c++ - uint32_t 值对的交换散列函数

Question

我需要一个快速、简单的散列函数，它为一对值创建一个唯一标识符- 因此和uint32_t的散列值相同。(2,7)(7,2)

任何想法？

Answer 1

To answer my own question, the solution is:

uint64_t hash(uint32_t x, uint32_t y)
{
    const uint64_t a = static_cast<uint64_t>(x);
    const uint64_t b = static_cast<uint64_t>(y);

    if (x < y) return (b << 32) | a;
    else return (a << 32) | b;
}

Which can be improved to the branchless version

uint64_t hash(uint32_t x, uint32_t y)
{
    const uint64_t a = static_cast<uint64_t>(x);
    const uint64_t b = static_cast<uint64_t>(y);

    const uint64_t h0 = (b << 32) | a;
    const uint64_t h1 = (a << 32) | b;

    return (x < y) ? h0 : h1; // conditional move (CMOV) instruction
}

These methods are perfect hash functions - they guarantee zero collisions. However, they have the disadvantage that you cannot hash values above 2^32 - 1.

Answer 2

constexpr uint32_t hash_max = ...;    

constexpr uint32_t commutative_hash(uint32_t i, uint32_t j) {
   return (i*j + (i*i)*(j*j) + (i*i*i)*(j*j*j)) % hash_max;
};

额外的括号用于编译器 - 优化此表达式会更容易。

如果要创建快速函数，请不要使用任何会破坏 CPU 管道（并且速度很慢）的条件指令（或std::max/ ）。std::min