From e9324d926a9189e222741fce6e676f0944661a72 Mon Sep 17 00:00:00 2001 From: Abseil Team Date: Fri, 21 Jun 2019 13:11:42 -0700 Subject: Export of internal Abseil changes. -- 7a6ff16a85beb730c172d5d25cf1b5e1be885c56 by Laramie Leavitt : Internal change. PiperOrigin-RevId: 254454546 -- ff8f9bafaefc26d451f576ea4a06d150aed63f6f by Andy Soffer : Internal changes PiperOrigin-RevId: 254451562 -- deefc5b651b479ce36f0b4ef203e119c0c8936f2 by CJ Johnson : Account for subtracting unsigned values from the size of InlinedVector PiperOrigin-RevId: 254450625 -- 3c677316a27bcadc17e41957c809ca472d5fef14 by Andy Soffer : Add C++17's std::make_from_tuple to absl/utility/utility.h PiperOrigin-RevId: 254411573 -- 4ee3536a918830eeec402a28fc31a62c7c90b940 by CJ Johnson : Adds benchmark for the rest of the InlinedVector public API PiperOrigin-RevId: 254408378 -- e5a21a00700ee83498ff1efbf649169756463ee4 by CJ Johnson : Updates the definition of InlinedVector::shrink_to_fit() to be exception safe and adds exception safety tests for it. PiperOrigin-RevId: 254401387 -- 2ea82e72b86d82d78b4e4712a63a55981b53c64b by Laramie Leavitt : Use absl::InsecureBitGen in place of std::mt19937 in tests absl/random/...distribution_test.cc PiperOrigin-RevId: 254289444 -- fa099e02c413a7ffda732415e8105cad26a90337 by Andy Soffer : Internal changes PiperOrigin-RevId: 254286334 -- ce34b7f36933b30cfa35b9c9a5697a792b5666e4 by Andy Soffer : Internal changes PiperOrigin-RevId: 254273059 -- 6f9c473da7c2090c2e85a37c5f00622e8a912a89 by Jorg Brown : Change absl::container_internal::CompressedTuple to instantiate its internal Storage class with the name of the type it's holding, rather than the name of the Tuple. This is not an externally-visible change, other than less compiler memory is used and less debug information is generated. PiperOrigin-RevId: 254269285 -- 8bd3c186bf2fc0c55d8a2dd6f28a5327502c9fba by Andy Soffer : Adding short-hand IntervalClosed for IntervalClosedClosed and IntervalOpen for IntervalOpenOpen. PiperOrigin-RevId: 254252419 -- ea957f99b6a04fccd42aa05605605f3b44b1ecfd by Abseil Team : Do not directly use __SIZEOF_INT128__. In order to avoid linker errors when building with clang-cl (__fixunsdfti, __udivti3 and __fixunssfti are undefined), this CL uses ABSL_HAVE_INTRINSIC_INT128 which is not defined for clang-cl. PiperOrigin-RevId: 254250739 -- 89ab385cd26b34d64130bce856253aaba96d2345 by Andy Soffer : Internal changes PiperOrigin-RevId: 254242321 -- cffc793d93eca6d6bdf7de733847b6ab4a255ae9 by CJ Johnson : Adds benchmark for InlinedVector::reserve(size_type) PiperOrigin-RevId: 254199226 -- c90c7a9fa3c8f0c9d5114036979548b055ea2f2a by Gennadiy Rozental : Import of CCTZ from GitHub. PiperOrigin-RevId: 254072387 -- c4c388beae016c9570ab54ffa1d52660e4a85b7b by Laramie Leavitt : Internal cleanup. PiperOrigin-RevId: 254062381 -- d3c992e221cc74e5372d0c8fa410170b6a43c062 by Tom Manshreck : Update distributions.h to Abseil standards PiperOrigin-RevId: 254054946 -- d15ad0035c34ef11b14fadc5a4a2d3ec415f5518 by CJ Johnson : Removes functions with only one caller from the implementation details of InlinedVector by manually inlining the definitions PiperOrigin-RevId: 254005427 -- 2f37e807efc3a8ef1f4b539bdd379917d4151520 by Andy Soffer : Initial release of Abseil Random PiperOrigin-RevId: 253999861 -- 24ed1694b6430791d781ed533a8f8ccf6cac5856 by CJ Johnson : Updates the definition of InlinedVector::assign(...)/InlinedVector::operator=(...) to new, exception-safe implementations with exception safety tests to boot PiperOrigin-RevId: 253993691 -- 5613d95f5a7e34a535cfaeadce801441e990843e by CJ Johnson : Adds benchmarks for InlinedVector::shrink_to_fit() PiperOrigin-RevId: 253989647 -- 2a96ddfdac40bbb8cb6a7f1aeab90917067c6e63 by Abseil Team : Initial release of Abseil Random PiperOrigin-RevId: 253927497 -- bf1aff8fc9ffa921ad74643e9525ecf25b0d8dc1 by Andy Soffer : Initial release of Abseil Random PiperOrigin-RevId: 253920512 -- bfc03f4a3dcda3cf3a4b84bdb84cda24e3394f41 by Laramie Leavitt : Internal change. PiperOrigin-RevId: 253886486 -- 05036cfcc078ca7c5f581a00dfb0daed568cbb69 by Eric Fiselier : Don't include `winsock2.h` because it drags in `windows.h` and friends, and they define awful macros like OPAQUE, ERROR, and more. This has the potential to break abseil users. Instead we only forward declare `timeval` and require Windows users include `winsock2.h` themselves. This is both inconsistent and poor QoI, but so including 'windows.h' is bad too. PiperOrigin-RevId: 253852615 GitOrigin-RevId: 7a6ff16a85beb730c172d5d25cf1b5e1be885c56 Change-Id: Icd6aff87da26f29ec8915da856f051129987cef6 --- absl/random/internal/fast_uniform_bits.h | 299 +++++++++++++++++++++++++++++++ 1 file changed, 299 insertions(+) create mode 100644 absl/random/internal/fast_uniform_bits.h (limited to 'absl/random/internal/fast_uniform_bits.h') diff --git a/absl/random/internal/fast_uniform_bits.h b/absl/random/internal/fast_uniform_bits.h new file mode 100644 index 0000000..23eabbc --- /dev/null +++ b/absl/random/internal/fast_uniform_bits.h @@ -0,0 +1,299 @@ +// Copyright 2017 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_INTERNAL_FAST_UNIFORM_BITS_H_ +#define ABSL_RANDOM_INTERNAL_FAST_UNIFORM_BITS_H_ + +#include +#include +#include +#include + +namespace absl { +namespace random_internal { +// Computes the length of the range of values producible by the URBG, or returns +// zero if that would encompass the entire range of representable values in +// URBG::result_type. +template +constexpr typename URBG::result_type constexpr_range() { + using result_type = typename URBG::result_type; + return ((URBG::max)() == (std::numeric_limits::max)() && + (URBG::min)() == std::numeric_limits::lowest()) + ? result_type{0} + : (URBG::max)() - (URBG::min)() + result_type{1}; +} + +// FastUniformBits implements a fast path to acquire uniform independent bits +// from a type which conforms to the [rand.req.urbg] concept. +// Parameterized by: +// `UIntType`: the result (output) type +// `Width`: binary output width +// +// The std::independent_bits_engine [rand.adapt.ibits] adaptor can be +// instantiated from an existing generator through a copy or a move. It does +// not, however, facilitate the production of pseudorandom bits from an un-owned +// generator that will outlive the std::independent_bits_engine instance. +template ::digits> +class FastUniformBits { + static_assert(std::is_unsigned::value, + "Class-template FastUniformBits<> must be parameterized using " + "an unsigned type."); + + // `kWidth` is the width, in binary digits, of the output. By default it is + // the number of binary digits in the `result_type`. + static constexpr size_t kWidth = Width; + static_assert(kWidth > 0, + "Class-template FastUniformBits<> Width argument must be > 0"); + + static_assert(kWidth <= std::numeric_limits::digits, + "Class-template FastUniformBits<> Width argument must be <= " + "width of UIntType."); + + static constexpr bool kIsMaxWidth = + (kWidth >= std::numeric_limits::digits); + + // Computes a mask of `n` bits for the `UIntType`. + static constexpr UIntType constexpr_mask(size_t n) { + return (UIntType(1) << n) - 1; + } + + public: + using result_type = UIntType; + + static constexpr result_type(min)() { return 0; } + static constexpr result_type(max)() { + return kIsMaxWidth ? (std::numeric_limits::max)() + : constexpr_mask(kWidth); + } + + template + result_type operator()(URBG& g); // NOLINT(runtime/references) + + private: + // Variate() generates a single random variate, always returning a value + // in the closed interval [0 ... FastUniformBitsURBGConstants::kRangeMask] + // (kRangeMask+1 is a power of 2). + template + typename URBG::result_type Variate(URBG& g); // NOLINT(runtime/references) + + // generate() generates a random value, dispatched on whether + // the underlying URNG must loop over multiple calls or not. + template + result_type Generate(URBG& g, // NOLINT(runtime/references) + std::true_type /* avoid_looping */); + + template + result_type Generate(URBG& g, // NOLINT(runtime/references) + std::false_type /* avoid_looping */); +}; + +// FastUniformBitsURBGConstants computes the URBG-derived constants used +// by FastUniformBits::Generate and FastUniformBits::Variate. +// Parameterized by the FastUniformBits parameter: +// `URBG`: The underlying UniformRandomNumberGenerator. +// +// The values here indicate the URBG range as well as providing an indicator +// whether the URBG output is a power of 2, and kRangeMask, which allows masking +// the generated output to kRangeBits. +template +class FastUniformBitsURBGConstants { + // Computes the floor of the log. (i.e., std::floor(std::log2(N)); + static constexpr size_t constexpr_log2(size_t n) { + return (n <= 1) ? 0 : 1 + constexpr_log2(n / 2); + } + + // Computes a mask of n bits for the URBG::result_type. + static constexpr typename URBG::result_type constexpr_mask(size_t n) { + return (typename URBG::result_type(1) << n) - 1; + } + + public: + using result_type = typename URBG::result_type; + + // The range of the URNG, max - min + 1, or zero if that result would cause + // overflow. + static constexpr result_type kRange = constexpr_range(); + + static constexpr bool kPowerOfTwo = + (kRange == 0) || ((kRange & (kRange - 1)) == 0); + + // kRangeBits describes the number number of bits suitable to mask off of URNG + // variate, which is: + // kRangeBits = floor(log2(kRange)) + static constexpr size_t kRangeBits = + kRange == 0 ? std::numeric_limits::digits + : constexpr_log2(kRange); + + // kRangeMask is the mask used when sampling variates from the URNG when the + // width of the URNG range is not a power of 2. + // Y = (2 ^ kRange) - 1 + static constexpr result_type kRangeMask = + kRange == 0 ? (std::numeric_limits::max)() + : constexpr_mask(kRangeBits); + + static_assert((URBG::max)() != (URBG::min)(), + "Class-template FastUniformBitsURBGConstants<> " + "URBG::max and URBG::min may not be equal."); + + static_assert(std::is_unsigned::value, + "Class-template FastUniformBitsURBGConstants<> " + "URBG::result_type must be unsigned."); + + static_assert(kRangeMask > 0, + "Class-template FastUniformBitsURBGConstants<> " + "URBG does not generate sufficient random bits."); + + static_assert(kRange == 0 || + kRangeBits < std::numeric_limits::digits, + "Class-template FastUniformBitsURBGConstants<> " + "URBG range computation error."); +}; + +// FastUniformBitsLoopingConstants computes the looping constants used +// by FastUniformBits::Generate. These constants indicate how multiple +// URBG::result_type values are combined into an output_value. +// Parameterized by the FastUniformBits parameters: +// `UIntType`: output type. +// `Width`: binary output width, +// `URNG`: The underlying UniformRandomNumberGenerator. +// +// The looping constants describe the sets of loop counters and mask values +// which control how individual variates are combined the final output. The +// algorithm ensures that the number of bits used by any individual call differs +// by at-most one bit from any other call. This is simplified into constants +// which describe two loops, with the second loop parameters providing one extra +// bit per variate. +// +// See [rand.adapt.ibits] for more details on the use of these constants. +template +class FastUniformBitsLoopingConstants { + private: + static constexpr size_t kWidth = Width; + using urbg_result_type = typename URBG::result_type; + using uint_result_type = UIntType; + + public: + using result_type = + typename std::conditional<(sizeof(urbg_result_type) <= + sizeof(uint_result_type)), + uint_result_type, urbg_result_type>::type; + + private: + // Estimate N as ceil(width / urng width), and W0 as (width / N). + static constexpr size_t kRangeBits = + FastUniformBitsURBGConstants::kRangeBits; + + // The range of the URNG, max - min + 1, or zero if that result would cause + // overflow. + static constexpr result_type kRange = constexpr_range(); + static constexpr size_t kEstimateN = + kWidth / kRangeBits + (kWidth % kRangeBits != 0); + static constexpr size_t kEstimateW0 = kWidth / kEstimateN; + static constexpr result_type kEstimateY0 = (kRange >> kEstimateW0) + << kEstimateW0; + + public: + // Parameters for the two loops: + // kN0, kN1 are the number of underlying calls required for each loop. + // KW0, kW1 are shift widths for each loop. + // + static constexpr size_t kN1 = (kRange - kEstimateY0) > + (kEstimateY0 / kEstimateN) + ? kEstimateN + 1 + : kEstimateN; + static constexpr size_t kN0 = kN1 - (kWidth % kN1); + static constexpr size_t kW0 = kWidth / kN1; + static constexpr size_t kW1 = kW0 + 1; + + static constexpr result_type kM0 = (result_type(1) << kW0) - 1; + static constexpr result_type kM1 = (result_type(1) << kW1) - 1; + + static_assert( + kW0 <= kRangeBits, + "Class-template FastUniformBitsLoopingConstants::kW0 too large."); + + static_assert( + kW0 > 0, + "Class-template FastUniformBitsLoopingConstants::kW0 too small."); +}; + +template +template +typename FastUniformBits::result_type +FastUniformBits::operator()( + URBG& g) { // NOLINT(runtime/references) + using constants = FastUniformBitsURBGConstants; + return Generate( + g, std::integral_constant= (max)()>{}); +} + +template +template +typename URBG::result_type FastUniformBits::Variate( + URBG& g) { // NOLINT(runtime/references) + using constants = FastUniformBitsURBGConstants; + if (constants::kPowerOfTwo) { + return g() - (URBG::min)(); + } + + // Use rejection sampling to ensure uniformity across the range. + typename URBG::result_type u; + do { + u = g() - (URBG::min)(); + } while (u > constants::kRangeMask); + return u; +} + +template +template +typename FastUniformBits::result_type +FastUniformBits::Generate( + URBG& g, // NOLINT(runtime/references) + std::true_type /* avoid_looping */) { + // The width of the result_type is less than than the width of the random bits + // provided by URNG. Thus, generate a single value and then simply mask off + // the required bits. + return Variate(g) & (max)(); +} + +template +template +typename FastUniformBits::result_type +FastUniformBits::Generate( + URBG& g, // NOLINT(runtime/references) + std::false_type /* avoid_looping */) { + // The width of the result_type is wider than the number of random bits + // provided by URNG. Thus we merge several variates of URNG into the result + // using a shift and mask. The constants type generates the parameters used + // ensure that the bits are distributed across all the invocations of the + // underlying URNG. + using constants = FastUniformBitsLoopingConstants; + + result_type s = 0; + for (size_t n = 0; n < constants::kN0; ++n) { + auto u = Variate(g); + s = (s << constants::kW0) + (u & constants::kM0); + } + for (size_t n = constants::kN0; n < constants::kN1; ++n) { + auto u = Variate(g); + s = (s << constants::kW1) + (u & constants::kM1); + } + return s; +} + +} // namespace random_internal +} // namespace absl + +#endif // ABSL_RANDOM_INTERNAL_FAST_UNIFORM_BITS_H_ -- cgit v1.2.3