object_cache.h

// File:        object_cache.h
// Description: A string indexed object cache.
// Author:      David Eger
// Created:     Fri Jan 27 12:08:00 PST 2012
//
// (C) Copyright 2012, Google Inc.
// 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
// http://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 TESSERACT_CCUTIL_OBJECT_CACHE_H_
#define TESSERACT_CCUTIL_OBJECT_CACHE_H_

#include "ccutil.h"
#include "errcode.h"
#include "genericvector.h"
#include "tesscallback.h"

namespace tesseract {

// A simple object cache which maps a string to an object of type T.
// Usually, these are expensive objects that are loaded from disk.
// Reference counting is performed, so every Get() needs to be followed later
// by a Free().  Actual deletion is accomplished by DeleteUnusedObjects().
template<typename T>
class ObjectCache {
 public:
  ObjectCache() = default;
  ~ObjectCache() {
    mu_.Lock();
    for (int i = 0; i < cache_.size(); i++) {
      if (cache_[i].count > 0) {
        tprintf("ObjectCache(%p)::~ObjectCache(): WARNING! LEAK! object %p "
                "still has count %d (id %s)\n",
                this, cache_[i].object, cache_[i].count,
                cache_[i].id.string());
      } else {
        delete cache_[i].object;
        cache_[i].object = nullptr;
      }
    }
    mu_.Unlock();
  }

  // Return a pointer to the object identified by id.
  // If we haven't yet loaded the object, use loader to load it.
  // If loader fails to load it, record a nullptr entry in the cache
  // and return nullptr -- further attempts to load will fail (even
  // with a different loader) until DeleteUnusedObjects() is called.
  // We delete the given loader.
  T *Get(STRING id,
         TessResultCallback<T *> *loader) {
    T *retval = nullptr;
    mu_.Lock();
    for (int i = 0; i < cache_.size(); i++) {
      if (id == cache_[i].id) {
        retval = cache_[i].object;
        if (cache_[i].object != nullptr) {
          cache_[i].count++;
        }
        mu_.Unlock();
        delete loader;
        return retval;
      }
    }
    cache_.push_back(ReferenceCount());
    ReferenceCount &rc = cache_.back();
    rc.id = id;
    retval = rc.object = loader->Run();
    rc.count = (retval != nullptr) ? 1 : 0;
    mu_.Unlock();
    return retval;
  }

  // Decrement the count for t.
  // Return whether we knew about the given pointer.
  bool Free(T *t) {
    if (t == nullptr) return false;
    mu_.Lock();
    for (int i = 0; i < cache_.size(); i++) {
      if (cache_[i].object == t) {
        --cache_[i].count;
        mu_.Unlock();
        return true;
      }
    }
    mu_.Unlock();
    return false;
  }

  void DeleteUnusedObjects() {
    mu_.Lock();
    for (int i = cache_.size() - 1; i >= 0; i--) {
      if (cache_[i].count <= 0) {
        delete cache_[i].object;
        cache_.remove(i);
      }
    }
    mu_.Unlock();
  }

 private:
  struct ReferenceCount {
    STRING id;  // A unique ID to identify the object (think path on disk)
    T *object;  // A copy of the object in memory.  Can be delete'd.
    int count;  // A count of the number of active users of this object.
  };

  CCUtilMutex mu_;
  GenericVector<ReferenceCount> cache_;
};

}  // namespace tesseract


#endif  // TESSERACT_CCUTIL_OBJECT_CACHE_H_