Abseil Common Libraries (C++) (grcp 依赖)
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74 lines
2.5 KiB
74 lines
2.5 KiB
// Copyright 2017 The Abseil Authors. |
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// |
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// Licensed under the Apache License, Version 2.0 (the "License"); |
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// you may not use this file except in compliance with the License. |
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// You may obtain a copy of the License at |
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// |
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// https://www.apache.org/licenses/LICENSE-2.0 |
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// |
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// Unless required by applicable law or agreed to in writing, software |
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// distributed under the License is distributed on an "AS IS" BASIS, |
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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// See the License for the specific language governing permissions and |
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// limitations under the License. |
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#ifndef ABSL_RANDOM_INTERNAL_FASTMATH_H_ |
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#define ABSL_RANDOM_INTERNAL_FASTMATH_H_ |
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// This file contains fast math functions (bitwise ops as well as some others) |
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// which are implementation details of various absl random number distributions. |
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#include <cassert> |
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#include <cmath> |
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#include <cstdint> |
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#include "absl/base/internal/bits.h" |
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namespace absl { |
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ABSL_NAMESPACE_BEGIN |
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namespace random_internal { |
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// Returns the position of the first bit set. |
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inline int LeadingSetBit(uint64_t n) { |
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return 64 - base_internal::CountLeadingZeros64(n); |
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} |
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// Compute log2(n) using integer operations. |
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// While std::log2 is more accurate than std::log(n) / std::log(2), for |
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// very large numbers--those close to std::numeric_limits<uint64_t>::max() - 2, |
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// for instance--std::log2 rounds up rather than down, which introduces |
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// definite skew in the results. |
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inline int IntLog2Floor(uint64_t n) { |
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return (n <= 1) ? 0 : (63 - base_internal::CountLeadingZeros64(n)); |
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} |
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inline int IntLog2Ceil(uint64_t n) { |
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return (n <= 1) ? 0 : (64 - base_internal::CountLeadingZeros64(n - 1)); |
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} |
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inline double StirlingLogFactorial(double n) { |
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assert(n >= 1); |
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// Using Stirling's approximation. |
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constexpr double kLog2PI = 1.83787706640934548356; |
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const double logn = std::log(n); |
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const double ninv = 1.0 / static_cast<double>(n); |
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return n * logn - n + 0.5 * (kLog2PI + logn) + (1.0 / 12.0) * ninv - |
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(1.0 / 360.0) * ninv * ninv * ninv; |
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} |
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// Rotate value right. |
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// |
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// We only implement the uint32_t / uint64_t versions because |
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// 1) those are the only ones we use, and |
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// 2) those are the only ones where clang detects the rotate idiom correctly. |
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inline constexpr uint32_t rotr(uint32_t value, uint8_t bits) { |
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return (value >> (bits & 31)) | (value << ((-bits) & 31)); |
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} |
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inline constexpr uint64_t rotr(uint64_t value, uint8_t bits) { |
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return (value >> (bits & 63)) | (value << ((-bits) & 63)); |
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} |
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} // namespace random_internal |
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ABSL_NAMESPACE_END |
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} // namespace absl |
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#endif // ABSL_RANDOM_INTERNAL_FASTMATH_H_
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