Can C Achieve Compile-Time String Hashing, and What Are Its Advantages?

Compile-Time String Hashing in C

The notion of computing a string's hash at compile time has sparked interest in the C community. This article explores the possibility and implications of this innovative approach.

Is Compile-Time String Hashing Feasible?

According to recent advancements in C standards, it is indeed possible to compute string hashes at compile time. This exciting development opens up new possibilities for more efficient code execution.

Syntax and Operators

The syntax for compile-time string hashing is not yet standardized. However, potential operators that could be considered include:

• std::hash(std::string_literal)
• meta_hash<"string"_meta>::value

Use Cases

Compile-time string hashing offers several valuable use cases, including:

• Efficient switch statements, as mentioned in the original question:

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;
   }
}

• Static table lookups
• Data structure optimization
• Symbolic constants generation

Example Implementation Using GCC

Although not fully standardized, it is possible to implement compile-time CRC32 hashing using GCC. Here's an example:

constexpr uint32_t crc32(const char *str)
{
    // Recursively compute CRC32 for each character
    return (crc32(str) >> 8) ^ crc_table[(crc32(str) ^ str[idx]) & 0x000000FF];
}

#define COMPILE_TIME_CRC32_STR(x) (crc32(x) ^ 0xFFFFFFFF)

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

In this example, the resulting CrcVal01 will be computed at compile time and assigned the calculated value.

Conclusion

Compile-time string hashing is an exciting area of development in C . Its potential for efficiency and flexibility is evident in the use cases mentioned above. As the C standard matures, standardized syntax and operators for this powerful technique will likely emerge.

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