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Date: Fri, 18 Mar 2005 21:42:42 +0300
Message-ID: <opsnujlgpklf5mcf@valyala>
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Subject: about mt_srand() function
From: valyala@gmail.com ("Alexander Valyalkin")

mt_srand() is used to seed initialisation vector (IV) for Mersenne Twister
pseudorandom generator. But there is a "little" problem: the actual length
of IV is 19968 bits, but mt_srand() accepts only 32-bit integers, which  
even
reduced to 31 bit in function php_mt_srand() at /ext/standard/rand.c at
string:
php_uint32 x = (seed | 1U) & 0xFFFFFFFFU
hint: lower bit of x is always equal to 1.

So, entorpy of current implementation of MT generator is 31 bit (i.e. it  
can
generate only 2^31-1 = 2147483647 different sequences), while its potential
entropy is significant higher - up 19937 bit (it is impossible to imagine
the number of different sequences 2^19937-1).

So, I think, it will be good idea to change mt_srand() function in that way
to accept not only one integer, but array of integers with length up to 624
elements to use Mersenne Twister generator in a full power. This change do
not break backward compatibility.

I can make nesessary changes and post unified diff, if you agree with me.

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