// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TASK_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_
#define BASE_TASK_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_

#include <stddef.h>

#include <vector>

#include "base/base_export.h"
#include "base/macros.h"

namespace base {
namespace internal {

class SchedulerWorker;

// A stack of SchedulerWorkers which has custom logic to treat the worker on top
// of the stack as being "in-use" (so its time in that position doesn't count
// towards being inactive / reclaimable). Supports removal of arbitrary
// SchedulerWorkers. DCHECKs when a SchedulerWorker is inserted multiple times.
// SchedulerWorkers are not owned by the stack. Push() is amortized O(1). Pop(),
// Peek(), Size() and Empty() are O(1). Contains() and Remove() are O(n). This
// class is NOT thread-safe.
class BASE_EXPORT SchedulerWorkerStack {
 public:
  SchedulerWorkerStack();
  ~SchedulerWorkerStack();

  // Inserts |worker| at the top of the stack. |worker| must not already be on
  // the stack. Flags the SchedulerWorker previously on top of the stack, if
  // any, as unused.
  void Push(SchedulerWorker* worker);

  // Removes the top SchedulerWorker from the stack and returns it. Returns
  // nullptr if the stack is empty. Flags the SchedulerWorker now on top of the
  // stack, if any, as being in-use.
  SchedulerWorker* Pop();

  // Returns the top SchedulerWorker from the stack, nullptr if empty.
  SchedulerWorker* Peek() const;

  // Returns true if |worker| is already on the stack.
  bool Contains(const SchedulerWorker* worker) const;

  // Removes |worker| from the stack. Must not be invoked for the first worker
  // on the stack.
  void Remove(const SchedulerWorker* worker);

  // Returns the number of SchedulerWorkers on the stack.
  size_t Size() const { return stack_.size(); }

  // Returns true if the stack is empty.
  bool IsEmpty() const { return stack_.empty(); }

 private:
  std::vector<SchedulerWorker*> stack_;

  DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerStack);
};

}  // namespace internal
}  // namespace base

#endif  // BASE_TASK_TASK_SCHEDULER_SCHEDULER_WORKER_STACK_H_