    case decode_csum_file_entry(One) of
        error ->
            split_checksum_list_blob_decode(Rest, Acc);
        DecOne ->
            split_checksum_list_blob_decode(Rest, [DecOne|Acc])
    end;
split_checksum_list_blob_decode(<<$t, Offset:64/unsigned-big, Size:32/unsigned-big, Rest/binary>>, Acc) ->
    %% trimmed offset
    split_checksum_list_blob_decode(Rest, [{Offset, Size, trimmed}|Acc]);
split_checksum_list_blob_decode(Rest, Acc) ->
    {lists:reverse(Acc), Rest}.

-spec build_unwritten_bytes_list( CsumData   :: [{ Offset   :: non_neg_integer(),
                                                   Size     :: pos_integer(),
                                                   Checksum :: binary() }],
                                  LastOffset :: non_neg_integer(),
                                  Acc        :: list() ) -> [byte_sequence()].
% @private Given a <b>sorted</b> list of checksum data tuples, return a sorted
% list of unwritten byte ranges. The output list <b>always</b> has at least one
% entry: the last tuple in the list is guaranteed to be the current end of
% bytes written to a particular file with the special space moniker
% `infinity'.
build_unwritten_bytes_list([], Last, Acc) ->
    NewAcc = [ {Last, infinity} | Acc ],
    lists:reverse(NewAcc);
build_unwritten_bytes_list([{CurrentOffset, CurrentSize, _Csum}|Rest], LastOffset, Acc) when
      CurrentOffset /= LastOffset ->
    Hole = CurrentOffset - LastOffset,
    build_unwritten_bytes_list(Rest, (CurrentOffset+CurrentSize), [{LastOffset, Hole}|Acc]);
build_unwritten_bytes_list([{CO, CS, _Ck}|Rest], _LastOffset, Acc) ->
    build_unwritten_bytes_list(Rest, CO + CS, Acc).