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648 lines (576 loc) · 20.7 KB
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%%
%% %CopyrightBegin%
%%
%%
8000
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%
%%
%% The new version 2 has moved to use maps under the roof whenever a
%% map is given.
%% The previous version (version 1) uses 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 in a bucket.
-module(sets).
-compile([{nowarn_deprecated_function, [{erlang,phash,2}]}]).
%% Standard interface.
-export([new/0,is_set/1,size/1,is_empty/1,to_list/1,from_list/1]).
-export([is_element/2,add_element/2,del_element/2]).
-export([union/2,union/1,intersection/2,intersection/1]).
-export([is_disjoint/2]).
-export([subtract/2,is_subset/2]).
-export([fold/3,filter/2]).
-export([new/1, from_list/2]).
-export_type([set/0, set/1]).
%% This is the value used when sets are represented as maps.
%% We use an empty list instead of an atom as it is cheaper
%% to serialize.
-define(VALUE, []).
%% 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).
-compile({no_auto_import,[size/1]}).
%%------------------------------------------------------------------------------
-type seg() :: tuple().
-type segs(_Element) :: tuple().
%% Define a hash set. The default values are the standard ones.
-record(set,
{size=0 :: non_neg_integer(), % Number of elements
n=?seg_size :: non_neg_integer(), % Number of active slots
maxn=?seg_size :: pos_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 :: seg(), % Empty segment
segs :: segs(_) % Segments
}).
-type set() :: set(_).
-opaque set(Element) :: #set{segs :: segs(Element)} | #{Element => ?VALUE}.
%%------------------------------------------------------------------------------
%% new() -> Set
-spec new() -> set().
new() ->
Empty = mk_seg(?seg_size),
#set{empty = Empty, segs = {Empty}}.
-spec new([{version, 1..2}]) -> set().
new([{version, 2}]) ->
#{};
new(Opts) ->
case proplists:get_value(version, Opts, 1) of
1 -> new();
2 -> new([{version, 2}])
end.
%% from_list([Elem]) -> Set.
%% Build a set from the elements in List.
-spec from_list(List) -> Set when
List :: [Element],
Set :: set(Element).
from_list(Ls) ->
lists:foldl(fun (E, S) -> add_element(E, S) end, new(), Ls).
-spec from_list(List, [{version, 1..2}]) -> Set when
List :: [Element],
Set :: set(Element).
from_list(Ls, [{version, 2}]) ->
maps:from_keys(Ls, ?VALUE);
from_list(Ls, Opts) ->
case proplists:get_value(version, Opts, 1) of
1 -> from_list(Ls);
2 -> from_list(Ls, [{version, 2}])
end.
%%------------------------------------------------------------------------------
%% is_set(Set) -> boolean().
%% Return 'true' if Set is a set of elements, else 'false'.
-spec is_set(Set) -> boolean() when
Set :: term().
is_set(#{}) -> true;
is_set(#set{}) -> true;
is_set(_) -> false.
%% size(Set) -> int().
%% Return the number of elements in Set.
-spec size(Set) -> non_neg_integer() when
Set :: set().
size(#{}=S) -> map_size(S);
size(#set{size=Size}) -> Size.
%% is_empty(Set) -> boolean().
%% Return 'true' if Set is an empty set, otherwise 'false'.
-spec is_empty(Set) -> boolean() when
Set :: set().
is_empty(#{}=S) -> map_size(S)=:=0;
is_empty(#set{size=Size}) -> Size=:=0.
%% to_list(Set) -> [Elem].
%% Return the elements in Set as a list.
-spec to_list(Set) -> List when
Set :: set(Element),
List :: [Element].
to_list(#{}=S) ->
maps:keys(S);
to_list(#set{} = S) ->
fold(fun (Elem, List) -> [Elem|List] end, [], S).
%% is_element(Element, Set) -> boolean().
%% Return 'true' if Element is an element of Set, else 'false'.
-spec is_element(Element, Set) -> boolean() when
Set :: set(Element).
is_element(E, #{}=S) ->
case S of
#{E := _} -> true;
_ -> false
end;
is_element(E, #set{}=S) ->
Slot = get_slot(S, E),
Bkt = get_bucket(S, Slot),
lists:member(E, Bkt).
%% add_element(Element, Set) -> Set.
%% Return Set with Element inserted in it.
-spec add_element(Element, Set1) -> Set2 when
Set1 :: set(Element),
Set2 :: set(Element).
add_element(E, #{}=S) ->
S#{E=>?VALUE};
add_element(E, #set{}=S0) ->
Slot = get_slot(S0, E),
Bkt = get_bucket(S0, Slot),
case lists:member(E, Bkt) of
true ->
S0;
false ->
S1 = update_bucket(S0, Slot, [E | Bkt]),
maybe_expand(S1)
end.
%% del_element(Element, Set) -> Set.
%% Return Set but with Element removed.
-spec del_element(Element, Set1) -> Set2 when
Set1 :: set(Element),
Set2 :: set(Element).
del_element(E, #{}=S) ->
maps:remove(E, S);
del_element(E, #set{}=S0) ->
Slot = get_slot(S0, E),
Bkt = get_bucket(S0, Slot),
case lists:member(E, Bkt) of
false ->
S0;
true ->
S1 = update_bucket(S0, Slot, lists:delete(E, Bkt)),
maybe_contract(S1, 1)
end.
%% update_bucket(Set, Slot, NewBucket) -> UpdatedSet.
%% Replace bucket in Slot by NewBucket
-spec update_bucket(Set1, Slot, Bkt) -> Set2 when
Set1 :: set(Element),
Set2 :: set(Element),
Slot :: non_neg_integer(),
Bkt :: [Element].
update_bucket(Set, Slot, NewBucket) ->
SegI = ((Slot-1) div ?seg_size) + 1,
BktI = ((Slot-1) rem ?seg_size) + 1,
Segs = Set#set.segs,
Seg = element(SegI, Segs),
Set#set{segs = setelement(SegI, Segs, setelement(BktI, Seg, NewBucket))}.
%% union(Set1, Set2) -> Set
%% Return the union of Set1 and Set2.
-spec union(Set1, Set2) -> Set3 when
Set1 :: set(Element),
Set2 :: set(Element),
Set3 :: set(Element).
union(#{}=S1, #{}=S2) ->
maps:merge(S1,S2);
union(S1, S2) ->
case size(S1) < size(S2) of
true ->
fold(fun (E, S) -> add_element(E, S) end, S2, S1);
false ->
fold(fun (E, S) -> add_element(E, S) end, S1, S2)
end.
%% union([Set]) -> Set
%% Return the union of the list of sets.
-spec union(SetList) -> Set when
SetList :: [set(Element)],
Set :: set(Element).
union([S1,S2|Ss]) ->
union1(union(S1, S2), Ss);
union([S]) -> S;
union([]) -> new().
-spec union1(set(E), [set(E)]) -> set(E).
union1(S1, [S2|Ss]) ->
union1(union(S1, S2), Ss);
union1(S1, []) -> S1.
%% intersection(Set1, Set2) -> Set.
%% Return the intersection of Set1 and Set2.
-spec intersection(Set1, Set2) -> Set3 when
Set1 :: set(Element),
Set2 :: set(Element),
Set3 :: set(Element).
intersection(#{}=S1, #{}=S2) ->
case map_size(S1) < map_size(S2) of
true ->
Next = maps:next(maps:iterator(S1)),
intersection_heuristic(Next, [], [], floor(map_size(S1) * 0.75), S1, S2);
false ->
Next = maps:next(maps:iterator(S2)),
intersection_heuristic(Next, [], [], floor(map_size(S2) * 0.75), S2, S1)
end;
intersection(S1, S2) ->
case size(S1) < size(S2) of
true ->
filter(fun (E) -> is_element(E, S2) end, S1);
false ->
filter(fun (E) -> is_element(E, S1) end, S2)
end.
%% If we are keeping more than 75% of the keys, then it is
%% cheaper to delete them. Stop accumulating and start deleting.
intersection_heuristic(Next, _Keep, Delete, 0, Acc, Reference) ->
intersection_decided(Next, remove_keys(Delete, Acc), Reference);
intersection_heuristic({Key, _Value, Iterator}, Keep, Delete, KeepCount, Acc, Reference) ->
Next = maps:next(Iterator),
case Reference of
#{Key := _} ->
intersection_heuristic(Next, [Key | Keep], Delete, KeepCount - 1, Acc, Reference);
_ ->
intersection_heuristic(Next, Keep, [Key | Delete], KeepCount, Acc, Reference)
end;
intersection_heuristic(none, Keep, _Delete, _Count, _Acc, _Reference) ->
maps:from_keys(Keep, ?VALUE).
intersection_decided({Key, _Value, Iterator}, Acc0, Reference) ->
Acc1 = case Reference of
#{Key := _} -> Acc0;
#{} -> maps:remove(Key, Acc0)
end,
intersection_decided(maps:next(Iterator), Acc1, Reference);
intersection_decided(none, Acc, _Reference) ->
Acc.
remove_keys([K | Ks], Map) -> remove_keys(Ks, maps:remove(K, Map));
remove_keys([], Map) -> Map.
%% intersection([Set]) -> Set.
%% Return the intersection of the list of sets.
-spec intersection(SetList) -> Set when
SetList :: [set(Element),...],
Set :: set(Element).
intersection([S1,S2|Ss]) ->
intersection1(intersection(S1, S2), Ss);
intersection([S]) -> S.
-spec intersection1(set(E), [set(E)]) -> set(E).
intersection1(S1, [S2|Ss]) ->
intersection1(intersection(S1, S2), Ss);
intersection1(S1, []) -> S1.
%% is_disjoint(Set1, Set2) -> boolean().
%% Check whether Set1 and Set2 are disjoint.
-spec is_disjoint(Set1, Set2) -> boolean() when
Set1 :: set(Element),
Set2 :: set(Element).
is_disjoint(#{}=S1, #{}=S2) ->
if
map_size(S1) < map_size(S2) ->
is_disjoint_1(S2, maps:iterator(S1));
true ->
is_disjoint_1(S1, maps:iterator(S2))
end;
is_disjoint(S1, S2) ->
case size(S1) < size(S2) of
true ->
fold(fun (_, false) -> false;
(E, true) -> not is_element(E, S2)
end, true, S1);
false ->
fold(fun (_, false) -> false;
(E, true) -> not is_element(E, S1)
end, true, S2)
end.
is_disjoint_1(Set, Iter) ->
case maps:next(Iter) of
{K, _, NextIter} ->
case Set of
#{K := _} -> false;
#{} -> is_disjoint_1(Set, NextIter)
end;
none ->
true
end.
%% subtract(Set1, Set2) -> Set.
%% Return all and only the elements of Set1 which are not also in
%% Set2.
-spec subtract(Set1, Set2) -> Set3 when
Set1 :: set(Element),
Set2 :: set(Element),
Set3 :: set(Element).
subtract(#{}=S1, #{}=S2) ->
Next = maps:next(maps:iterator(S1)),
subtract_heuristic(Next, [], [], floor(map_size(S1) * 0.75), S1, S2);
subtract(S1, S2) ->
filter(fun (E) -> not is_element(E, S2) end, S1).
%% If we are keeping more than 75% of the keys, then it is
%% cheaper to delete them. Stop accumulating and start deleting.
subtract_heuristic(Next, _Keep, Delete, 0, Acc, Reference) ->
subtract_decided(Next, remove_keys(Delete, Acc), Reference);
subtract_heuristic({Key, _Value, Iterator}, Keep, Delete, KeepCount, Acc, Reference) ->
Next = maps:next(Iterator),
case Reference of
#{Key := _} ->
subtract_heuristic(Next, Keep, [Key | Delete], KeepCount, Acc, Reference);
_ ->
subtract_heuristic(Next, [Key | Keep], Delete, KeepCount - 1, Acc, Reference)
end;
subtract_heuristic(none, Keep, _Delete, _Count, _Acc, _Reference) ->
maps:from_keys(Keep, ?VALUE).
subtract_decided({Key, _Value, Iterator}, Acc, Reference) ->
case Reference of
#{Key := _} ->
subtract_decided(maps:next(Iterator), maps:remove(Key, Acc), Reference);
_ ->
subtract_decided(maps:next(Iterator), Acc, Reference)
end;
8000
subtract_decided(none, Acc, _Reference) ->
Acc.
%% is_subset(Set1, Set2) -> boolean().
%% Return 'true' when every element of Set1 is also a member of
%% Set2, else 'false'.
-spec is_subset(Set1, Set2) -> boolean() when
Set1 :: set(Element),
Set2 :: set(Element).
is_subset(#{}=S1, #{}=S2) ->
if
map_size(S1) > map_size(S2) ->
false;
true ->
is_subset_1(S2, maps:iterator(S1))
end;
is_subset(S1, S2) ->
fold(fun (E, Sub) -> Sub andalso is_element(E, S2) end, true, S1).
is_subset_1(Set, Iter) ->
case maps:next(Iter) of
{K, _, NextIter} ->
case Set of
#{K := _} -> is_subset_1(Set, NextIter);
#{} -> false
end;
none ->
true
end.
%% fold(Fun, Accumulator, Set) -> Accumulator.
%% Fold function Fun over all elements in Set and return Accumulator.
-spec fold(Function, Acc0, Set) -> Acc1 when
Function :: fun((Element, AccIn) -> AccOut),
Set :: set(Element),
Acc0 :: Acc,
Acc1 :: Acc,
AccIn :: Acc,
AccOut :: Acc.
fold(F, Acc, #{}=D) when is_function(F, 2)->
fold_1(F, Acc, maps:iterator(D));
fold(F, Acc, #set{}=D) when is_function(F, 2)->
fold_set(F, Acc, D).
fold_1(Fun, Acc, Iter) ->
case maps:next(Iter) of
{K, _, NextIter} ->
fold_1(Fun, Fun(K,Acc), NextIter);
none ->
Acc
end.
%% filter(Fun, Set) -> Set.
%% Filter Set with Fun.
-spec filter(Pred, Set1) -> Set2 when
Pred :: fun((Element) -> boolean()),
Set1 :: set(Element),
Set2 :: set(Element).
filter(F, #{}=D) when is_function(F, 1)->
maps:from_keys(filter_1(F, maps:iterator(D)), ?VALUE);
filter(F, #set{}=D) when is_function(F, 1)->
filter_set(F, D).
filter_1(Fun, Iter) ->
case maps:next(Iter) of
{K, _, NextIter} ->
case Fun(K) of
true ->
[K | filter_1(Fun, NextIter)];
false ->
filter_1(Fun, NextIter)
end;
none ->
[]
end.
%% 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.
-spec get_slot(set(E), E) -> non_neg_integer().
get_slot(T, Key) ->
H = erlang:phash(Key, T#set.maxn),
if
H > T#set.n -> H - T#set.bso;
true -> H
end.
%% get_bucket(Hashdb, Slot) -> Bucket.
-spec get_bucket(set(), non_neg_integer()) -> term().
get_bucket(T, Slot) -> get_bucket_s(T#set.segs, Slot).
%% fold_set(Fun, Acc, Dictionary) -> Dictionary.
%% filter_set(Fun, Dictionary) -> Dictionary.
%% Work functions for fold and filter operations. These traverse the
%% hash structure rebuilding as necessary. Note we could have
%% implemented map and hash using fold but these should be faster.
%% We hope!
fold_set(F, Acc, D) ->
Segs = D#set.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(_, Acc, _, _) -> 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(_, Acc, _, _) -> Acc.
fold_bucket(F, Acc, [E|Bkt]) ->
fold_bucket(F, F(E, Acc), Bkt);
fold_bucket(_, Acc, []) -> Acc.
filter_set(F, D) ->
Segs0 = tuple_to_list(D#set.segs),
{Segs1,Fc} = filter_seg_list(F, Segs0, [], 0),
maybe_contract(D#set{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(_, [], Fss, Fc) ->
{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(_, [], Fbs, Fc) ->
{lists:reverse(Fbs),Fc}.
filter_bucket(F, [E|Bkt], Fb, Fc) ->
case F(E) of
true -> filter_bucket(F, Bkt, [E|Fb], Fc);
false -> filter_bucket(F, Bkt, Fb, Fc+1)
end;
filter_bucket(_, [], Fb, Fc) -> {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).
-spec maybe_expand(set(E)) -> set(E).
maybe_expand(T0) when T0#set.size + 1 > T0#set.exp_size ->
T = maybe_expand_segs(T0), %Do we need more segments.
N = T#set.n + 1, %Next slot to expand into
Segs0 = T#set.segs,
Slot1 = N - T#set.bso,
B = get_bucket_s(Segs0, Slot1),
Slot2 = N,
{B1,B2} = rehash(B, Slot1, Slot2, T#set.maxn),
Segs1 = put_bucket_s(Segs0, Slot1, B1),
Segs2 = put_bucket_s(Segs1, Slot2, B2),
T#set{size = T#set.size + 1,
n = N,
exp_size = N * ?expand_load,
con_size = N * ?contract_load,
segs = Segs2};
maybe_expand(T) -> T#set{size = T#set.size + 1}.
-spec maybe_expand_segs(set(E)) -> set(E).
maybe_expand_segs(T) when T#set.n =:= T#set.maxn ->
T#set{maxn = 2 * T#set.maxn,
bso = 2 * T#set.bso,
segs = expand_segs(T#set.segs, T#set.empty)};
maybe_expand_segs(T) -> T.
-spec maybe_contract(set(E), non_neg_integer()) -> set(E).
maybe_contract(T, Dc) when T#set.size - Dc < T#set.con_size,
T#set.n > ?seg_size ->
N = T#set.n,
Slot1 = N - T#set.bso,
Segs0 = T#set.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#set{size = T#set.size - Dc,
n = N1,
exp_size = N1 * ?expand_load,
con_size = N1 * ?contract_load,
segs = Segs2});
maybe_contract(T, Dc) -> T#set{size = T#set.size - Dc}.
-spec maybe_contract_segs(set(E)) -> set(E).
maybe_contract_segs(T) when T#set.n =:= T#set.bso ->
T#set{maxn = T#set.maxn div 2,
bso = T#set.bso div 2,
segs = contract_segs(T#set.segs)};
maybe_contract_segs(T) -> T.
%% rehash(Bucket, Slot1, Slot2, MaxN) -> {Bucket1,Bucket2}.
-spec rehash([T], integer(), pos_integer(), pos_integer()) -> {[T],[T]}.
rehash([E|T], Slot1, Slot2, MaxN) ->
{L1,L2} = rehash(T, Slot1, Slot2, MaxN),
case erlang:phash(E, MaxN) of
Slot1 -> {[E|L1],L2};
Slot2 -> {L1,[E|L2]}
end;
rehash([], _, _, _) -> {[],[]}.
%% mk_seg(Size) -> Segment.
-spec mk_seg(16) -> seg().
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.
-spec expand_segs(segs(E), seg()) -> segs(E).
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)).
-spec contract_segs(segs(E)) -> segs(E).
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)).