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69 lines
2.2 KiB
69 lines
2.2 KiB
/* Written in 2016 by David Blackman and Sebastiano Vigna (vigna@acm.org)
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To the extent possible under law, the author has dedicated all copyright
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and related and neighboring rights to this software to the public domain
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worldwide. This software is distributed without any warranty.
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See <http://creativecommons.org/publicdomain/zero/1.0/>. */
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#include <stdint.h>
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/* This is the successor to xorshift128+. It is the fastest full-period
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generator passing BigCrush without systematic failures, but due to the
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relatively short period it is acceptable only for applications with a
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mild amount of parallelism; otherwise, use a xorshift1024* generator.
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Beside passing BigCrush, this generator passes the PractRand test suite
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up to (and included) 16TB, with the exception of binary rank tests,
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which fail due to the lowest bit being an LFSR; all other bits pass all
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tests. We suggest to use a sign test to extract a random Boolean value.
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Note that the generator uses a simulated rotate operation, which most C
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compilers will turn into a single instruction. In Java, you can use
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Long.rotateLeft(). In languages that do not make low-level rotation
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instructions accessible xorshift128+ could be faster.
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The state must be seeded so that it is not everywhere zero. If you have
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a 64-bit seed, we suggest to seed a splitmix64 generator and use its
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output to fill s. */
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uint64_t s[2];
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static inline uint64_t rotl(const uint64_t x, int k) {
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return (x << k) | (x >> (64 - k));
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}
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uint64_t next(void) {
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const uint64_t s0 = s[0];
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uint64_t s1 = s[1];
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const uint64_t result = s0 + s1;
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s1 ^= s0;
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s[0] = rotl(s0, 55) ^ s1 ^ (s1 << 14); // a, b
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s[1] = rotl(s1, 36); // c
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return result;
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}
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/* This is the jump function for the generator. It is equivalent
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to 2^64 calls to next(); it can be used to generate 2^64
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non-overlapping subsequences for parallel computations. */
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void jump(void) {
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static const uint64_t JUMP[] = { 0xbeac0467eba5facb, 0xd86b048b86aa9922 };
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uint64_t s0 = 0;
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uint64_t s1 = 0;
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for(int i = 0; i < sizeof JUMP / sizeof *JUMP; i++)
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for(int b = 0; b < 64; b++) {
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if (JUMP[i] & 1ULL << b) {
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s0 ^= s[0];
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s1 ^= s[1];
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}
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next();
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}
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s[0] = s0;
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s[1] = s1;
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}
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