# View Source Appendix C: Fragmented Table Hashing Callback Interface

## mnesia_frag_hash Callback Behavior

```
-module(mnesia_frag_hash).
-compile([{nowarn_deprecated_function, [{erlang,phash,2}]}]).
%% 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}).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec init_state(Tab, State) -> NewState when
Tab :: atom(),
State :: term(),
NewState :: term().
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}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec add_frag(State :: term()) -> {NewState, IterFrags, AdditionalLockFrags} when
NewState :: term(),
IterFrags :: [integer()],
AdditionalLockFrags :: [integer()].
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).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec del_frag(State :: term()) -> {NewState, IterFrags, AdditionalLockFrags} when
NewState :: term(),
IterFrags :: [integer()],
AdditionalLockFrags :: [integer()].
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).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec key_to_frag_number(State, Key) -> Fragnum when
State :: term(),
Key :: term(),
Fragnum :: integer().
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).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-spec match_spec_to_frag_numbers(State, MatchSpec) -> Fragnums when
State :: term(),
MatchSpec :: ets:match_spec(),
Fragnums :: [integer()].
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)).
```

Built using ExDoc (v0.32.2) for the Erlang programming language

Copyright © 1996-2023 Ericsson AB