# 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, n_fragments = N, n_doubles = L}, Key) -> A = erlang:phash(Key, power2(L + 1)), if A > N -> A - power2(L); true -> A end; key_to_frag_number(#hash_state{function = phash2, n_fragments = N, n_doubles = L}, Key) -> A = erlang:phash2(Key, power2(L + 1)) + 1, if A > N -> A - power2(L); 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 is_tuple(HeadPat), tuple_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)).