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Diffstat (limited to 'absl/random/internal/generate_real.h')
| -rw-r--r-- | absl/random/internal/generate_real.h | 146 |
1 files changed, 146 insertions, 0 deletions
diff --git a/absl/random/internal/generate_real.h b/absl/random/internal/generate_real.h new file mode 100644 index 00000000..20f6d208 --- /dev/null +++ b/absl/random/internal/generate_real.h @@ -0,0 +1,146 @@ +// 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_GENERATE_REAL_H_ +#define ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_ + +// This file contains some implementation details which are used by one or more +// of the absl random number distributions. + +#include <cstdint> +#include <cstring> +#include <limits> +#include <type_traits> + +#include "absl/base/internal/bits.h" +#include "absl/meta/type_traits.h" +#include "absl/random/internal/fastmath.h" +#include "absl/random/internal/traits.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace random_internal { + +// Tristate tag types controlling the output of GenerateRealFromBits. +struct GeneratePositiveTag {}; +struct GenerateNegativeTag {}; +struct GenerateSignedTag {}; + +// GenerateRealFromBits generates a single real value from a single 64-bit +// `bits` with template fields controlling the output. +// +// The `SignedTag` parameter controls whether positive, negative, +// or either signed/unsigned may be returned. +// When SignedTag == GeneratePositiveTag, range is U(0, 1) +// When SignedTag == GenerateNegativeTag, range is U(-1, 0) +// When SignedTag == GenerateSignedTag, range is U(-1, 1) +// +// When the `IncludeZero` parameter is true, the function may return 0 for some +// inputs, otherwise it never returns 0. +// +// When a value in U(0,1) is required, use: +// Uniform64ToReal<double, PositiveValueT, true>; +// +// When a value in U(-1,1) is required, use: +// Uniform64ToReal<double, SignedValueT, false>; +// +// This generates more distinct values than the mathematical equivalent +// `U(0, 1) * 2.0 - 1.0`. +// +// Scaling the result by powers of 2 (and avoiding a multiply) is also possible: +// GenerateRealFromBits<double>(..., -1); => U(0, 0.5) +// GenerateRealFromBits<double>(..., 1); => U(0, 2) +// +template <typename RealType, // Real type, either float or double. + typename SignedTag = GeneratePositiveTag, // Whether a positive, + // negative, or signed + // value is generated. + bool IncludeZero = true> +inline RealType GenerateRealFromBits(uint64_t bits, int exp_bias = 0) { + using real_type = RealType; + using uint_type = absl::conditional_t<std::is_same<real_type, float>::value, + uint32_t, uint64_t>; + + static_assert( + (std::is_same<double, real_type>::value || + std::is_same<float, real_type>::value), + "GenerateRealFromBits must be parameterized by either float or double."); + + static_assert(sizeof(uint_type) == sizeof(real_type), + "Mismatched unsinged and real types."); + + static_assert((std::numeric_limits<real_type>::is_iec559 && + std::numeric_limits<real_type>::radix == 2), + "RealType representation is not IEEE 754 binary."); + + static_assert((std::is_same<SignedTag, GeneratePositiveTag>::value || + std::is_same<SignedTag, GenerateNegativeTag>::value || + std::is_same<SignedTag, GenerateSignedTag>::value), + ""); + + static constexpr int kExp = std::numeric_limits<real_type>::digits - 1; + static constexpr uint_type kMask = (static_cast<uint_type>(1) << kExp) - 1u; + static constexpr int kUintBits = sizeof(uint_type) * 8; + + int exp = exp_bias + int{std::numeric_limits<real_type>::max_exponent - 2}; + + // Determine the sign bit. + // Depending on the SignedTag, this may use the left-most bit + // or it may be a constant value. + uint_type sign = std::is_same<SignedTag, GenerateNegativeTag>::value + ? (static_cast<uint_type>(1) << (kUintBits - 1)) + : 0; + if (std::is_same<SignedTag, GenerateSignedTag>::value) { + if (std::is_same<uint_type, uint64_t>::value) { + sign = bits & uint64_t{0x8000000000000000}; + } + if (std::is_same<uint_type, uint32_t>::value) { + const uint64_t tmp = bits & uint64_t{0x8000000000000000}; + sign = static_cast<uint32_t>(tmp >> 32); + } + // adjust the bits and the exponent to account for removing + // the leading bit. + bits = bits & uint64_t{0x7FFFFFFFFFFFFFFF}; + exp++; + } + if (IncludeZero) { + if (bits == 0u) return 0; + } + + // Number of leading zeros is mapped to the exponent: 2^-clz + // bits is 0..01xxxxxx. After shifting, we're left with 1xxx...0..0 + int clz = base_internal::CountLeadingZeros64(bits); + bits <<= (IncludeZero ? clz : (clz & 63)); // remove 0-bits. + exp -= clz; // set the exponent. + bits >>= (63 - kExp); + + // Construct the 32-bit or 64-bit IEEE 754 floating-point value from + // the individual fields: sign, exp, mantissa(bits). + uint_type val = + (std::is_same<SignedTag, GeneratePositiveTag>::value ? 0u : sign) | + (static_cast<uint_type>(exp) << kExp) | + (static_cast<uint_type>(bits) & kMask); + + // bit_cast to the output-type + real_type result; + memcpy(static_cast<void*>(&result), static_cast<const void*>(&val), + sizeof(result)); + return result; +} + +} // namespace random_internal +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_ |