blob: 71516ee667dcc0362a30e7a437b4824040fc7612 [file] [log] [blame]
// Copyright 2016 The Fuchsia Authors
// Copyright (c) 2008-2014 Travis Geiselbrecht
//
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT
#include <stdlib.h>
#include <ktl/atomic.h>
namespace {
template <size_t = sizeof(uintptr_t)>
struct Prng;
template <>
struct Prng<4> {
[[maybe_unused]] static constexpr uint32_t kMultiplier = 1664525;
[[maybe_unused]] static constexpr uint32_t kIncrement = 1013904223;
[[maybe_unused]] static constexpr int kRandShift = 0;
};
template <>
struct Prng<8> {
[[maybe_unused]] static constexpr uint64_t kMultiplier = 6364136223846793005;
[[maybe_unused]] static constexpr uint64_t kIncrement = 1;
[[maybe_unused]] static constexpr int kRandShift = 33;
};
uintptr_t TrivialPrng(uintptr_t old_seed) {
return (Prng<>::kMultiplier * old_seed) + Prng<>::kIncrement;
}
int TrivialRand(uintptr_t prng) { return static_cast<int>(prng >> Prng<>::kRandShift); }
ktl::atomic<uintptr_t> gPrng;
} // namespace
void srand(unsigned int seed) { gPrng.store(seed - 1, ktl::memory_order::relaxed); }
// rand_r doesn't need to access the state atomically.
int rand_r(uintptr_t* seed) {
*seed = TrivialPrng(*seed);
return TrivialRand(*seed);
}
int rand() {
uintptr_t new_seed, old_seed = gPrng.load(ktl::memory_order::relaxed);
do {
new_seed = TrivialPrng(old_seed);
} while (!gPrng.compare_exchange_weak(old_seed, new_seed, ktl::memory_order::relaxed));
return TrivialRand(new_seed);
}