siwenzhang/sparsepp

A fast, memory efficient hash map for C++

Sparsepp: A fast, memory efficient hash map for C++

Sparsepp is derived from Google's excellent sparsehash implementation. It aims to achieve the following objectives:

• A drop-in alternative for unordered_map and unordered_set.
• Extremely low memory usage (typically about one byte overhead per entry).
• Very efficient, typically faster than your compiler's unordered map/set or Boost's.
• C++11 support (if supported by compiler).
• Single header implementation - just copy sparsepp.h to your project and include it.
• Tested on Windows (vs2010-2015, g++), linux (g++, clang++) and MacOS (clang++).

We believe Sparsepp provides an unparalleled combination of performance and memory usage, and will outperform your compiler's unordered_map on both counts. Only Google's dense_hash_map is consistently faster, at the cost of much greater memory usage (especially when the final size of the map is not known in advance).

For a detailed comparison of various hash implementations, including Sparsepp, please see our write-up.

Example

#include <iostream>
#include <string>
#include <sparsepp.h>

using spp::sparse_hash_map;
 
int main()
{
    // Create an unordered_map of three strings (that map to strings)
    sparse_hash_map<std::string, std::string> email = 
    {
        { "tom",  "[email protected]"},
        { "jeff", "[email protected]"},
        { "jim",  "[email protected]"}
    };
 
    // Iterate and print keys and values 
    for (const auto& n : email) 
        std::cout << n.first << "'s email is: " << n.second << "\n";
 
    // Add a new entry
    email["bill"] = "[email protected]";
 
    // and print it
    std::cout << "bill's email is: " << email["bill"] << "\n";
 
    return 0;
}

Installation

Since the full Sparsepp implementation is contained in a single header file sparsepp.h, the installation consist in copying this header file wherever it will be convenient to include in your project(s).

Optionally, a second header file spp_utils.h is provided, which implements only the spp::hash_combine() functionality. This is useful when we want to specify a hash function for a user-defined class in an header file, without including the full sparsepp.h header (this is demonstrated in example 2 below).

Usage

As shown in the example above, you need to include the header file: #include <sparsepp.h>

This provides the implementation for the following classes:

namespace spp
{
    template <class Key, 
              class T,
              class HashFcn  = spp_hash<Key>,  
              class EqualKey = std::equal_to<Key>,
              class Alloc    = libc_allocator_with_realloc<std::pair<const Key, T>>>
    class sparse_hash_map;

    template <class Value,
              class HashFcn  = spp_hash<Value>,
              class EqualKey = std::equal_to<Value>,
              class Alloc    = libc_allocator_with_realloc<Value>>
    class sparse_hash_set;
};

These classes provide the same interface as std::unordered_map and std::unordered_set, with the following differences:

• Calls to erase() may invalidate iterators. However, conformant to the C++11 standard, the position and range erase functions return an iterator pointing to the position immediately following the last of the elements erased. This makes it easy to traverse a sparse hash table and delete elements matching a condition. For example to delete odd values:

        for (auto it = c.begin(); it != c.end(); )
        if (it->first % 2 == 1)
            it = c.erase(it);
        else
            ++it;

• Since items are not grouped into buckets, Bucket APIs have been adapted: max_bucket_count is equivalent to max_size, and bucket_count returns the sparsetable size, which is normally at least twice the number of items inserted into the hash_map.

Example 2 - providing a hash function for a user-defined class

In order to use a sparse_hash_set or sparse_hash_map, a hash function should be provided. Even though a the hash function can be provided via the HashFcn template parameter, we recommend injecting a specialization of std::hash for the class into the "std" namespace. For example:

#include <iostream>
#include <functional>
#include <string>
#include "sparsepp.h"

using std::string;

struct Person 
{
    bool operator==(const Person &o) const 
    { return _first == o._first && _last == o._last; }

    string _first;
    string _last;
};

namespace std
{
    // inject specialization of std::hash for Person into namespace std
    // ----------------------------------------------------------------
    template<> 
    struct hash<Person>
    {
        std::size_t operator()(Person const &p) const
        {
            std::size_t seed = 0;
            spp::hash_combine(seed, p._first);
            spp::hash_combine(seed, p._last);
            return seed;
        }
    };
}
 
int main()
{
    // As we have defined a specialization of std::hash() for Person, 
    // we can now create sparse_hash_set or sparse_hash_map of Persons
    // ----------------------------------------------------------------
    spp::sparse_hash_set<Person> persons = { { "John", "Galt" }, 
                                             { "Jane", "Doe" } };
    for (auto& p: persons)
        std::cout << p._first << ' ' << p._last << '\n';
}

The std::hash specialization for Person combines the hash values for both first and last name using the convenient spp::hash_combine function, and returns the combined hash value.

spp::hash_combine is provided by the header sparsepp.h. However, class definitions often appear in header files, and it is desirable to limit the size of headers included in such header files, so we provide the very small header spp_utils.h for that purpose:

#include <string>
#include "spp_utils.h"

using std::string;
 
struct Person 
{
    bool operator==(const Person &o) const 
    { 
        return _first == o._first && _last == o._last && _age == o._age; 
    }

    string _first;
    string _last;
    int    _age;
};

namespace std
{
    // inject specialization of std::hash for Person into namespace std
    // ----------------------------------------------------------------
    template<> 
    struct hash<Person>
    {
        std::size_t operator()(Person const &p) const
        {
            std::size_t seed = 0;
            spp::hash_combine(seed, p._first);
            spp::hash_combine(seed, p._last);
            spp::hash_combine(seed, p._age);
            return seed;
        }
    };
}

Example 3 - serialization

sparse_hash_set and sparse_hash_map can easily be serialized/unserialized to a file or network connection. This support is implemented in the following APIs:

    template <typename Serializer, typename OUTPUT>
    bool serialize(Serializer serializer, OUTPUT *stream);

    template <typename Serializer, typename INPUT>
    bool unserialize(Serializer serializer, INPUT *stream);

The following example demontrates how a simple sparse_hash_map can be written to a file, and then read back. The serializer we use read and writes to a file using the stdio APIs, but it would be equally simple to write a serialized using the stream APIS:

#include <cstdio>

#include "sparsepp.h"

using spp::sparse_hash_map;
using namespace std;

class FileSerializer 
{
public:
    // serialize basic types to FILE
    // -----------------------------
    template <class T>
    bool operator()(FILE *fp, const T& value) 
    {
        return fwrite((const void *)&value, sizeof(value), 1, fp) == 1;
    }

    template <class T>
    bool operator()(FILE *fp, T* value) 
    {
        return fread((void *)value, sizeof(*value), 1, fp) == 1;
    }

    // serialize std::string to FILE
    // -----------------------------
    bool operator()(FILE *fp, const string& value) 
    {
        const size_t size = value.size();
        return (*this)(fp, size) && fwrite(value.c_str(), size, 1, fp) == 1;
    }

    bool operator()(FILE *fp, string* value) 
    {
        size_t size;
        if (!(*this)(fp, &size)) 
            return false;
        char* buf = new char[size];
        if (fread(buf, size, 1, fp) != 1) 
        {
            delete [] buf;
            return false;
        }
        new (value) string(buf, (size_t)size);
        delete[] buf;
        return true;
    }

    // serialize std::pair<const A, B> to FILE - needed for maps
    // ---------------------------------------------------------
    template <class A, class B>
    bool operator()(FILE *fp, const std::pair<const A, B>& value)
    {
        return (*this)(fp, value.first) && (*this)(fp, value.second);
    }

    template <class A, class B>
    bool operator()(FILE *fp, std::pair<const A, B> *value) 
    {
        return (*this)(fp, (A *)&value->first) && (*this)(fp, &value->second);
    }
};

int main(int argc, char* argv[]) 
{
    sparse_hash_map<string, int> age{ { "John", 12 }, {"Jane", 13 }, { "Fred", 8 } };

    // serialize age hash_map to "ages.dmp" file
    FILE *out = fopen("ages.dmp", "wb");
    age.serialize(FileSerializer(), out);
    fclose(out);

    sparse_hash_map<string, int> age_read;

    // read from "ages.dmp" file into age_read hash_map 
    FILE *input = fopen("ages.dmp", "rb");
    age_read.unserialize(FileSerializer(), input);
    fclose(input);

    // print out contents of age_read to verify correct serialization
    for (auto& v : age_read)
        printf("age_read: %s -> %d\n", v.first.c_str(), v.second);
}