Add comments from reference impl of xoroshiro128+

src/recgen.cpp

@@ -27,6 +27,46 @@
 namespace {  // anonymous namespace
 
 
+/**
+ * Pseudo random number generator "xoroshiro128+"
+ *
+ * This code is based on the reference implementation,
+ * modified by Joris van Rantwijk to fit in a C++ class.
+ *
+ * Source: http://prng.di.unimi.it/
+ *
+ * The following commpents apply to the reference implementation:
+ *
+ * Written in 2016-2018 by David Blackman and Sebastiano Vigna (vigna@acm.org)
+ *
+ * To the extent possible under law, the author has dedicated all copyright
+ * and related and neighboring rights to this software to the public domain
+ * worldwide. This software is distributed without any warranty.
+ *
+ * See <http://creativecommons.org/publicdomain/zero/1.0/>.
+ *
+ * This is xoroshiro128+ 1.0, our best and fastest small-state generator
+ * for floating-point numbers. We suggest to use its upper bits for
+ * floating-point generation, as it is slightly faster than
+ * xoroshiro128++/xoroshiro128**. It passes all tests we are aware of
+ * except for the four lower bits, which might fail linearity tests (and
+ * just those), so if low linear complexity is not considered an issue (as
+ * it is usually the case) it can be used to generate 64-bit outputs, too;
+ * moreover, this generator has a very mild Hamming-weight dependency
+ * making our test (http://prng.di.unimi.it/hwd.php) fail after 5 TB of
+ * output; we believe this slight bias cannot affect any application. If
+ * you are concerned, use xoroshiro128++, xoroshiro128** or xoshiro256+.
+ *
+ * We suggest to use a sign test to extract a random Boolean value, and
+ * right shifts to extract subsets of bits.
+ *
+ * The state must be seeded so that it is not everywhere zero. If you have
+ * a 64-bit seed, we suggest to seed a splitmix64 generator and use its
+ * output to fill s.
+ *
+ * NOTE: the parameters (a=24, b=16, b=37) of this version give slightly
+ * better results in our test than the 2016 version (a=55, b=14, c=36).
+ */
 class Xoroshiro128plus
 {
 public: