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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_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 #define ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 
 #ifndef _WIN32
 #include <sys/types.h>
 #endif
 
 #include <algorithm>
 #include <chrono>  // NOLINT(build/c++11)
 #include <cstdint>
 #include <ctime>
 #include <limits>
 
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // An optional timeout, with nanosecond granularity.
 //
 // This is a private low-level API for use by a handful of low-level
 // components. Higher-level components should build APIs based on
 // absl::Time and absl::Duration.
 class KernelTimeout {
  public:
   // Construct an absolute timeout that should expire at `t`.
   explicit KernelTimeout(absl::Time t);
 
   // Construct a relative timeout that should expire after `d`.
   explicit KernelTimeout(absl::Duration d);
 
   // Infinite timeout.
   constexpr KernelTimeout() : rep_(kNoTimeout) {}
 
   // A more explicit factory for those who prefer it.
   // Equivalent to `KernelTimeout()`.
   static constexpr KernelTimeout Never() { return KernelTimeout(); }
 
   // Returns true if there is a timeout that will eventually expire.
   // Returns false if the timeout is infinite.
   bool has_timeout() const { return rep_ != kNoTimeout; }
 
   // If `has_timeout()` is true, returns true if the timeout was provided as an
   // `absl::Time`. The return value is undefined if `has_timeout()` is false
   // because all indefinite timeouts are equivalent.
   bool is_absolute_timeout() const { return (rep_ & 1) == 0; }
 
   // If `has_timeout()` is true, returns true if the timeout was provided as an
   // `absl::Duration`. The return value is undefined if `has_timeout()` is false
   // because all indefinite timeouts are equivalent.
   bool is_relative_timeout() const { return (rep_ & 1) == 1; }
 
   // Convert to `struct timespec` for interfaces that expect an absolute
   // timeout. If !has_timeout() or is_relative_timeout(), attempts to convert to
   // a reasonable absolute timeout, but callers should to test has_timeout() and
   // is_relative_timeout() and prefer to use a more appropriate interface.
   struct timespec MakeAbsTimespec() const;
 
   // Convert to `struct timespec` for interfaces that expect a relative
   // timeout. If !has_timeout() or is_absolute_timeout(), attempts to convert to
   // a reasonable relative timeout, but callers should to test has_timeout() and
   // is_absolute_timeout() and prefer to use a more appropriate interface. Since
   // the return value is a relative duration, it should be recomputed by calling
   // this method in the case of a spurious wakeup.
   struct timespec MakeRelativeTimespec() const;
 
 #ifndef _WIN32
   // Convert to `struct timespec` for interfaces that expect an absolute timeout
   // on a specific clock `c`. This is similar to `MakeAbsTimespec()`, but
   // callers usually want to use this method with `CLOCK_MONOTONIC` when
   // relative timeouts are requested, and when the appropriate interface expects
   // an absolute timeout relative to a specific clock (for example,
   // pthread_cond_clockwait() or sem_clockwait()). If !has_timeout(), attempts
   // to convert to a reasonable absolute timeout, but callers should to test
   // has_timeout() prefer to use a more appropriate interface.
   struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
 #endif
 
   // Convert to unix epoch nanos for interfaces that expect an absolute timeout
   // in nanoseconds. If !has_timeout() or is_relative_timeout(), attempts to
   // convert to a reasonable absolute timeout, but callers should to test
   // has_timeout() and is_relative_timeout() and prefer to use a more
   // appropriate interface.
   int64_t MakeAbsNanos() const;
 
   // Converts to milliseconds from now, or INFINITE when
   // !has_timeout(). For use by SleepConditionVariableSRW on
   // Windows. Callers should recognize that the return value is a
   // relative duration (it should be recomputed by calling this method
   // in the case of a spurious wakeup).
   // This header file may be included transitively by public header files,
   // so we define our own DWORD and INFINITE instead of getting them from
   // <intsafe.h> and <WinBase.h>.
   typedef unsigned long DWord;  // NOLINT
   DWord InMillisecondsFromNow() const;
 
   // Convert to std::chrono::time_point for interfaces that expect an absolute
   // timeout, like std::condition_variable::wait_until(). If !has_timeout() or
   // is_relative_timeout(), attempts to convert to a reasonable absolute
   // timeout, but callers should test has_timeout() and is_relative_timeout()
   // and prefer to use a more appropriate interface.
   std::chrono::time_point<std::chrono::system_clock> ToChronoTimePoint() const;
 
   // Convert to std::chrono::time_point for interfaces that expect a relative
   // timeout, like std::condition_variable::wait_for(). If !has_timeout() or
   // is_absolute_timeout(), attempts to convert to a reasonable relative
   // timeout, but callers should test has_timeout() and is_absolute_timeout()
   // and prefer to use a more appropriate interface. Since the return value is a
   // relative duration, it should be recomputed by calling this method in the
   // case of a spurious wakeup.
   std::chrono::nanoseconds ToChronoDuration() const;
 
   // Returns true if steady (aka monotonic) clocks are supported by the system.
   // This method exists because go/btm requires synchronized clocks, and
   // thus requires we use the system (aka walltime) clock.
   static constexpr bool SupportsSteadyClock() { return true; }
 
  private:
   // Returns the current time, expressed as a count of nanoseconds since the
   // epoch used by an arbitrary clock. The implementation tries to use a steady
   // (monotonic) clock if one is available.
   static int64_t SteadyClockNow();
 
   // Internal representation.
   //   - If the value is kNoTimeout, then the timeout is infinite, and
   //     has_timeout() will return true.
   //   - If the low bit is 0, then the high 63 bits is the number of nanoseconds
   //     after the unix epoch.
   //   - If the low bit is 1, then the high 63 bits is the number of nanoseconds
   //     after the epoch used by SteadyClockNow().
   //
   // In all cases the time is stored as an absolute time, the only difference is
   // the clock epoch. The use of absolute times is important since in the case
   // of a relative timeout with a spurious wakeup, the program would have to
   // restart the wait, and thus needs a way of recomputing the remaining time.
   uint64_t rep_;
 
   // Returns the number of nanoseconds stored in the internal representation.
   // When combined with the clock epoch indicated by the low bit (which is
   // accessed through is_absolute_timeout() and is_relative_timeout()), the
   // return value is used to compute when the timeout should occur.
   int64_t RawAbsNanos() const { return static_cast<int64_t>(rep_ >> 1); }
 
   // Converts to nanoseconds from now. Since the return value is a relative
   // duration, it should be recomputed by calling this method in the case of a
   // spurious wakeup.
   int64_t InNanosecondsFromNow() const;
 
   // A value that represents no timeout (or an infinite timeout).
   static constexpr uint64_t kNoTimeout = (std::numeric_limits<uint64_t>::max)();
 
   // The maximum value that can be stored in the high 63 bits.
   static constexpr int64_t kMaxNanos = (std::numeric_limits<int64_t>::max)();
 };
 
 }  // namespace synchronization_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_

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