// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef ABSL_RANDOM_PCG_ENGINE_H_
#define ABSL_RANDOM_PCG_ENGINE_H_
#include <type_traits>
#include "absl/base/config.h"
#include "absl/meta/type_traits.h"
#include "absl/numeric/int128.h"
#include "absl/random/internal/fastmath.h"
#include "absl/random/internal/iostream_state_saver.h"
namespace absl {
namespace random_internal {
// pcg_engine is a simplified implementation of Melissa O'Neil's PCG engine in
// C++. PCG combines a linear congruential generator (LCG) with output state
// mixing functions to generate each random variate. pcg_engine supports only a
// single sequence (oneseq), and does not support streams.
//
// pcg_engine is parameterized by two types:
// Params, which provides the multiplier and increment values;
// Mix, which mixes the state into the result.
//
template <typename Params, typename Mix>
class pcg_engine {
static_assert(std::is_same<typename Params::state_type,
typename Mix::state_type>::value,
"Class-template absl::pcg_engine must be parameterized by "
"Params and Mix with identical state_type");
static_assert(std::is_unsigned<typename Mix::result_type>::value,
"Class-template absl::pcg_engine must be parameterized by "
"an unsigned Mix::result_type");
using params_type = Params;
using mix_type = Mix;
using state_type = typename Mix::state_type;
public:
// C++11 URBG interface:
using result_type = typename Mix::result_type;
static constexpr result_type(min)() {
return (std::numeric_limits<result_type>::min)();
}
static constexpr result_type(max)() {
return (std::numeric_limits<result_type>::max)();
}
explicit pcg_engine(uint64_t seed_value = 0) { seed(seed_value); }
template <class SeedSequence,
typename = typename absl::enable_if_t<
!std::is_same<SeedSequence, pcg_engine>::value>>
explicit pcg_engine(SeedSequence&& seq) {
seed(seq);
}
pcg_engine(const pcg_engine&) = default;
pcg_engine& operator=(const pcg_engine&) = default;
pcg_engine(pcg_engine&&) = default;
pcg_engine& operator=(pcg_engine&&) = default;
result_type operator()() {
// Advance the LCG state, always using the new value to generate the output.
state_ = lcg(state_);
return Mix{}(state_);
}
void seed(uint64_t seed_value = 0) {
state_type tmp = seed_value;
state_ = lcg(tmp + Params::increment());
}
template <class SeedSequence>
typename absl::enable_if_t<
!std::is_convertible<SeedSequence, uint64_t>::value, void>
seed(SeedSequence&& seq) {
reseed(seq);
}
void discard(uint64_t count) { state_ = advance(state_, count); }
bool operator==(const pcg_engine& other) const {
return state_ == other.state_;
}
bool operator!=(const pcg_engine& other) const { return !(*this == other); }
template <class CharT, class Traits>
friend typename absl::enable_if_t<(sizeof(state_type) == 16),
std::basic_ostream<CharT, Traits>&>
operator<<(
std::basic_ostream<CharT, Traits>& os, // NOLINT(runtime/references)
const pcg_engine& engine) {
auto saver = random_internal::make_ostream_state_saver(os);
random_internal::stream_u128_helper<state_type> helper;
helper.write(pcg_engine::params_type::multiplier(), os);
os << os.fill();
helper.write(pcg_engine::params_type::increment(), os);
os << os.fill();
helper.write(engine.state_, os);
return os;
}
template <class CharT, class Traits>
friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
std::basic_ostream<CharT, Traits>&>
operator<<(
std::basic_ostream<CharT, Traits>& os, // NOLINT(runtime/references)
const pcg_engine& engine) {
auto saver = random_internal::make_ostream_state_saver(os);
os << pcg_engine::params_type::multiplier() << os.fill();
os << pcg_engine::params_type::increment() << os.fill();
os << engine.state_;
return os;
}
template <class CharT, class Traits>
friend typename absl::enable_if_t<(sizeof(state_type) == 16),
std::basic_istream<CharT, Traits>&>
operator>>(
std::basic_istream<CharT, Traits>& is, // NOLINT(runtime/references)
pcg_engine& engine) { // NOLINT(runtime/references)
random_internal::stream_u128_helper<state_type> helper;
auto mult = helper.read(is);
auto inc = helper.read(is);
auto tmp = helper.read(is);
if (mult != pcg_engine::params_type::multiplier() ||
inc != pcg_engine::params_type::increment()) {
// signal failure by setting the failbit.
is.setstate(is.rdstate() | std::ios_base::failbit);
}
if (!is.fail()) {
engine.state_ = tmp;
}
return is;
}
template <class CharT, class Traits>
friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
std::basic_istream<CharT, Traits>&>
operator>>(
std::basic_istream<CharT, Traits>& is, // NOLINT(runtime/references)
pcg_engine& engine) { // NOLINT(runtime/references)
state_type mult{}, inc{}, tmp{};
is >> mult >> inc >> tmp;
if (mult != pcg_engine::params_type::multiplier() ||
inc != pcg_engine::params_type::increment()) {
// signal failure by setting the failbit.
is.setstate(is.rdstate() | std::ios_base::failbit);
}
if (!is.fail()) {
engine.state_ = tmp;
}
return is;
}
private:
state_type state_;
// Returns the linear-congruential generator next state.
static inline constexpr state_type lcg(state_type s) {
return s * Params::multiplier() + Params::increment();
}
// Returns the linear-congruential arbitrary seek state.
inline state_type advance(state_type s, uint64_t n) const {
state_type mult = Params::multiplier();
state_type inc = Params::increment();
state_type m = 1;
state_type i = 0;
while (n > 0) {
if (n & 1) {
m *= mult;
i = i * mult + inc;
}
inc = (mult + 1) * inc;
mult *= mult;
n >>= 1;
}
return m * s + i;
}
template <class SeedSequence>
void reseed(SeedSequence& seq) {
using sequence_result_type = typename SeedSequence::result_type;
constexpr size_t kBufferSize =
sizeof(state_type) / sizeof(sequence_result_type);
sequence_result_type buffer[kBufferSize];
seq.generate(std::begin(buffer), std::end(buffer));
// Convert the seed output to a single state value.
state_type tmp = buffer[0];
for (size_t i = 1; i < kBufferSize; i++) {
tmp <<= (sizeof(sequence_result_type) * 8);
tmp |= buffer[i];
}
state_ = lcg(tmp + params_type::increment());
}
};
// Parameterized implementation of the PCG 128-bit oneseq state.
// This provides state_type, multiplier, and increment for pcg_engine.
template <uint64_t kMultA, uint64_t kMultB, uint64_t kIncA, uint64_t kIncB>
class pcg128_params {
public:
#if ABSL_HAVE_INTRINSIC_INT128
using state_type = __uint128_t;
static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
return (static_cast<__uint128_t>(a) << 64) | b;
}
#else
using state_type = absl::uint128;
static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
return absl::MakeUint128(a, b);
}
#endif
static inline constexpr state_type multiplier() {
return make_u128(kMultA, kMultB);
}
static inline constexpr state_type increment() {
return make_u128(kIncA, kIncB);
}
};
// Implementation of the PCG xsl_rr_128_64 128-bit mixing function, which
// accepts an input of state_type and mixes it into an output of result_type.
struct pcg_xsl_rr_128_64 {
#if ABSL_HAVE_INTRINSIC_INT128
using state_type = __uint128_t;
#else
using state_type = absl::uint128;
#endif
using result_type = uint64_t;
inline uint64_t operator()(state_type state) {
// This is equivalent to the xsl_rr_128_64 mixing function.
#if ABSL_HAVE_INTRINSIC_INT128
uint64_t rotate = static_cast<uint64_t>(state >> 122u);
state ^= state >> 64;
uint64_t s = static_cast<uint64_t>(state);
#else
uint64_t h = Uint128High64(state);
uint64_t rotate = h >> 58u;
uint64_t s = Uint128Low64(state) ^ h;
#endif
return random_internal::rotr(s, rotate);
}
};
// Parameterized implementation of the PCG 64-bit oneseq state.
// This provides state_type, multiplier, and increment for pcg_engine.
template <uint64_t kMult, uint64_t kInc>
class pcg64_params {
public:
using state_type = uint64_t;
static inline constexpr state_type multiplier() { return kMult; }
static inline constexpr state_type increment() { return kInc; }
};
// Implementation of the PCG xsh_rr_64_32 64-bit mixing function, which accepts
// an input of state_type and mixes it into an output of result_type.
struct pcg_xsh_rr_64_32 {
using state_type = uint64_t;
using result_type = uint32_t;
inline uint32_t operator()(uint64_t state) {
return random_internal::rotr(
static_cast<uint32_t>(((state >> 18) ^ state) >> 27), state >> 59);
}
};
// Stable pcg_engine implementations:
// This is a 64-bit generator using 128-bits of state.
// The output sequence is equivalent to Melissa O'Neil's pcg64_oneseq.
using pcg64_2018_engine = pcg_engine<
random_internal::pcg128_params<0x2360ed051fc65da4ull, 0x4385df649fccf645ull,
0x5851f42d4c957f2d, 0x14057b7ef767814f>,
random_internal::pcg_xsl_rr_128_64>;
// This is a 32-bit generator using 64-bits of state.
// This is equivalent to Melissa O'Neil's pcg32_oneseq.
using pcg32_2018_engine = pcg_engine<
random_internal::pcg64_params<0x5851f42d4c957f2dull, 0x14057b7ef767814full>,
random_internal::pcg_xsh_rr_64_32>;
} // namespace random_internal
} // namespace absl
#endif // ABSL_RANDOM_PCG2018_ENGINE_H_
