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<li title="mnesia_frag_hash callback behavior"><a href="Mnesia_App_D.html#id80315">mnesia_frag_hash callback behavior</a></li>
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<h1>11 Appendix D: The Fragmented Table Hashing Call Back Interface</h1>
<h3><a name="id80315">11.1
mnesia_frag_hash callback behavior</a></h3>
<p></p>
<div class="example"><pre>
-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
]).
</pre></div> <p></p>
<div class="example"><pre>
-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)).
</pre></div>
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