[Ericsson AB]

11 Appendix D: The Fragmented Table Hashing Call Back Interface

11.1 mnesia_frag_hash callback behavior

-module(mnesia_frag_hash).

%% Fragmented Table Hashing callback functions
-export([
         init_state/2,
         add_frag/1,
         del_frag/1,
         key_to_frag_number/2,
         match_spec_to_frag_numbers/2
        ]).


-record(hash_state,
        {n_fragments,
         next_n_to_split,
         n_doubles,
         function}).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

init_state(_Tab, State) when State == undefined ->
    #hash_state{n_fragments     = 1,
                next_n_to_split = 1,
                n_doubles       = 0,
                function        = phash2}.

convert_old_state({hash_state, N, P, L}) ->
    #hash_state{n_fragments     = N,
                next_n_to_split = P,
                n_doubles       = L,
                function        = phash}.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

add_frag(#hash_state{next_n_to_split = SplitN, n_doubles = L, n_fragments = N} = State) ->
    P = SplitN + 1,
    NewN = N + 1,
    State2 = case power2(L) + 1 of
                 P2 when P2 == P ->
                     State#hash_state{n_fragments      = NewN,
                                      n_doubles        = L + 1,
                                      next_n_to_split = 1};
                 _ ->
                     State#hash_state{n_fragments     = NewN,
                                      next_n_to_split = P}
             end,
    {State2, [SplitN], [NewN]};
add_frag(OldState) ->
    State = convert_old_state(OldState),
    add_frag(State).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

del_frag(#hash_state{next_n_to_split = SplitN, n_doubles = L, n_fragments = N} = State) ->
    P = SplitN - 1,
    if
        P < 1 ->
            L2 = L - 1,
            MergeN = power2(L2),
            State2 = State#hash_state{n_fragments     = N - 1,
                                      next_n_to_split = MergeN,
                                      n_doubles       = L2},
            {State2, [N], [MergeN]};
        true ->
            MergeN = P,
            State2 = State#hash_state{n_fragments     = N - 1,
                                      next_n_to_split = MergeN},
            {State2, [N], [MergeN]}
        end;
del_frag(OldState) ->
    State = convert_old_state(OldState),
    del_frag(State).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

key_to_frag_number(#hash_state{function = phash, next_n_to_split = SplitN, n_doubles = L}, Key) ->
    P = SplitN,
    A = erlang:phash(Key, power2(L)),
    if
        A < P ->
            erlang:phash(Key, power2(L + 1));
        true ->
            A
    end;
key_to_frag_number(#hash_state{function = phash2, next_n_to_split = SplitN, n_doubles = L}, Key) ->
    P = SplitN,
    A = erlang:phash2(Key, power2(L)) + 1,
    if
        A < P ->
            erlang:phash2(Key, power2(L + 1)) + 1;
        true ->
            A
    end;
key_to_frag_number(OldState, Key) ->
    State = convert_old_state(OldState),
    key_to_frag_number(State, Key).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

match_spec_to_frag_numbers(#hash_state{n_fragments = N} = State, MatchSpec) ->
    case MatchSpec of
        [{HeadPat, _, _}] when tuple(HeadPat), size(HeadPat) > 2 ->
            KeyPat = element(2, HeadPat),
            case has_var(KeyPat) of
                false ->
                    [key_to_frag_number(State, KeyPat)];
                true ->
                    lists:seq(1, N)
            end;
        _ -> 
            lists:seq(1, N)
    end;
match_spec_to_frag_numbers(OldState, MatchSpec) ->
    State = convert_old_state(OldState),
    match_spec_to_frag_numbers(State, MatchSpec).

power2(Y) ->
    1 bsl Y. % trunc(math:pow(2, Y)).



mnesia 4.4
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