// 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 BASE_CONTAINERS_SMALL_MAP_H_
#define BASE_CONTAINERS_SMALL_MAP_H_
#include <stddef.h>
#include <map>
#include <string>
#include <utility>
#include "base/containers/hash_tables.h"
#include "base/logging.h"
#include "base/memory/manual_constructor.h"
namespace base {
// An STL-like associative container which starts out backed by a simple
// array but switches to some other container type if it grows beyond a
// fixed size.
//
// WHAT TYPE OF MAP SHOULD YOU USE?
// --------------------------------
//
// - std::map should be the default if you're not sure, since it's the most
// difficult to mess up. Generally this is backed by a red-black tree. It
// will generate a lot of code (if you use a common key type like int or
// string the linker will probably emiminate the duplicates). It will
// do heap allocations for each element.
//
// - If you only ever keep a couple of items and have very simple usage,
// consider whether a using a vector and brute-force searching it will be
// the most efficient. It's not a lot of generated code (less than a
// red-black tree if your key is "weird" and not eliminated as duplicate of
// something else) and will probably be faster and do fewer heap allocations
// than std::map if you have just a couple of items.
//
// - base::hash_map should be used if you need O(1) lookups. It may waste
// space in the hash table, and it can be easy to write correct-looking
// code with the default hash function being wrong or poorly-behaving.
//
// - SmallMap combines the performance benefits of the brute-force-searched
// vector for small cases (no extra heap allocations), but can efficiently
// fall back if you end up adding many items. It will generate more code
// than std::map (at least 160 bytes for operator[]) which is bad if you
// have a "weird" key where map functions can't be
// duplicate-code-eliminated. If you have a one-off key and aren't in
// performance-critical code, this bloat may negate some of the benefits and
// you should consider on of the other options.
//
// SmallMap will pick up the comparator from the underlying map type. In
// std::map (and in MSVC additionally hash_map) only a "less" operator is
// defined, which requires us to do two comparisons per element when doing the
// brute-force search in the simple array.
//
// We define default overrides for the common map types to avoid this
// double-compare, but you should be aware of this if you use your own
// operator< for your map and supply yor own version of == to the SmallMap.
// You can use regular operator== by just doing:
//
// base::SmallMap<std::map<MyKey, MyValue>, 4, std::equal_to<KyKey> >
//
//
// USAGE
// -----
//
// NormalMap: The map type to fall back to. This also defines the key
// and value types for the SmallMap.
// kArraySize: The size of the initial array of results. This will be
// allocated with the SmallMap object rather than separately on
// the heap. Once the map grows beyond this size, the map type
// will be used instead.
// EqualKey: A functor which tests two keys for equality. If the wrapped
// map type has a "key_equal" member (hash_map does), then that will
// be used by default. If the wrapped map type has a strict weak
// ordering "key_compare" (std::map does), that will be used to
// implement equality by default.
// MapInit: A functor that takes a ManualConstructor<NormalMap>* and uses it to
// initialize the map. This functor will be called at most once per
// SmallMap, when the map exceeds the threshold of kArraySize and we
// are about to copy values from the array to the map. The functor
// *must* call one of the Init() methods provided by
// ManualConstructor, since after it runs we assume that the NormalMap
// has been initialized.
//
// example:
// base::SmallMap< std::map<string, int> > days;
// days["sunday" ] = 0;
// days["monday" ] = 1;
// days["tuesday" ] = 2;
// days["wednesday"] = 3;
// days["thursday" ] = 4;
// days["friday" ] = 5;
// days["saturday" ] = 6;
//
// You should assume that SmallMap might invalidate all the iterators
// on any call to erase(), insert() and operator[].
namespace internal {
template <typename NormalMap>
class SmallMapDefaultInit {
public:
void operator()(ManualConstructor<NormalMap>* map) const {
map->Init();
}
};
// has_key_equal<M>::value is true iff there exists a type M::key_equal. This is
// used to dispatch to one of the select_equal_key<> metafunctions below.
template <typename M>
struct has_key_equal {
typedef char sml; // "small" is sometimes #defined so we use an abbreviation.
typedef struct { char dummy[2]; } big;
// Two functions, one accepts types that have a key_equal member, and one that
// accepts anything. They each return a value of a different size, so we can
// determine at compile-time which function would have been called.
template <typename U> static big test(typename U::key_equal*);
template <typename> static sml test(...);
// Determines if M::key_equal exists by looking at the size of the return
// type of the compiler-chosen test() function.
static const bool value = (sizeof(test<M>(0)) == sizeof(big));
};
template <typename M> const bool has_key_equal<M>::value;
// Base template used for map types that do NOT have an M::key_equal member,
// e.g., std::map<>. These maps have a strict weak ordering comparator rather
// than an equality functor, so equality will be implemented in terms of that
// comparator.
//
// There's a partial specialization of this template below for map types that do
// have an M::key_equal member.
template <typename M, bool has_key_equal_value>
struct select_equal_key {
struct equal_key {
bool operator()(const typename M::key_type& left,
const typename M::key_type& right) {
// Implements equality in terms of a strict weak ordering comparator.
typename M::key_compare comp;
return !comp(left, right) && !comp(right, left);
}
};
};
// Provide overrides to use operator== for key compare for the "normal" map and
// hash map types. If you override the default comparator or allocator for a
// map or hash_map, or use another type of map, this won't get used.
//
// If we switch to using std::unordered_map for base::hash_map, then the
// hash_map specialization can be removed.
template <typename KeyType, typename ValueType>
struct select_equal_key< std::map<KeyType, ValueType>, false> {
struct equal_key {
bool operator()(const KeyType& left, const KeyType& right) {
return left == right;
}
};
};
template <typename KeyType, typename ValueType>
struct select_equal_key< base::hash_map<KeyType, ValueType>, false> {
struct equal_key {
bool operator()(const KeyType& left, const KeyType& right) {
return left == right;
}
};
};
// Partial template specialization handles case where M::key_equal exists, e.g.,
// hash_map<>.
template <typename M>
struct select_equal_key<M, true> {
typedef typename M::key_equal equal_key;
};
} // namespace internal
template <typename NormalMap,
int kArraySize = 4,
typename EqualKey =
typename internal::select_equal_key<
NormalMap,
internal::has_key_equal<NormalMap>::value>::equal_key,
typename MapInit = internal::SmallMapDefaultInit<NormalMap> >
class SmallMap {
// We cannot rely on the compiler to reject array of size 0. In
// particular, gcc 2.95.3 does it but later versions allow 0-length
// arrays. Therefore, we explicitly reject non-positive kArraySize
// here.
static_assert(kArraySize > 0, "default initial size should be positive");
public:
typedef typename NormalMap::key_type key_type;
typedef typename NormalMap::mapped_type data_type;
typedef typename NormalMap::mapped_type mapped_type;
typedef typename NormalMap::value_type value_type;
typedef EqualKey key_equal;
SmallMap() : size_(0), functor_(MapInit()) {}
explicit SmallMap(const MapInit& functor) : size_(0), functor_(functor) {}
// Allow copy-constructor and assignment, since STL allows them too.
SmallMap(const SmallMap& src) {
// size_ and functor_ are initted in InitFrom()
InitFrom(src);
}
void operator=(const SmallMap& src) {
if (&src == this) return;
// This is not optimal. If src and dest are both using the small
// array, we could skip the teardown and reconstruct. One problem
// to be resolved is that the value_type itself is pair<const K,
// V>, and const K is not assignable.
Destroy();
InitFrom(src);
}
~SmallMap() {
Destroy();
}
class const_iterator;
class iterator {
public:
typedef typename NormalMap::iterator::iterator_category iterator_category;
typedef typename NormalMap::iterator::value_type value_type;
typedef typename NormalMap::iterator::difference_type difference_type;
typedef typename NormalMap::iterator::pointer pointer;
typedef typename NormalMap::iterator::reference reference;
inline iterator(): array_iter_(NULL) {}
inline iterator& operator++() {
if (array_iter_ != NULL) {
++array_iter_;
} else {
++hash_iter_;
}
return *this;
}
inline iterator operator++(int /*unused*/) {
iterator result(*this);
++(*this);
return result;
}
inline iterator& operator--() {
if (array_iter_ != NULL) {
--array_iter_;
} else {
--hash_iter_;
}
return *this;
}
inline iterator operator--(int /*unused*/) {
iterator result(*this);
--(*this);
return result;
}
inline value_type* operator->() const {
if (array_iter_ != NULL) {
return array_iter_->get();
} else {
return hash_iter_.operator->();
}
}
inline value_type& operator*() const {
if (array_iter_ != NULL) {
return *array_iter_->get();
} else {
return *hash_iter_;
}
}
inline bool operator==(const iterator& other) const {
if (array_iter_ != NULL) {
return array_iter_ == other.array_iter_;
} else {
return other.array_iter_ == NULL && hash_iter_ == other.hash_iter_;
}
}
inline bool operator!=(const iterator& other) const {
return !(*this == other);
}
bool operator==(const const_iterator& other) const;
bool operator!=(const const_iterator& other) const;
private:
friend class SmallMap;
friend class const_iterator;
inline explicit iterator(ManualConstructor<value_type>* init)
: array_iter_(init) {}
inline explicit iterator(const typename NormalMap::iterator& init)
: array_iter_(NULL), hash_iter_(init) {}
ManualConstructor<value_type>* array_iter_;
typename NormalMap::iterator hash_iter_;
};
class const_iterator {
public:
typedef typename NormalMap::const_iterator::iterator_category
iterator_category;
typedef typename NormalMap::const_iterator::value_type value_type;
typedef typename NormalMap::const_iterator::difference_type difference_type;
typedef typename NormalMap::const_iterator::pointer pointer;
typedef typename NormalMap::const_iterator::reference reference;
inline const_iterator(): array_iter_(NULL) {}
// Non-explicit ctor lets us convert regular iterators to const iterators
inline const_iterator(const iterator& other)
: array_iter_(other.array_iter_), hash_iter_(other.hash_iter_) {}
inline const_iterator& operator++() {
if (array_iter_ != NULL) {
++array_iter_;
} else {
++hash_iter_;
}
return *this;
}
inline const_iterator operator++(int /*unused*/) {
const_iterator result(*this);
++(*this);
return result;
}
inline const_iterator& operator--() {
if (array_iter_ != NULL) {
--array_iter_;
} else {
--hash_iter_;
}
return *this;
}
inline const_iterator operator--(int /*unused*/) {
const_iterator result(*this);
--(*this);
return result;
}
inline const value_type* operator->() const {
if (array_iter_ != NULL) {
return array_iter_->get();
} else {
return hash_iter_.operator->();
}
}
inline const value_type& operator*() const {
if (array_iter_ != NULL) {
return *array_iter_->get();
} else {
return *hash_iter_;
}
}
inline bool operator==(const const_iterator& other) const {
if (array_iter_ != NULL) {
return array_iter_ == other.array_iter_;
} else {
return other.array_iter_ == NULL && hash_iter_ == other.hash_iter_;
}
}
inline bool operator!=(const const_iterator& other) const {
return !(*this == other);
}
private:
friend class SmallMap;
inline explicit const_iterator(
const ManualConstructor<value_type>* init)
: array_iter_(init) {}
inline explicit const_iterator(
const typename NormalMap::const_iterator& init)
: array_iter_(NULL), hash_iter_(init) {}
const ManualConstructor<value_type>* array_iter_;
typename NormalMap::const_iterator hash_iter_;
};
iterator find(const key_type& key) {
key_equal compare;
if (size_ >= 0) {
for (int i = 0; i < size_; i++) {
if (compare(array_[i]->first, key)) {
return iterator(array_ + i);
}
}
return iterator(array_ + size_);
} else {
return iterator(map()->find(key));
}
}
const_iterator find(const key_type& key) const {
key_equal compare;
if (size_ >= 0) {
for (int i = 0; i < size_; i++) {
if (compare(array_[i]->first, key)) {
return const_iterator(array_ + i);
}
}
return const_iterator(array_ + size_);
} else {
return const_iterator(map()->find(key));
}
}
// Invalidates iterators.
data_type& operator[](const key_type& key) {
key_equal compare;
if (size_ >= 0) {
// operator[] searches backwards, favoring recently-added
// elements.
for (int i = size_-1; i >= 0; --i) {
if (compare(array_[i]->first, key)) {
return array_[i]->second;
}
}
if (size_ == kArraySize) {
ConvertToRealMap();
return (*map_)[key];
} else {
array_[size_].Init(key, data_type());
return array_[size_++]->second;
}
} else {
return (*map_)[key];
}
}
// Invalidates iterators.
std::pair<iterator, bool> insert(const value_type& x) {
key_equal compare;
if (size_ >= 0) {
for (int i = 0; i < size_; i++) {
if (compare(array_[i]->first, x.first)) {
return std::make_pair(iterator(array_ + i), false);
}
}
if (size_ == kArraySize) {
ConvertToRealMap(); // Invalidates all iterators!
std::pair<typename NormalMap::iterator, bool> ret = map_->insert(x);
return std::make_pair(iterator(ret.first), ret.second);
} else {
array_[size_].Init(x);
return std::make_pair(iterator(array_ + size_++), true);
}
} else {
std::pair<typename NormalMap::iterator, bool> ret = map_->insert(x);
return std::make_pair(iterator(ret.first), ret.second);
}
}
// Invalidates iterators.
template <class InputIterator>
void insert(InputIterator f, InputIterator l) {
while (f != l) {
insert(*f);
++f;
}
}
iterator begin() {
if (size_ >= 0) {
return iterator(array_);
} else {
return iterator(map_->begin());
}
}
const_iterator begin() const {
if (size_ >= 0) {
return const_iterator(array_);
} else {
return const_iterator(map_->begin());
}
}
iterator end() {
if (size_ >= 0) {
return iterator(array_ + size_);
} else {
return iterator(map_->end());
}
}
const_iterator end() const {
if (size_ >= 0) {
return const_iterator(array_ + size_);
} else {
return const_iterator(map_->end());
}
}
void clear() {
if (size_ >= 0) {
for (int i = 0; i < size_; i++) {
array_[i].Destroy();
}
} else {
map_.Destroy();
}
size_ = 0;
}
// Invalidates iterators. Returns iterator following the last removed element.
iterator erase(const iterator& position) {
if (size_ >= 0) {
int i = position.array_iter_ - array_;
array_[i].Destroy();
--size_;
if (i != size_) {
array_[i].InitFromMove(std::move(array_[size_]));
array_[size_].Destroy();
return iterator(array_ + i);
}
return end();
}
return iterator(map_->erase(position.hash_iter_));
}
size_t erase(const key_type& key) {
iterator iter = find(key);
if (iter == end()) return 0u;
erase(iter);
return 1u;
}
size_t count(const key_type& key) const {
return (find(key) == end()) ? 0 : 1;
}
size_t size() const {
if (size_ >= 0) {
return static_cast<size_t>(size_);
} else {
return map_->size();
}
}
bool empty() const {
if (size_ >= 0) {
return (size_ == 0);
} else {
return map_->empty();
}
}
// Returns true if we have fallen back to using the underlying map
// representation.
bool UsingFullMap() const {
return size_ < 0;
}
inline NormalMap* map() {
CHECK(UsingFullMap());
return map_.get();
}
inline const NormalMap* map() const {
CHECK(UsingFullMap());
return map_.get();
}
private:
int size_; // negative = using hash_map
MapInit functor_;
// We want to call constructors and destructors manually, but we don't
// want to allocate and deallocate the memory used for them separately.
// So, we use this crazy ManualConstructor class. Since C++11 it's possible
// to use objects in unions like this, but the ManualDestructor syntax is
// a bit better and doesn't have limitations on object type.
//
// Since array_ and map_ are mutually exclusive, we'll put them in a
// union.
union {
ManualConstructor<value_type> array_[kArraySize];
ManualConstructor<NormalMap> map_;
};
void ConvertToRealMap() {
// Move the current elements into a temporary array.
ManualConstructor<value_type> temp_array[kArraySize];
for (int i = 0; i < kArraySize; i++) {
temp_array[i].InitFromMove(std::move(array_[i]));
array_[i].Destroy();
}
// Initialize the map.
size_ = -1;
functor_(&map_);
// Insert elements into it.
for (int i = 0; i < kArraySize; i++) {
map_->insert(std::move(*temp_array[i]));
temp_array[i].Destroy();
}
}
// Helpers for constructors and destructors.
void InitFrom(const SmallMap& src) {
functor_ = src.functor_;
size_ = src.size_;
if (src.size_ >= 0) {
for (int i = 0; i < size_; i++) {
array_[i].Init(*src.array_[i]);
}
} else {
functor_(&map_);
(*map_.get()) = (*src.map_.get());
}
}
void Destroy() {
if (size_ >= 0) {
for (int i = 0; i < size_; i++) {
array_[i].Destroy();
}
} else {
map_.Destroy();
}
}
};
template <typename NormalMap, int kArraySize, typename EqualKey,
typename Functor>
inline bool SmallMap<NormalMap, kArraySize, EqualKey,
Functor>::iterator::operator==(
const const_iterator& other) const {
return other == *this;
}
template <typename NormalMap, int kArraySize, typename EqualKey,
typename Functor>
inline bool SmallMap<NormalMap, kArraySize, EqualKey,
Functor>::iterator::operator!=(
const const_iterator& other) const {
return other != *this;
}
} // namespace base
#endif // BASE_CONTAINERS_SMALL_MAP_H_