A C++ implementation of a fast hash map and hash set using robin hood hashing

The robin-map library is a C++ implementation of a fast hash map and hash set using open-addressing and linear robin hood hashing with backward shift deletion to resolve collisions.

Four classes are provided: tsl::robin_map, tsl::robin_set, tsl::robin_pg_map and tsl::robin_pg_set. The first two are faster and use a power of two growth policy, the last two use a prime growth policy instead and are able to cope better with a poor hash function. Use the prime version if there is a chance of repeating patterns in the lower bits of your hash (e.g. you are storing pointers with an identity hash function). See GrowthPolicy for details.

A benchmark of tsl::robin_map against other hash maps may be found here. This page also gives some advices on which hash table structure you should try for your use case (useful if you are a bit lost with the multiple hash tables implementations in the tsl namespace).

Key features

• Header-only library, just include the project to your include path and you are ready to go.
• Fast hash table, see the benchmark for some numbers.
• Support for move-only and non-default constructible key/value.
• 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 as key parameter to find, see example).
• No need to reserve any sentinel value from the keys.
• Possibility to store the hash value alongside the stored key-value for faster rehash and lookup if the hash or the key equal functions are expensive to compute. Note that hash may be stored even if not asked explicitly when the library can detect that it will have no impact on the size of the structure in memory due to alignment. See the StoreHash template parameter for details.
• If the hash is known before a lookup, it is possible to pass it as parameter to speed-up the lookup.
• API closely similar to std::unordered_map and std::unordered_set.

Differences compare to std::unordered_map

tsl::robin_map tries to have an interface similar to std::unordered_map, but some differences exist.

• The strong exception guarantee only holds if the following statement is true std::is_nothrow_swappable<value_type>::value && std::is_nothrow_move_constructible<value_type>::value (where value_type is Key for tsl::robin_set and std::pair<Key, T> for tsl::robin_map). Otherwise if an exception is thrown during the swap or the move, the structure may end up in a undefined state. Note that per the standard, a value_type with a noexcept copy constructor and no move constructor also satisfies this condition and will thus guarantee the strong exception guarantee for the structure (see API for details).
• Iterator invalidation doesn't behave in the same way, any operation modifying the hash table invalidate them (see API for details).
• References and pointers to keys or values in the map are invalidated in the same way as iterators to these keys-values.
• 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::robin_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
}

• No support for some buckets related methods (like bucket_size, bucket, ...).

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

Thread-safety guarantees are the same as std::unordered_map/set (i.e. possible to have multiple readers with no writer).

Growth policy

The library supports multiple growth policies through the GrowthPolicy template parameter. Three policies are provided by the library but you can easly implement your own if needed.

• tsl::rh::power_of_two_growth_policy. Default policy used by tsl::robin_map/set. This policy keeps the size of the bucket array of the hash table to a power of two. This constraint allows the policy to avoid the usage of the slow modulo operation to map a hash to a bucket, instead of hash % 2n, it uses hash & (2n - 1) (see fast modulo). Fast but this may cause a lot of collisions with a poor hash function as the modulo with a power of two only masks the most significant bits in the end.
• tsl::rh::prime_growth_policy. Default policy used by tsl::robin_pg_map/set. The policy keeps the size of the bucket array of the hash table to a prime number. When mapping a hash to a bucket, using a prime number as modulo will result in a better distribution of the hash across the buckets even with a poor hash function. To allow the compiler to optimize the modulo operation, the policy use a lookup table with constant primes modulos (see API for details). Slower than tsl::rh::power_of_two_growth_policy but more secure.
• tsl::rh::mod_growth_policy. The policy grows the map by a customizable growth factor passed in parameter. It then just use the modulo operator to map a hash to a bucket. Slower but more flexible.

To implement your own policy, you have to implement the following interface.

struct custom_policy {
    // Called on hash table construction, min_bucket_count_in_out is the minimum size
    // that the hash table needs. The policy can change it to a higher bucket count if needed
    custom_policy(std::size_t& min_bucket_count_in_out);
    
    // Return the bucket for the corresponding hash
    std::size_t bucket_for_hash(std::size_t hash) const noexcept;
    
    // Return the number of buckets that should be used on next growth
    std::size_t next_bucket_count() const;
    
    // Maximum number of buckets supported by the policy
    std::size_t max_bucket_count() const;
}

Installation

To use robin-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/robin-map.git
cd robin-map
mkdir build
cd build
cmake ..
make
./test_robin_map

Usage

The API can be found here.

All methods are not documented yet, but they replicate the behavior of the ones in std::unordered_map and std::unordered_set, except if specified otherwise.

Example

#include <cstdint>
#include <iostream>
#include <string>
#include <tsl/robin_map.h>
#include <tsl/robin_set.h>

int main() {
    tsl::robin_map<std::string, int> map = {{"a", 1}, {"b", 2}};
    map["c"] = 3;
    map["d"] = 4;
    
    map.insert({"e", 5});
    map.erase("b");
    
    for(auto it = map.begin(); it != map.end(); ++it) {
        //it->second += 2; // Not valid.
        it.value() += 2;
    }
    
    // {d, 6} {a, 3} {e, 7} {c, 5}
    for(const auto& key_value : map) {
        std::cout << "{" << key_value.first << ", " << key_value.second << "}" << std::endl;
    }
    
    
    
    
    /*
     * Calculating the hash and comparing two std::string may be slow. 
     * We can store the hash of each std::string in the hash map to make 
     * the inserts and lookups faster by setting StoreHash to true.
     */ 
    tsl::robin_map<std::string, int, std::hash<std::string>, 
                   std::equal_to<std::string>,
                   std::allocator<std::pair<std::string, int>>,
                   true> map2;
                       
    map2["a"] = 1;
    map2["b"] = 2;
    
    // {a, 1} {b, 2}
    for(const auto& key_value : map2) {
        std::cout << "{" << key_value.first << ", " << key_value.second << "}" << std::endl;
    }
    
    
    
    
    tsl::robin_set<int> set;
    set.insert({1, 9, 0});
    set.insert({2, -1, 9});
    
    // {0} {1} {2} {9} {-1}
    for(const auto& key : set) {
        std::cout << "{" << key << "}" << std::endl;
    }
}  

Heterogeneous lookups

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::robin_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/robin_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::robin_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::robin_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.