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+// 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_FASTMATH_H_
+#define ABSL_RANDOM_INTERNAL_FASTMATH_H_
+
+// This file contains fast math functions (bitwise ops as well as some others)
+// which are implementation details of various absl random number distributions.
+
+#include <cassert>
+#include <cmath>
+#include <cstdint>
+
+#include "absl/base/internal/bits.h"
+
+namespace absl {
+namespace random_internal {
+
+// Returns the position of the first bit set.
+inline int LeadingSetBit(uint64_t n) {
+  return 64 - base_internal::CountLeadingZeros64(n);
+}
+
+// Compute log2(n) using integer operations.
+// While std::log2 is more accurate than std::log(n) / std::log(2), for
+// very large numbers--those close to std::numeric_limits<uint64_t>::max() - 2,
+// for instance--std::log2 rounds up rather than down, which introduces
+// definite skew in the results.
+inline int IntLog2Floor(uint64_t n) {
+  return (n <= 1) ? 0 : (63 - base_internal::CountLeadingZeros64(n));
+}
+inline int IntLog2Ceil(uint64_t n) {
+  return (n <= 1) ? 0 : (64 - base_internal::CountLeadingZeros64(n - 1));
+}
+
+inline double StirlingLogFactorial(double n) {
+  assert(n >= 1);
+  // Using Stirling's approximation.
+  constexpr double kLog2PI = 1.83787706640934548356;
+  const double logn = std::log(n);
+  const double ninv = 1.0 / static_cast<double>(n);
+  return n * logn - n + 0.5 * (kLog2PI + logn) + (1.0 / 12.0) * ninv -
+         (1.0 / 360.0) * ninv * ninv * ninv;
+}
+
+// Rotate value right.
+//
+// We only implement the uint32_t / uint64_t versions because
+// 1) those are the only ones we use, and
+// 2) those are the only ones where clang detects the rotate idiom correctly.
+inline constexpr uint32_t rotr(uint32_t value, uint8_t bits) {
+  return (value >> (bits & 31)) | (value << ((-bits) & 31));
+}
+inline constexpr uint64_t rotr(uint64_t value, uint8_t bits) {
+  return (value >> (bits & 63)) | (value << ((-bits) & 63));
+}
+
+}  // namespace random_internal
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_FASTMATH_H_