编译时字符串散列 [英] Compile time string hashing

问题描述

我在几个不同的地方读到，使用 C++11 的新字符串文字可能可以在编译时计算字符串的哈希值.然而，似乎没有人准备好站出来说这将是可能的或将如何完成.

I have read in few different places that using C++11's new string literals it might be possible to compute a string's hash at compile time. However, no one seems to be ready to come out and say that it will be possible or how it would be done.

• 这可能吗?
• 操作员会是什么样子?

我对这样的用例特别感兴趣.

I'm particularly interested use cases like this.

void foo( const std::string& value )
{
   switch( std::hash(value) )
   {
      case "one"_hash: one(); break;
      case "two"_hash: two(); break;
      /*many more cases*/
      default: other(); break;
   }
}

注意:编译时哈希函数不必与我编写的完全一样.我尽力猜测最终的解决方案会是什么样子，但 meta_hash<"string"_meta>::value 也可能是一个可行的解决方案.

Note: the compile time hash function doesn't have to look exactly as I've written it. I did my best to guess what the final solution would look like, but meta_hash<"string"_meta>::value could also be a viable solution.

推荐答案

这有点晚了，但我成功地使用 constexpr 实现了编译时 CRC32 函数.它的问题在于，在撰写本文时，它仅适用于 GCC，不适用于 MSVC 或 Intel 编译器.

This is a little bit late, but I succeeded in implementing a compile-time CRC32 function with the use of constexpr. The problem with it is that at the time of writing, it only works with GCC and not MSVC nor Intel compiler.

这是代码片段:

// CRC32 Table (zlib polynomial)
static constexpr uint32_t crc_table[256] = {
    0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L,
    0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L,
    0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L,
...
};
template<size_t idx>
constexpr uint32_t crc32(const char * str)
{
    return (crc32<idx-1>(str) >> 8) ^ crc_table[(crc32<idx-1>(str) ^ str[idx]) & 0x000000FF];
}

// This is the stop-recursion function
template<>
constexpr uint32_t crc32<size_t(-1)>(const char * str)
{
    return 0xFFFFFFFF;
}

// This doesn't take into account the nul char
#define COMPILE_TIME_CRC32_STR(x) (crc32<sizeof(x) - 2>(x) ^ 0xFFFFFFFF)

enum TestEnum
{
    CrcVal01 = COMPILE_TIME_CRC32_STR("stack-overflow"),
};

CrcVal01 等于 0x335CC04A

CrcVal01 is equal to 0x335CC04A

希望对你有帮助！

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