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Avoid system_limit crash on very large BVV gaps #3

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Avoid system_limit crash on very large BVV gaps #3

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16 changes: 13 additions & 3 deletions src/swc_node.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -125,13 +125,22 @@ add(BVV, {Id, Counter}) ->
Entry = maps:get(Id, BVV, {0,0}),
maps:put(Id, add_aux(Entry, Counter), BVV).

%% Maximum bitmap shift, kept under OTP's bignum system_limit (~4 Mbit).
%% Above this gap we advance the base directly instead of building a giant
%% bitmap, which would crash with {system_limit, ...} on `bsl`.
-define(MAX_BSL_BITS, 3000000).

%% @doc Adds a dot to a BVV entry, returning the normalized entry.
-spec add_aux(entry(), counter()) -> entry().
add_aux({N,B}, M) ->
case N < M of
false -> norm(N, B);
true -> M2 = B bor (1 bsl (M-N-1)),
norm(N, M2)
true ->
Gap = M - N - 1,
case Gap > ?MAX_BSL_BITS of
true -> {M, 0};
false -> norm(N, B bor (1 bsl Gap))
end
end.

%% @doc Merges all entries from the two BVVs.
Expand Down Expand Up @@ -253,7 +262,8 @@ add_aux_test() ->
?assertEqual( add_aux({5,3}, 4), {7,0} ),
?assertEqual( add_aux({2,5}, 4), {5,0} ),
?assertEqual( add_aux({2,5}, 6), {3,6} ),
?assertEqual( add_aux({2,4}, 6), {2,12} ).
?assertEqual( add_aux({2,4}, 6), {2,12} ),
?assertEqual( add_aux({0,0}, 5000000), {5000000, 0} ).

merge_test() ->
?assertEqual( merge( #{"a" => {5,3}} , #{"a" => {2,4}} ), #{"a" => {7,0}} ),
Expand Down