C++ hash map and hash set which preserves the order of insertion

The ordered-map library provides a hash map and a hash set which preserve the order of insertion in a way similar to Python's OrderedDict. When iterating over the map, the values will be returned in the same order as they were inserted.

The values are stored contiguously in an underlying structure, no holes in-between values even after an erase operation. By default a std::deque is used for this structure, but it's also possible to use a std::vector. This structure is directly accessible through the values_container() method and if the structure is a std::vector, a data() method is also provided to easily interact with C APIs.

To resolve collisions on hashes, the library uses robin hood probing with backward shift deletion.

The library provides a behaviour similar to a std::deque/std::vector with unique values but with an average time complexity of O(1) for lookups and an amortised time complexity of O(1) for insertions. This comes at the price of a little higher memory footprint (8 bytes per entry if the load factor is 1, around 16 bytes per entry for a 0.5 load factor).

Two classes are provided: tsl::ordered_map and tsl::ordered_set.

Note: The library uses a power of two for the size of its buckets array to take advantage of the fast modulo. For good performance, it requires the hash table to have a well-distributed hash function. If you encounter performance issues check your hash function.

Key features

• Header-only library, just add the project to your include path and you are ready to go.
• Values are stored in the same order as the insertion order. The library provides a direct access to the underlying structure which stores the values.
• O(1) average time complexity for lookups with performances similar to std::unordered_map but with faster insertions and reduced memory usage (see benchmark for details).
• Provide random access iterators and also reverse iterators.
• Support for heterogeneous lookups (e.g. if you have a map that uses std::unique_ptr<int> as key, you could use an int* or a std::uintptr_t for example as key parameter for find, see example).
• API closely similar to std::unordered_map and std::unordered_set.

Differences compare to std::unordered_map

• The iterators are RandomAccessIterator.
• Iterator invalidation behaves in a way closer to std::vector and std::deque (see API for details). If you use std::vector as ValueTypeContainer, you can use reserve() to preallocate some space and avoid the invalidation of the iterators on insert.
• Slow erase() operation, it has a complexity of O(n). A faster O(1) version unordered_erase() exists, but it breaks the insertion order (see API for details). An O(1) pop_back() is also available.
• For iterators, operator*() and operator->() return a reference and a pointer to const std::pair<Key, T> instead of std::pair<const Key, T> making the value T not modifiable. To modify the value you have to call the value() method of the iterator to get a mutable reference. Example:

tsl::ordered_map<int, int> map = {{1, 1}, {2, 1}, {3, 1}};
for(auto it = map.begin(); it != map.end(); ++it) {
    //it->second = 2; // Illegal
    it.value() = 2; // Ok
}

• The map can only hold up to 2³² - 1 values, that is 4 294 967 295 values.
• No support for some bucket related methods (like bucket_size, bucket, ...).

Thread-safety guarantee is the same as std::unordered_map (possible to have multiple readers). Concerning the strong exception guarantee, it holds only if ValueContainer::emplace_back has the strong exception guarantee (which is true for std::vector and std::deque as long as the type T is not a move-only type with a move constructor that may throw an exception, see details).

These differences also apply between std::unordered_set and tsl::ordered_set.

Installation

To use ordered-map, just add the project to your include path. It is a header-only library.

The code should work with any C++11 standard-compliant compiler and has been tested with GCC 4.8.4, Clang 3.5.0 and Visual Studio 2015.

To run the tests you will need the Boost Test library and CMake.

git clone https://github.com/Tessil/ordered-map.git
cd ordered-map
mkdir build
cd build
cmake ..
make
./test_ordered_map

Usage

The API can be found here.

Example

#include <iostream>
#include <string>
#include <cstdlib>
#include <tsl/ordered_map.h>
#include <tsl/ordered_set.h>

int main() {
    tsl::ordered_map<char, int> map = {{'d', 1}, {'a', 2}, {'g', 3}};
    map.insert({'b', 4});
    map['h'] = 5;
    map['e'] = 6;
    
    map.erase('a');
    
    
    // {d, 1} {g, 3} {b, 4} {h, 5} {e, 6}
    for(const auto& key_value : map) {
        std::cout << "{" << key_value.first << ", " << key_value.second << "}" << std::endl;
    }
    
    
    map.unordered_erase('b');
    
    // Break order: {d, 1} {g, 3} {e, 6} {h, 5}
    for(const auto& key_value : map) {
        std::cout << "{" << key_value.first << ", " << key_value.second << "}" << std::endl;
    }
    
    
    tsl::ordered_set<char, std::hash<char>, std::equal_to<char>,
                     std::allocator<char>, std::vector<char>> set;
    set.reserve(6);
    
    set = {'3', '4', '9', '2'};
    set.erase('2');
    set.insert('1');
    set.insert('\0');
    
    set.pop_back();
    set.insert({'0', '\0'});
    
    // Get raw buffer for C API: 34910
    std::cout << atoi(set.data()) << std::endl;
}

Heterogeneous lookup

Heterogeneous overloads allow the usage of other types than Key for lookup and erase operations as long as the used types are hashable and comparable to Key.

To activate the heterogeneous overloads in tsl::ordered_map/set, the qualified-id KeyEqual::is_transparent must be valid. It works the same way as for std::map::find. You can either use std::equal_to<> or define your own function object.

Both KeyEqual and Hash will need to be able to deal with the different types.

#include <functional>
#include <iostream>
#include <string>
#include <tsl/ordered_map.h>


struct employee {
    employee(int id, std::string name) : m_id(id), m_name(std::move(name)) {
    }
    
    friend bool operator==(const employee& empl, int empl_id) {
        return empl.m_id == empl_id;
    }
    
    friend bool operator==(int empl_id, const employee& empl) {
        return empl_id == empl.m_id;
    }
    
    friend bool operator==(const employee& empl1, const employee& empl2) {
        return empl1.m_id == empl2.m_id;
    }
    
    
    int m_id;
    std::string m_name;
};

struct hash_employee {
    std::size_t operator()(const employee& empl) const {
        return std::hash<int>()(empl.m_id);
    }
    
    std::size_t operator()(int id) const {
        return std::hash<int>()(id);
    }
};

struct equal_employee {
    using is_transparent = void;
    
    bool operator()(const employee& empl, int empl_id) const {
        return empl.m_id == empl_id;
    }
    
    bool operator()(int empl_id, const employee& empl) const {
        return empl_id == empl.m_id;
    }
    
    bool operator()(const employee& empl1, const employee& empl2) const {
        return empl1.m_id == empl2.m_id;
    }
};


int main() {
    // Use std::equal_to<> which will automatically deduce and forward the parameters
    tsl::ordered_map<employee, int, hash_employee, std::equal_to<>> map; 
    map.insert({employee(1, "John Doe"), 2001});
    map.insert({employee(2, "Jane Doe"), 2002});
    map.insert({employee(3, "John Smith"), 2003});

    // John Smith 2003
    auto it = map.find(3);
    if(it != map.end()) {
        std::cout << it->first.m_name << " " << it->second << std::endl;
    }

    map.erase(1);



    // Use a custom KeyEqual which has an is_transparent member type
    tsl::ordered_map<employee, int, hash_employee, equal_employee> map2;
    map2.insert({employee(4, "Johnny Doe"), 2004});

    // 2004
    std::cout << map2.at(4) << std::endl;
} 

License

The code is licensed under the MIT license, see the LICENSE file for details.