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Improve default seed to spread out the entropy over all 3 words #11165

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Improve default seed to spread out the entropy over all 3 words #11165

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16 changes: 13 additions & 3 deletions lib/stdlib/src/rand.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -1006,9 +1006,19 @@ seed_s(Alg_or_State) ->
end.

default_seed() ->
{erlang:phash2([{node(),self()}]),
erlang:system_time(),
erlang:unique_integer()}.
%% The purpose of this seed is that it shall be different
%% for different calls in a cluster even if they coincide in time.
%% High entropy is hard to find and not a primary goal.
%%
%% Luckily self() in external term format contains the node name
%% so the seed will differ between nodes as well as between processes.
%% unique_integer makes the seed differ between calls. system_time
%% gives some more entropy. MD5 is used to spread out the entropy
%% over all 3 seed integers.
%%
SeedTerm = {self(), erlang:unique_integer(), erlang:system_time()},
<<A:43, B:43, C:42>> = erlang:md5(term_to_binary(SeedTerm)),
{A, B, C}.

%% seed/2: seeds RNG with the algorithm and given values
%% and returns the NEW state.
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