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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef mozilla_MruCache_h
#define mozilla_MruCache_h
#include <cstdint>
#include <type_traits>
#include <utility>
#include "mozilla/Attributes.h"
#include "mozilla/HashFunctions.h"
namespace mozilla {
namespace detail {
// Helper struct for checking if a value is empty.
//
// `IsNotEmpty` will return true if `Value` is not a pointer type or if the
// pointer value is not null.
template <typename Value, bool IsPtr = std::is_pointer<Value>::value>
struct EmptyChecker {
static bool IsNotEmpty(const Value&) { return true; }
};
// Template specialization for the `IsPtr == true` case.
template <typename Value>
struct EmptyChecker<Value, true> {
static bool IsNotEmpty(const Value& aVal) { return aVal != nullptr; }
};
} // namespace detail
// Provides a most recently used cache that can be used as a layer on top of
// a larger container where lookups can be expensive. The default size is 31,
// which as a prime number provides a better distrubution of cached entries.
//
// Users are expected to provide a `Cache` class that defines two required
// methods:
// - A method for providing the hash of a key:
//
// static HashNumber Hash(const KeyType& aKey)
//
// - A method for matching a key to a value, for pointer types the value
// is guaranteed not to be null.
//
// static bool Match(const KeyType& aKey, const ValueType& aVal)
//
// For example:
// class MruExample : public MruCache<void*, PtrInfo*, MruExample>
// {
// static HashNumber Hash(const KeyType& aKey)
// {
// return HashGeneric(aKey);
// }
// static Match(const KeyType& aKey, const ValueType& aVal)
// {
// return aVal->mPtr == aKey;
// }
// };
template <class Key, class Value, class Cache, size_t Size = 31>
class MruCache {
// Best distribution is achieved with a prime number. Ideally the closest
// to a power of two will be the most efficient use of memory. This
// assertion is pretty weak, but should catch the common inclination to
// use a power-of-two.
static_assert(Size % 2 != 0, "Use a prime number");
// This is a stronger assertion but significantly limits the values to just
// those close to a power-of-two value.
// static_assert(Size == 7 || Size == 13 || Size == 31 || Size == 61 ||
// Size == 127 || Size == 251 || Size == 509 || Size == 1021,
// "Use a prime number less than 1024");
public:
using KeyType = Key;
using ValueType = Value;
MruCache() = default;
MruCache(const MruCache&) = delete;
MruCache(const MruCache&&) = delete;
// Inserts the given value into the cache. Potentially overwrites an
// existing entry.
template <typename U>
void Put(const KeyType& aKey, U&& aVal) {
*RawEntry(aKey) = std::forward<U>(aVal);
}
// Removes the given entry if it is in the cache.
void Remove(const KeyType& aKey) { Lookup(aKey).Remove(); }
// Clears all cached entries and resets them to a default value.
void Clear() {
for (ValueType& val : mCache) {
val = ValueType{};
}
}
// Helper that holds an entry that matched a lookup key. Usage:
//
// auto p = mCache.Lookup(aKey);
// if (p) {
// return p.Data();
// }
//
// auto foo = new Foo();
// p.Set(foo);
// return foo;
class Entry {
public:
Entry(ValueType* aEntry, bool aMatch) : mEntry(aEntry), mMatch(aMatch) {
MOZ_ASSERT(mEntry);
}
explicit operator bool() const { return mMatch; }
ValueType& Data() const {
MOZ_ASSERT(mMatch);
return *mEntry;
}
template <typename U>
void Set(U&& aValue) {
mMatch = true;
Data() = std::forward<U>(aValue);
}
void Remove() {
if (mMatch) {
Data() = ValueType{};
mMatch = false;
}
}
private:
ValueType* mEntry; // Location of the entry in the cache.
bool mMatch; // Whether the value matched.
};
// Retrieves an entry from the cache. Can be used to test if an entry is
// present, update the entry to a new value, or remove the entry if one was
// matched.
Entry Lookup(const KeyType& aKey) {
using EmptyChecker = detail::EmptyChecker<ValueType>;
auto entry = RawEntry(aKey);
bool match = EmptyChecker::IsNotEmpty(*entry) && Cache::Match(aKey, *entry);
return Entry(entry, match);
}
private:
MOZ_ALWAYS_INLINE ValueType* RawEntry(const KeyType& aKey) {
return &mCache[Cache::Hash(aKey) % Size];
}
ValueType mCache[Size] = {};
};
} // namespace mozilla
#endif // mozilla_mrucache_h
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