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Change evutil_weakrand_() to avoid platform random()
This change allows us to avoid perturbing the platform's random(), and to avoid hitting locks on random() in the platform's libc. evutil_weakrand_() is, well, weak, so we choose here an algorithm that favors speed over a number of other possibly desirable properties. We're using a linear congruential generator, and taking our parameters from those shared by the OpenBSD random() implementation, and Glibc's fastest random() implementation. The low bits of a LCG of modulus 2^32 are (notoriously) less random than the higher bits. So to generate a random value in a range, using the % operator is no good; we ought to divide. We add an evutil_weakrand_range_() function to do that. This code also changes the interface of evutil_weakrand_() so that it now manipulates an explicit seed, rather than having the seed in a static variable. This change enables us to use existing locks to achieve thread-safety, rather than having to rely on an additional lock. (Patch by Nicholas Marriott; commit message by Nick Mathewson.)
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