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MongoDB Shard ID hash 算法 std::hash 的跨平台性 

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#include <functional>
#include <iomanip>
#include <iostream>
#include <string>


int main()
{
    std::string str = "Meet the new boss...";
    std::size_t str_hash = std::hash<std::string>{}(str);
    std::cout << "hash(" << std::quoted(str) << ") = " << str_hash << std::endl;

    str = "Meet the new boss..;";
    str_hash = std::hash<std::string>{}(str);
    std::cout << "hash(" << std::quoted(str) << ") = " << str_hash << std::endl;

    str = "Meet the new boss../";
    str_hash = std::hash<std::string>{}(str);
    std::cout << "hash(" << std::quoted(str) << ") = " << str_hash << std::endl;

    str = "Meet the new boss..,";
    str_hash = std::hash<std::string>{}(str);
    std::cout << "hash(" << std::quoted(str) << ") = " << str_hash << std::endl;

    return 0;
}

Windows, VS 2017 的结果：

hash(“Meet the new boss…”) = 5935324269489717502

hash(“Meet the new boss…;”) = 5935347359233909933

hash(“Meet the new boss…/”) = 5935325369001345713

hash(“Meet the new boss…,”) = 5935322070466461080

Ubuntu 16.04, g++ 5.4.0 20160609 的结果：

hash(“Meet the new boss…”) = 10656026664466977650

hash(“Meet the new boss…;”) = 12509209616339026574

hash(“Meet the new boss…/”) = 6552276210272946664

hash(“Meet the new boss…,”) = 15639609178671340058

还好我们不会在生产环境，使用 Windows 部署 MongoDB……

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std::size_t ShardId::Hasher::operator()(const ShardId& shardId) const {
     return std::hash<std::string>()(shardId._shardId); 
}

详见：https://github.com/mongodb/mongo/blob/master/src/mongo/s/shard_id.cpp

这个 std::hash 在 x86 和 x64 下都不一样，所以，让我们看看 MongoDB 如何解决这个问题：

MongoDB 3.4 no longer supports 32-bit x86 platforms.

好样的！