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-- dab5caab05d89d03066ef92584660688595a3aaf by Mark Barolak <mbar@google.com>: Add absl::Status and absl::StatusOr to absl/README.md Import of https://github.com/abseil/abseil-cpp/pull/863 PiperOrigin-RevId: 347857368 -- 1ca3c7a96417cd6e6d62f4dc36fd5ddaa61cfa20 by Chris Kennelly <ckennelly@google.com>: Leverage integer power-of-2 functions and bit counting library in Abseil. PiperOrigin-RevId: 347816486 -- e5cbe05879fd65dce7875e2e0105331a1615d89b by Chris Kennelly <ckennelly@google.com>: Mitigate narrowing warning on MSVC. If sizeof(x) <= sizeof(uint32_t), no truncation occurs when casting to uint32_t, but the compiler cannot always determine this. PiperOrigin-RevId: 347696526 -- 079dff64cb175d282d9e22dfb4a522199ffdae2e by Benjamin Barenblat <bbaren@google.com>: Avoid libgcc -NaN narrowing bug When testing -NaN parsing, avoid narrowing -NaN from double to float. This avoids a bug in libgcc (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98251). PiperOrigin-RevId: 347654751 -- 2e78a7634865aeef6765e1f447e96cf8d9985059 by Chris Kennelly <ckennelly@google.com>: Mark popcount helpers as inline. These are conditionally constexpr, so we need to add inline to cover the non-constexpr builds to avoid ODR violations. PiperOrigin-RevId: 347620138 -- 437fbb363aea1654179f102dcdd607ec33c1af1e by Chris Kennelly <ckennelly@google.com>: Use explicit narrowing cast. This is never invoked in practice, but compilers with -Wimplicit-int-conversion may trigger when sizeof(T) > sizeof(uint16_t) prior to determining this never runs. PiperOrigin-RevId: 347609857 GitOrigin-RevId: dab5caab05d89d03066ef92584660688595a3aaf Change-Id: I6296ddffe7ec646f8ce121138f21e1e85a2cff4b
58 lines
1.9 KiB
C++
58 lines
1.9 KiB
C++
// 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/numeric/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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// 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 - countl_zero(n));
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}
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inline int IntLog2Ceil(uint64_t n) {
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return (n <= 1) ? 0 : (64 - countl_zero(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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} // 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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