// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2023 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
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//
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// Author: yp@photonscore.de (Yury Prokazov)

#ifndef CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_
#define CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_

#include "ceres/concurrent_queue.h"
#include "ceres/internal/config.h"
#include "ceres/internal/export.h"

namespace ceres::internal {

// Helper for C++ thread number identification that is similar to
// omp_get_thread_num() behaviour. This is necessary to support C++
// threading with a sequential thread id. This is used to access preallocated
// resources in the parallelized code parts.  The sequence of tokens varies from
// 0 to num_threads - 1 that can be acquired to identify the thread in a thread
// pool.
//
//
// Example usage pseudocode:
//
// ThreadTokenProvider ttp(N); // allocate N tokens
// Spawn N threads {
//    int token = ttp.Acquire(); // get unique token
//    ...
//    ... use token to access resources bound to the thread
//    ...
//    ttp.Release(token); // return token to the pool
//  }
//
class CERES_NO_EXPORT ThreadTokenProvider {
 public:
  explicit ThreadTokenProvider(int num_threads);

  // Returns the first token from the queue. The acquired value must be
  // given back by Release().
  int Acquire();

  // Makes previously acquired token available for other threads.
  void Release(int thread_id);

 private:
  // This queue initially holds a sequence from 0..num_threads-1. Every
  // Acquire() call the first number is removed from here. When the token is not
  // needed anymore it shall be given back with corresponding Release()
  // call. This concurrent queue is more expensive than TBB's version, so you
  // should not acquire the thread ID on every for loop iteration.
  ConcurrentQueue<int> pool_;
  ThreadTokenProvider(ThreadTokenProvider&) = delete;
  ThreadTokenProvider& operator=(ThreadTokenProvider&) = delete;
};

}  // namespace ceres::internal

#endif  // CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_