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greg
81 lines
3.4 KiB
Markdown
Vendored
81 lines
3.4 KiB
Markdown
Vendored
# The Parallel Hashmap: Very fast, and memory friendly as well
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Click here [For a full writeup explaining the design and benefits of the Parallel Hashmap](https://greg7mdp.github.io/parallel-hashmap/).
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> **IMPORTANT:** This repository borrowed a lot of code from the [abseil-cpp](https://github.com/abseil/abseil-cpp) repository, notably for the implementations of `phmap::flat_hash_map`, `phmap::flat_hash_set`, `phmap::node_hash_map` and `phmap::node_hash_set`. Please be aware that the code from [abseil-cpp](https://github.com/abseil/abseil-cpp) has been modified, and may behave differently than the original.
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This repository should be construed as an independent work, with no guarantees of any kind implied or provided by the authors. Please go to [abseil-cpp](https://github.com/abseil/abseil-cpp) for the official Abseil libraries.
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# Work in progress - please do not use yet - should be ready before the end of March 2019
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This repository aims to provide an set of excellent hash map implementations, with the following characteristics:
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- **header only**: nothing to build, just copy the `parallel_hashmap` directory to your project and you are good to go.
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- compiler with **C++11 support** required, **C++14 and C++17 APIs are provided**
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- **Very efficient**, significantly faster than your compiler's unordered map/set or Boost's, or than [sparsepp](https://github.com/greg7mdp/sparsepp)
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- **Memory friendly**: low memory usage, although a little higher than [sparsepp](https://github.com/greg7mdp/sparsepp)
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- support heterogeneous lookup
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- Not yet **Tested** ~~on Windows (vs2010-2015, g++), linux (g++, clang++) and MacOS (clang++)~~.
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## Installation
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Copy the parallel_hashmap directory to your project. That's all.
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> cmake configuration files (CMakeLists.txt) are provided for building the tests and examples.
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## Example
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```
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#include <iostream>
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#include <string>
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#include <parallel_hashmap/phmap.h>
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using phmap::flat_hash_map;
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int main()
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{
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// Create an unordered_map of three strings (that map to strings)
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flat_hash_map<std::string, std::string> email =
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{
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{ "tom", "tom@gmail.com"},
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{ "jeff", "jk@gmail.com"},
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{ "jim", "jimg@microsoft.com"}
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};
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// Iterate and print keys and values
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for (const auto& n : email)
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std::cout << n.first << "'s email is: " << n.second << "\n";
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// Add a new entry
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email["bill"] = "bg@whatever.com";
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// and print it
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std::cout << "bill's email is: " << email["bill"] << "\n";
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return 0;
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}
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```
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## Various hash maps and their pros and cons
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Key points:
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- The `flat` hash maps may move the keys and values in memory. So if you keep a pointer to something inside a `flat` hash map, this pointer may become invalid when the map is mutated. The `node` hash maps don't, and should be used instead if this is a problem.
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- The `flat` hash maps will use less memory, and usually be faster than the `node` hash maps, so use them if you can. A possible exception is when the values inserted in the hash map are large (say more than 100 bytes [*needs testing*]).
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- The `parallel` hash maps are preferred when you have a few hash maps that will store a very large number of values. The `non-parallel` hash maps are preferred if you have a large number of hash maps, each storing a relatively small number of values.
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- The benefits of the `parallel` hash maps are:
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a. reduced peak memory usage (when resizing), and
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b. multithreading support (and inherent internal parallelism)
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