flutter_flutter/sky/engine/platform/PurgeableVector.h

/*
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#ifndef SKY_ENGINE_PLATFORM_PURGEABLEVECTOR_H_
#define SKY_ENGINE_PLATFORM_PURGEABLEVECTOR_H_

#include "flutter/sky/engine/platform/PlatformExport.h"
#include "flutter/sky/engine/wtf/Forward.h"
#include "flutter/sky/engine/wtf/Noncopyable.h"
#include "flutter/sky/engine/wtf/Vector.h"

namespace blink {

class WebDiscardableMemory;

// A simple vector implementation that supports purgeable memory. The vector is
// already locked at construction and locking uses an internal counter which
// means that N calls to lock() must be followed by N+1 calls to unlock() to
// actually make the vector purgeable.
class PLATFORM_EXPORT PurgeableVector {
  WTF_MAKE_NONCOPYABLE(PurgeableVector);

 public:
  enum PurgeableOption {
    NotPurgeable,
    Purgeable,
  };

  // Clients who know in advance that they will call unlock() should construct
  // the instance with the Purgeable option so that the instance uses
  // discardable memory from the start and unlock() doesn't cause a memcpy().
  PurgeableVector(PurgeableOption = Purgeable);

  ~PurgeableVector();

  // WARNING: This causes a memcpy() if the instance was constructed with the
  // Purgeable hint or had its internal vector moved to discardable memory
  // after a call to unlock().
  void adopt(Vector<char>& other);

  void append(const char* data, size_t length);

  void grow(size_t);

  void clear();

  // The instance must be locked before calling this.
  char* data();

  size_t size() const;

  // Returns whether the memory is still resident.
  bool lock();

  // WARNING: Calling unlock() on an instance that wasn't created with the
  // Purgeable option does an extra memcpy().
  void unlock();

  bool isLocked() const;

  // Note that this method should be used carefully since it may not use
  // exponential growth internally. This means that repeated/invalid uses of
  // it can result in O(N^2) append(). If you don't exactly know what you are
  // doing then you should probably not call this method.
  void reserveCapacity(size_t capacity);

 private:
  enum PurgeableAllocationStrategy {
    UseExactCapacity,
    UseExponentialGrowth,
  };

  // Copies data from the discardable buffer to the vector and clears the
  // discardable buffer.
  void moveDataFromDiscardableToVector();

  void clearDiscardable();

  bool reservePurgeableCapacity(size_t capacity, PurgeableAllocationStrategy);

  size_t adjustPurgeableCapacity(size_t capacity) const;

  // Vector used when the instance is constructed without the purgeability
  // hint or when discardable memory allocation fails.
  // Note that there can't be data both in |m_vector| and
  // |m_discardable|, i.e. only one of them is used at a given time.
  Vector<char> m_vector;
  OwnPtr<WebDiscardableMemory> m_discardable;
  size_t m_discardableCapacity;
  size_t m_discardableSize;
  bool m_isPurgeable;
  int m_locksCount;
};

}  // namespace blink

#endif  // SKY_ENGINE_PLATFORM_PURGEABLEVECTOR_H_