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Diffstat (limited to 'absl/random/internal/pcg_engine.h')
| -rw-r--r-- | absl/random/internal/pcg_engine.h | 305 |
1 files changed, 305 insertions, 0 deletions
diff --git a/absl/random/internal/pcg_engine.h b/absl/random/internal/pcg_engine.h new file mode 100644 index 000000000000..33fea0b9689f --- /dev/null +++ b/absl/random/internal/pcg_engine.h @@ -0,0 +1,305 @@ +// 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_ |