// Copyright (c) 2012 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 NET_BASE_EXPIRING_CACHE_H_
#define NET_BASE_EXPIRING_CACHE_H_
#include <map>
#include <utility>
#include "base/basictypes.h"
#include "base/gtest_prod_util.h"
#include "base/time/time.h"
namespace net {
template <typename KeyType,
typename ValueType,
typename ExpirationType>
class NoopEvictionHandler {
public:
void Handle(const KeyType& key,
const ValueType& value,
const ExpirationType& expiration,
const ExpirationType& now,
bool onGet) const {
}
};
// Cache implementation where all entries have an explicit expiration policy. As
// new items are added, expired items will be removed first.
// The template types have the following requirements:
// KeyType must be LessThanComparable, Assignable, and CopyConstructible.
// ValueType must be CopyConstructible and Assignable.
// ExpirationType must be CopyConstructible and Assignable.
// ExpirationCompare is a function class that takes two arguments of the
// type ExpirationType and returns a bool. If |comp| is an instance of
// ExpirationCompare, then the expression |comp(current, expiration)| shall
// return true iff |current| is still valid within |expiration|.
//
// A simple use of this class may use base::TimeTicks, which provides a
// monotonically increasing clock, for the expiration type. Because it's always
// increasing, std::less<> can be used, which will simply ensure that |now| is
// sorted before |expiration|:
//
// ExpiringCache<std::string, std::string, base::TimeTicks,
// std::less<base::TimeTicks> > cache(0);
// // Add a value that expires in 5 minutes
// cache.Put("key1", "value1", base::TimeTicks::Now(),
// base::TimeTicks::Now() + base::TimeDelta::FromMinutes(5));
// // Add another value that expires in 10 minutes.
// cache.Put("key2", "value2", base::TimeTicks::Now(),
// base::TimeTicks::Now() + base::TimeDelta::FromMinutes(10));
//
// Alternatively, there may be some more complex expiration criteria, at which
// point a custom functor may be used:
//
// struct ComplexExpirationFunctor {
// bool operator()(const ComplexExpiration& now,
// const ComplexExpiration& expiration) const;
// };
// ExpiringCache<std::string, std::string, ComplexExpiration,
// ComplexExpirationFunctor> cache(15);
// // Add a value that expires once the 'sprocket' has 'cog'-ified.
// cache.Put("key1", "value1", ComplexExpiration("sprocket"),
// ComplexExpiration("cog"));
template <typename KeyType,
typename ValueType,
typename ExpirationType,
typename ExpirationCompare,
typename EvictionHandler = NoopEvictionHandler<KeyType,
ValueType,
ExpirationType> >
class ExpiringCache {
private:
// Intentionally violate the C++ Style Guide so that EntryMap is known to be
// a dependent type. Without this, Clang's two-phase lookup complains when
// using EntryMap::const_iterator, while GCC and MSVC happily resolve the
// typename.
// Tuple to represent the value and when it expires.
typedef std::pair<ValueType, ExpirationType> Entry;
typedef std::map<KeyType, Entry> EntryMap;
public:
typedef KeyType key_type;
typedef ValueType value_type;
typedef ExpirationType expiration_type;
// This class provides a read-only iterator over items in the ExpiringCache
class Iterator {
public:
explicit Iterator(const ExpiringCache& cache)
: cache_(cache),
it_(cache_.entries_.begin()) {
}
~Iterator() {}
bool HasNext() const { return it_ != cache_.entries_.end(); }
void Advance() { ++it_; }
const KeyType& key() const { return it_->first; }
const ValueType& value() const { return it_->second.first; }
const ExpirationType& expiration() const { return it_->second.second; }
private:
const ExpiringCache& cache_;
// Use a second layer of type indirection, as both EntryMap and
// EntryMap::const_iterator are dependent types.
typedef typename ExpiringCache::EntryMap EntryMap;
typename EntryMap::const_iterator it_;
};
// Constructs an ExpiringCache that stores up to |max_entries|.
explicit ExpiringCache(size_t max_entries) : max_entries_(max_entries) {}
~ExpiringCache() {}
// Returns the value matching |key|, which must be valid at the time |now|.
// Returns NULL if the item is not found or has expired. If the item has
// expired, it is immediately removed from the cache.
// Note: The returned pointer remains owned by the ExpiringCache and is
// invalidated by a call to a non-const method.
const ValueType* Get(const KeyType& key, const ExpirationType& now) {
typename EntryMap::iterator it = entries_.find(key);
if (it == entries_.end())
return NULL;
// Immediately remove expired entries.
if (!expiration_comp_(now, it->second.second)) {
Evict(it, now, true);
return NULL;
}
return &it->second.first;
}
// Updates or replaces the value associated with |key|.
void Put(const KeyType& key,
const ValueType& value,
const ExpirationType& now,
const ExpirationType& expiration) {
typename EntryMap::iterator it = entries_.find(key);
if (it == entries_.end()) {
// Compact the cache if it grew beyond the limit.
if (entries_.size() == max_entries_ )
Compact(now);
// No existing entry. Creating a new one.
entries_.insert(std::make_pair(key, Entry(value, expiration)));
} else {
// Update an existing cache entry.
it->second.first = value;
it->second.second = expiration;
}
}
// Empties the cache.
void Clear() {
entries_.clear();
}
// Returns the number of entries in the cache.
size_t size() const { return entries_.size(); }
// Returns the maximum number of entries in the cache.
size_t max_entries() const { return max_entries_; }
bool empty() const { return entries_.empty(); }
private:
FRIEND_TEST_ALL_PREFIXES(ExpiringCacheTest, Compact);
FRIEND_TEST_ALL_PREFIXES(ExpiringCacheTest, CustomFunctor);
// Prunes entries from the cache to bring it below |max_entries()|.
void Compact(const ExpirationType& now) {
// Clear out expired entries.
typename EntryMap::iterator it;
for (it = entries_.begin(); it != entries_.end(); ) {
if (!expiration_comp_(now, it->second.second)) {
Evict(it++, now, false);
} else {
++it;
}
}
if (entries_.size() < max_entries_)
return;
// If the cache is still too full, start deleting items 'randomly'.
for (it = entries_.begin();
it != entries_.end() && entries_.size() >= max_entries_;) {
Evict(it++, now, false);
}
}
void Evict(typename EntryMap::iterator it,
const ExpirationType& now,
bool on_get) {
eviction_handler_.Handle(it->first, it->second.first, it->second.second,
now, on_get);
entries_.erase(it);
}
// Bound on total size of the cache.
size_t max_entries_;
EntryMap entries_;
ExpirationCompare expiration_comp_;
EvictionHandler eviction_handler_;
DISALLOW_COPY_AND_ASSIGN(ExpiringCache);
};
} // namespace net
#endif // NET_BASE_EXPIRING_CACHE_H_