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633 lines (517 loc) · 19.5 KB
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%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2000-2021. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%% http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%
%% %CopyrightEnd%
%%
%% We use the dynamic hashing techniques by Per-Åke Larsson as
%% described in "The Design and Implementation of Dynamic Hashing for
%% Sets and Tables in Icon" by Griswold and Townsend. Much of the
%% terminology comes from that paper as well.
%%
%% The segments are all of the same fixed size and we just keep
%% increasing the size of the top tuple as the table grows. At the
%% end of the segments tuple we keep an empty segment which we use
%% when we expand the segments. The segments are expanded by doubling
%% every time n reaches maxn instead of increasing the tuple one
%% element at a time. It is easier and does not seem detrimental to
%% speed. The same applies when contracting the segments.
%%
%% Note that as the order of the keys is undefined we may freely
%% reorder keys within a bucket.
-module(dict).
-compile([{nowarn_deprecated_function, [{erlang,phash,2}]}]).
%% Standard interface.
-export([new/0,is_key/2,to_list/1,from_list/1,size/1,is_empty/1]).
-export([fetch/2,find/2,fetch_keys/1,erase/2,take/2]).
-export([store/3,append/3,append_list/3,update/3,update/4,update_counter/3]).
-export([fold/3,map/2,filter/2,merge/3]).
-export_type([dict/0, dict/2]).
%% Low-level interface.
%%-export([get_slot/2,get_bucket/2,on_bucket/3,fold_dict/3,
%% maybe_expand/2,maybe_contract/2]).
%% Note: mk_seg/1 must be changed too if seg_size is changed.
-define(seg_size, 16).
-define(max_seg, 32).
-define(expand_load, 5).
-define(contract_load, 3).
-define(exp_size, (?seg_size * ?expand_load)).
-define(con_size, (?seg_size * ?contract_load)).
-type segs(_Key, _Value) :: tuple().
%% Define a hashtable. The default values are the standard ones.
-record(dict,
{size=0 :: non_neg_integer(), % Number of elements
n=?seg_size :: non_neg_integer(), % Number of active slots
maxn=?seg_size :: non_neg_integer(), % Maximum slots
bso=?seg_size div 2 :: non_neg_integer(), % Buddy slot offset
exp_size=?exp_size :: non_neg_integer(), % Size to expand at
con_size=?con_size :: non_neg_integer(), % Size to contract at
empty :: tuple(), % Empty segment
segs :: segs(_, _) % Segments
}).
-type dict() :: dict(_, _).
-opaque dict(Key, Value) :: #dict{segs :: segs(Key, Value)}.
-define(kv(K,V), [K|V]). % Key-Value pair format
%%-define(kv(K,V), {K,V}). % Key-Value pair format
-spec new() -> dict().
new() ->
Empty = mk_seg(?seg_size),
#dict{empty=Empty,segs={Empty}}.
-spec is_key(Key, Dict) -> boolean() when
Dict :: dict(Key, Value :: term()).
is_key(Key, D) ->
Slot = get_slot(D, Key),
Bkt = get_bucket(D, Slot),
find_key(Key, Bkt).
find_key(K, [?kv(K,_Val)|_]) -> true;
find_key(K, [_|Bkt]) -> find_key(K, Bkt);
find_key(_, []) -> false.
-spec to_list(Dict) -> List when
Dict :: dict(Key, Value),
List :: [{Key, Value}].
to_list(D) ->
fold(fun (Key, Val, List) -> [{Key,Val}|List] end, [], D).
-spec from_list(List) -> Dict when
Dict :: dict(Key, Value),
List :: [{Key, Value}].
from_list(L) ->
lists:foldl(fun ({K,V}, D) -> store(K, V, D) end, new(), L).
-spec size(Dict) -> non_neg_integer() when
Dict :: dict().
size(#dict{size=N}) when is_integer(N), N >= 0 -> N.
-spec is_empty(Dict) -> boolean() when
Dict :: dict().
is_empty(#dict{size=N}) -> N =:= 0.
-spec fetch(Key, Dict) -> Value when
Dict :: dict(Key, Value).
fetch(Key, D) ->
Slot = get_slot(D, Key),
Bkt = get_bucket(D, Slot),
try fetch_val(Key, Bkt)
catch
badarg -> erlang:error(badarg, [Key, D])
end.
fetch_val(K, [?kv(K,Val)|_]) -> Val;
fetch_val(K, [_|Bkt]) -> fetch_val(K, Bkt);
fetch_val(_, []) -> throw(badarg).
-spec find(Key, Dict) -> {'ok', Value} | 'error' when
Dict :: dict(Key, Value).
find(Key, D) ->
Slot = get_slot(D, Key),
Bkt = get_bucket(D, Slot),
10000
find_val(Key, Bkt).
find_val(K, [?kv(K,Val)|_]) -> {ok,Val};
find_val(K, [_|Bkt]) -> find_val(K, Bkt);
find_val(_, []) -> error.
-spec fetch_keys(Dict) -> Keys when
Dict :: dict(Key, Value :: term()),
Keys :: [Key].
fetch_keys(D) ->
fold(fun (Key, _Val, Keys) -> [Key|Keys] end, [], D).
-spec erase(Key, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value).
%% Erase all elements with key Key.
erase(Key, D0) ->
Slot = get_slot(D0, Key),
{D1,Dc} = on_bucket(fun (B0) -> erase_key(Key, B0) end,
D0, Slot),
maybe_contract(D1, Dc).
erase_key(Key, [?kv(Key,_Val)|Bkt]) -> {Bkt,1};
erase_key(Key, [E|Bkt0]) ->
{Bkt1,Dc} = erase_key(Key, Bkt0),
{[E|Bkt1],Dc};
erase_key(_, []) -> {[],0}.
-spec take(Key, Dict) -> {Value, Dict1} | error when
Dict :: dict(Key, Value),
Dict1 :: dict(Key, Value),
Key :: term(),
Value :: term().
take(Key, D0) ->
Slot = get_slot(D0, Key),
case on_bucket(fun (B0) -> take_key(Key, B0) end, D0, Slot) of
{D1,{Value,Dc}} ->
{Value, maybe_contract(D1, Dc)};
{_,error} -> error
end.
take_key(Key, [?kv(Key,Val)|Bkt]) ->
{Bkt,{Val,1}};
take_key(Key, [E|Bkt0]) ->
{Bkt1,Res} = take_key(Key, Bkt0),
{[E|Bkt1],Res};
take_key(_, []) -> {[],error}.
-spec store(Key, Value, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value).
store(Key, Val, D0) ->
Slot = get_slot(D0, Key),
{D1,Ic} = on_bucket(fun (B0) -> store_bkt_val(Key, Val, B0) end,
D0, Slot),
maybe_expand(D1, Ic).
%% store_bkt_val(Key, Val, Bucket) -> {NewBucket,PutCount}.
store_bkt_val(Key, New, [?kv(Key,_Old)|Bkt]) -> {[?kv(Key,New)|Bkt],0};
store_bkt_val(Key, New, [Other|Bkt0]) ->
{Bkt1,Ic} = store_bkt_val(Key, New, Bkt0),
{[Other|Bkt1],Ic};
store_bkt_val(Key, New, []) -> {[?kv(Key,New)],1}.
-spec append(Key, Value, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value).
append(Key, Val, D0) ->
Slot = get_slot(D0, Key),
{D1,Ic} = on_bucket(fun (B0) -> append_bkt(Key, Val, B0) end,
D0, Slot),
maybe_expand(D1, Ic).
%% append_bkt(Key, Val, Bucket) -> {NewBucket,PutCount}.
append_bkt(Key, Val, [?kv(Key,Bag)|Bkt]) -> {[?kv(Key,Bag ++ [Val])|Bkt],0};
append_bkt(Key, Val, [Other|Bkt0]) ->
{Bkt1,Ic} = append_bkt(Key, Val, Bkt0),
{[Other|Bkt1],Ic};
append_bkt(Key, Val, []) -> {[?kv(Key,[Val])],1}.
-spec append_list(Key, ValList, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value),
ValList :: [Value].
append_list(Key, L, D0) ->
Slot = get_slot(D0, Key),
{D1,Ic} = on_bucket(fun (B0) -> app_list_bkt(Key, L, B0) end,
D0, Slot),
maybe_expand(D1, Ic).
%% app_list_bkt(Key, L, Bucket) -> {NewBucket,PutCount}.
app_list_bkt(Key, L, [?kv(Key,Bag)|Bkt]) -> {[?kv(Key,Bag ++ L)|Bkt],0};
app_list_bkt(Key, L, [Other|Bkt0]) ->
{Bkt1,Ic} = app_list_bkt(Key, L, Bkt0),
{[Other|Bkt1],Ic};
app_list_bkt(Key, L, []) -> {[?kv(Key,L)],1}.
%% %% first_key(Table) -> {ok,Key} | error.
%% %% Find the "first" key in a Table.
%% first_key(T) ->
%% case next_bucket(T, 1) of
%% [?kv(K,Val)|Bkt] -> {ok,K};
%% [] -> error %No elements
%% end.
%% next_bucket(T, Slot) when Slot > T#dict.n -> [];
%% next_bucket(T, Slot) ->
%% case get_bucket(T, Slot) of
%% [] -> next_bucket(T, Slot+1); %Empty bucket
%% B -> B
%% end.
%% %% next_key(Table, Key) -> {ok,NextKey} | error.
%% next_key(T, Key) ->
%% Slot = get_slot(T, Key),
%% B = get_bucket(T, Slot),
%% %% Find a bucket with something in it.
%% Bkt = case bucket_after_key(Key, B) of
%% no_key -> exit(badarg);
%% [] -> next_bucket(T, Slot+1);
%% Rest -> Rest
%% end,
%% case Bkt of
%% [?kv(Next,Val)|_] -> {ok,Next};
%% [] -> error %We have reached the end!
%% end.
%% bucket_after_key(Key, [?kv(Key,Val)|Bkt]) -> Bkt;
%% bucket_after_key(Key, [Other|Bkt]) ->
%% bucket_after_key(Key, Bkt);
%% bucket_after_key(Key, []) -> no_key. %Key not found!
%% %% on_key(Fun, Key, Dictionary) -> Dictionary.
%% on_key(F, Key, D0) ->
%% Slot = get_slot(D0, Key),
%% {D1,Dc} = on_bucket(fun (B0) -> on_key_bkt(Key, F, B0) end,
%% D0, Slot),
%% maybe_contract(D1, Dc).
%% on_key_bkt(Key, F, [?kv(Key,Val)|Bkt]) ->
%% case F(Val) of
%% {ok,New} -> {[?kv(Key,New)|Bkt],0};
%% erase -> {Bkt,1}
%% end;
%% on_key_bkt(Key, F, [Other|Bkt0]) ->
%% {Bkt1,Dc} = on_key_bkt(Key, F, Bkt0),
%% {[Other|Bkt1],Dc}.
-spec update(Key, Fun, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value),
Fun :: fun((Value1 :: Value) -> Value2 :: Value).
update(Key, F, D0) ->
Slot = get_slot(D0, Key),
try on_bucket(fun (B0) -> update_bkt(Key, F, B0) end, D0, Slot) of
{D1,_Uv} -> D1
catch
badarg -> erlang:error(badarg, [Key, F, D0])
end.
update_bkt(Key, F, [?kv(Key,Val)|Bkt]) ->
Upd = F(Val),
{[?kv(Key,Upd)|Bkt],Upd};
update_bkt(Key, F, [Other|Bkt0]) ->
{Bkt1,Upd} = update_bkt(Key, F, Bkt0),
{[Other|Bkt1],Upd};
update_bkt(_Key, _F, []) ->
throw(badarg).
-spec update(Key, Fun, Initial, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value),
Fun :: fun((Value1 :: Value) -> Value2 :: Value),
Initial :: Value.
update(Key, F, Init, D0) ->
Slot = get_slot(D0, Key),
{D1,Ic} = on_bucket(fun (B0) -> update_bkt(Key, F, Init, B0) end,
D0, Slot),
maybe_expand(D1, Ic).
update_bkt(Key, F, _, [?kv(Key,Val)|Bkt]) ->
{[?kv(Key,F(Val))|Bkt],0};
update_bkt(Key, F, I, [Other|Bkt0]) ->
{Bkt1,Ic} = update_bkt(Key, F, I, Bkt0),
{[Other|Bkt1],Ic};
update_bkt(Key, F, I, []) when is_function(F, 1) -> {[?kv(Key,I)],1}.
-spec update_counter(Key, Increment, Dict1) -> Dict2 when
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value),
Increment :: number().
update_counter(Key, Incr, D0) when is_number(Incr) ->
Slot = get_slot(D0, Key),
{D1,Ic} = on_bucket(fun (B0) -> counter_bkt(Key, Incr, B0) end,
D0, Slot),
maybe_expand(D1, Ic).
-dialyzer({no_improper_lists, counter_bkt/3}).
counter_bkt(Key, I, [?kv(Key,Val)|Bkt]) ->
{[?kv(Key,Val+I)|Bkt],0};
counter_bkt(Key, I, [Other|Bkt0]) ->
{Bkt1,Ic} = counter_bkt(Key, I, Bkt0),
{[Other|Bkt1],Ic};
counter_bkt(Key, I, []) -> {[?kv(Key,I)],1}.
-spec fold(Fun, Acc0, Dict) -> Acc1 when
Fun :: fun((Key, Value, AccIn) -> AccOut),
Dict :: dict(Key, Value),
Acc0 :: Acc,
Acc1 :: Acc,
AccIn :: Acc,
AccOut :: Acc.
%% Fold function Fun over all "bags" in Table and return Accumulator.
fold(F, Acc, D) -> fold_dict(F, Acc, D).
-spec map(Fun, Dict1) -> Dict2 when
Fun :: fun((Key, Value1) -> Value2),
Dict1 :: dict(Key, Value1),
Dict2 :: dict(Key, Value2).
map(F, D) -> map_dict(F, D).
-spec filter(Pred, Dict1) -> Dict2 when
Pred :: fun((Key , Value) -> boolean()),
Dict1 :: dict(Key, Value),
Dict2 :: dict(Key, Value).
filter(F, D) -> filter_dict(F, D).
-spec merge(Fun, Dict1, Dict2) -> Dict3 when
Fun :: fun((Key, Value1, Value2) -> Value),
Dict1 :: dict(Key, Value1),
Dict2 :: dict(Key, Value2),
Dict3 :: dict(Key, Value).
merge(F, D1, D2) when D1#dict.size < D2#dict.size ->
fold_dict(fun (K, V1, D) ->
update(K, fun (V2) -> F(K, V1, V2) end, V1, D)
end, D2, D1);
merge(F, D1, D2) ->
fold_dict(fun (K, V2, D) ->
update(K, fun (V1) -> F(K, V1, V2) end, V2, D)
end, D1, D2).
%% get_slot(Hashdb, Key) -> Slot.
%% Get the slot. First hash on the new range, if we hit a bucket
%% which has not been split use the unsplit buddy bucket.
get_slot(T, Key) ->
H = erlang:phash(Key, T#dict.maxn),
if
H > T#dict.n -> H - T#dict.bso;
true -> H
end.
%% get_bucket(Hashdb, Slot) -> Bucket.
get_bucket(T, Slot) -> get_bucket_s(T#dict.segs, Slot).
%% on_bucket(Fun, Hashdb, Slot) -> {NewHashDb,Result}.
%% Apply Fun to the bucket in Slot and replace the returned bucket.
on_bucket(F, T, Slot) ->
SegI = ((Slot-1) div ?seg_size) + 1,
BktI = ((Slot-1) rem ?seg_size) + 1,
Segs = T#dict.segs,
Seg = element(SegI, Segs),
B0 = element(BktI, Seg),
{B1,Res} = F(B0), %Op on the bucket.
{T#dict{segs=setelement(SegI, Segs, setelement(BktI, Seg, B1))},Res}.
%% fold_dict(Fun, Acc, Dictionary) -> Acc.
%% map_dict(Fun, Dictionary) -> Dictionary.
%% filter_dict(Fun, Dictionary) -> Dictionary.
%%
%% Work functions for fold, map and filter operations. These
%% traverse the hash structure rebuilding as necessary. Note we
%% could have implemented map and filter using fold but these are
%% faster. We hope!
fold_dict(F, Acc, #dict{size=0}) when is_function(F, 3) ->
Acc;
fold_dict(F, Acc, D) ->
Segs = D#dict.segs,
fold_segs(F, Acc, Segs, tuple_size(Segs)).
fold_segs(F, Acc, Segs, I) when I >= 1 ->
Seg = element(I, Segs),
fold_segs(F, fold_seg(F, Acc, Seg, tuple_size(Seg)), Segs, I-1);
fold_segs(F, Acc, _, 0) when is_function(F, 3) -> Acc.
fold_seg(F, Acc, Seg, I) when I >= 1 ->
fold_seg(F, fold_bucket(F, Acc, element(I, Seg)), Seg, I-1);
fold_seg(F, Acc, _, 0) when is_function(F, 3) -> Acc.
fold_bucket(F, Acc, [?kv(Key,Val)|Bkt]) ->
fold_bucket(F, F(Key, Val, Acc), Bkt);
fold_bucket(F, Acc, []) when is_function(F, 3) -> Acc.
map_dict(F, #dict{size=0} = Dict) when is_function(F, 2) ->
Dict;
map_dict(F, D) ->
Segs0 = tuple_to_list(D#dict.segs),
Segs1 = map_seg_list(F, Segs0),
D#dict{segs=list_to_tuple(Segs1)}.
map_seg_list(F, [Seg|Segs]) ->
Bkts0 = tuple_to_list(Seg),
Bkts1 = map_bkt_list(F, Bkts0),
[list_to_tuple(Bkts1)|map_seg_list(F, Segs)];
map_seg_list(F, []) when is_function(F, 2) -> [].
map_bkt_list(F, [Bkt0|Bkts]) ->
[map_bucket(F, Bkt0)|map_bkt_list(F, Bkts)];
map_bkt_list(F, []) when is_function(F, 2) -> [].
map_bucket(F, [?kv(Key,Val)|Bkt]) ->
[?kv(Key,F(Key, Val))|map_bucket(F, Bkt)];
map_bucket(F, []) when is_function(F, 2) -> [].
filter_dict(F, #dict{size=0} = Dict) when is_function(F, 2) ->
Dict;
filter_dict(F, D) ->
Segs0 = tuple_to_list(D#dict.segs),
{Segs1,Fc} = filter_seg_list(F, Segs0, [], 0),
maybe_contract(D#dict{segs=list_to_tuple(Segs1)}, Fc).
filter_seg_list(F, [Seg|Segs], Fss, Fc0) ->
Bkts0 = tuple_to_list(Seg),
{Bkts1,Fc1} = filter_bkt_list(F, Bkts0, [], Fc0),
filter_seg_list(F, Segs, [list_to_tuple(Bkts1)|Fss], Fc1);
filter_seg_list(F, [], Fss, Fc) when is_function(F, 2) ->
{lists:reverse(Fss, []),Fc}.
filter_bkt_list(F, [Bkt0|Bkts], Fbs, Fc0) ->
{Bkt1,Fc1} = filter_bucket(F, Bkt0, [], Fc0),
filter_bkt_list(F, Bkts, [Bkt1|Fbs], Fc1);
filter_bkt_list(F, [], Fbs, Fc) when is_function(F, 2) ->
{lists:reverse(Fbs),Fc}.
filter_bucket(F, [?kv(Key,Val)=E|Bkt], Fb, Fc) ->
case F(Key, Val) of
true -> filter_bucket(F, Bkt, [E|Fb], Fc);
false -> filter_bucket(F, Bkt, Fb, Fc+1)
end;
filter_bucket(F, [], Fb, Fc) when is_function(F, 2) ->
{lists:reverse(Fb),Fc}.
%% get_bucket_s(Segments, Slot) -> Bucket.
%% put_bucket_s(Segments, Slot, Bucket) -> NewSegments.
get_bucket_s(Segs, Slot) ->
SegI = ((Slot-1) div ?seg_size) + 1,
BktI = ((Slot-1) rem ?seg_size) + 1,
element(BktI, element(SegI, Segs)).
put_bucket_s(Segs, Slot, Bkt) ->
SegI = ((Slot-1) div ?seg_size) + 1,
BktI = ((Slot-1) rem ?seg_size) + 1,
Seg = setelement(BktI, element(SegI, Segs), Bkt),
setelement(SegI, Segs, Seg).
%% In maybe_expand(), the variable Ic only takes the values 0 or 1,
%% but type inference is not strong enough to infer this. Thus, the
%% use of explicit pattern matching and an auxiliary function.
maybe_expand(T, 0) -> maybe_expand_aux(T, 0);
maybe_expand(T, 1) -> maybe_expand_aux(T, 1).
maybe_expand_aux(T0, Ic) when T0#dict.size + Ic > T0#dict.exp_size ->
T = maybe_expand_segs(T0), %Do we need more segments.
N = T#dict.n + 1, %Next slot to expand into
Segs0 = T#dict.segs,
Slot1 = N - T#dict.bso,
B = get_bucket_s(Segs0, Slot1),
Slot2 = N,
[B1|B2] = rehash(B, Slot1, Slot2, T#dict.maxn),
Segs1 = put_bucket_s(Segs0, Slot1, B1),
Segs2 = put_bucket_s(Segs1, Slot2, B2),
T#dict{size=T#dict.size + Ic,
n=N,
exp_size=N * ?expand_load,
con_size=N * ?contract_load,
segs=Segs2};
maybe_expand_aux(T, Ic) -> T#dict{size=T#dict.size + Ic}.
maybe_expand_segs(T) when T#dict.n =:= T#dict.maxn ->
T#dict{maxn=2 * T#dict.maxn,
bso=2 * T#dict.bso,
segs=expand_segs(T#dict.segs, T#dict.empty)};
maybe_expand_segs(T) -> T.
maybe_contract(T, Dc) when T#dict.size - Dc < T#dict.con_size,
T#dict.n > ?seg_size ->
N = T#dict.n,
Slot1 = N - T#dict.bso,
Segs0 = T#dict.segs,
B1 = get_bucket_s(Segs0, Slot1),
Slot2 = N,
B2 = get_bucket_s(Segs0, Slot2),
Segs1 = put_bucket_s(Segs0, Slot1, B1 ++ B2),
Segs2 = put_bucket_s(Segs1, Slot2, []), %Clear the upper bucket
N1 = N - 1,
maybe_contract_segs(T#dict{size=T#dict.size - Dc,
n=N1,
exp_size=N1 * ?expand_load,
con_size=N1 * ?contract_load,
segs=Segs2});
maybe_contract(T, Dc) -> T#dict{size=T#dict.size - Dc}.
maybe_contract_segs(T) when T#dict.n =:= T#dict.bso ->
T#dict{maxn=T#dict.maxn div 2,
bso=T#dict.bso div 2,
segs=contract_segs(T#dict.segs)};
maybe_contract_segs(T) -> T.
%% rehash(Bucket, Slot1, Slot2, MaxN) -> [Bucket1|Bucket2].
%% Yes, we should return a tuple, but this is more fun.
rehash([?kv(Key,_Bag)=KeyBag|T], Slot1, Slot2, MaxN) ->
[L1|L2] = rehash(T, Slot1, Slot2, MaxN),
case erlang:phash(Key, MaxN) of
Slot1 -> [[KeyBag|L1]|L2];
Slot2 -> [L1|[KeyBag|L2]]
end;
rehash([], _Slot1, _Slot2, _MaxN) -> [[]|[]].
%% mk_seg(Size) -> Segment.
mk_seg(16) -> {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}.
%% expand_segs(Segs, EmptySeg) -> NewSegs.
%% contract_segs(Segs) -> NewSegs.
%% Expand/contract the segment tuple by doubling/halving the number
%% of segments. We special case the powers of 2 upto 32, this should
%% catch most case. N.B. the last element in the segments tuple is
%% an extra element containing a default empty segment.
expand_segs({B1}, Empty) ->
{B1,Empty};
expand_segs({B1,B2}, Empty) ->
{B1,B2,Empty,Empty};
expand_segs({B1,B2,B3,B4}, Empty) ->
{B1,B2,B3,B4,Empty,Empty,Empty,Empty};
expand_segs({B1,B2,B3,B4,B5,B6,B7,B8}, Empty) ->
{B1,B2,B3,B4,B5,B6,B7,B8,
Empty,Empty,Empty,Empty,Empty,Empty,Empty,Empty};
expand_segs({B1,B2,B3,B4,B5,B6,B7,B8,B9,B10,B11,B12,B13,B14,B15,B16}, Empty) ->
{B1,B2,B3,B4,B5,B6,B7,B8,B9,B10,B11,B12,B13,B14,B15,B16,
Empty,Empty,Empty,Empty,Empty,Empty,Empty,Empty,
Empty,Empty,Empty,Empty,Empty,Empty,Empty,Empty};
expand_segs(Segs, Empty) ->
list_to_tuple(tuple_to_list(Segs)
++ lists:duplicate(tuple_size(Segs), Empty)).
contract_segs({B1,_}) ->
{B1};
contract_segs({B1,B2,_,_}) ->
{B1,B2};
contract_segs({B1,B2,B3,B4,_,_,_,_}) ->
{B1,B2,B3,B4};
contract_segs({B1,B2,B3,B4,B5,B6,B7,B8,_,_,_,_,_,_,_,_}) ->
{B1,B2,B3,B4,B5,B6,B7,B8};
contract_segs({B1,B2,B3,B4,B5,B6,B7,B8,B9,B10,B11,B12,B13,B14,B15,B16,
_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_}) ->
{B1,B2,B3,B4,B5,B6,B7,B8,B9,B10,B11,B12,B13,B14,B15,B16};
contract_segs(Segs) ->
Ss = tuple_size(Segs) div 2,
list_to_tuple(lists:sublist(tuple_to_list(Segs), 1, Ss)).