calculations.h File Reference

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Macros
#define	REGISTER_HASH(hash_name)

Detailed Description

Each hash algorithm supported by the target needs to be declared as a functor. This is how you declare one:

struct hash_ex {
  uint32_t operator()(const char *buf, size_t s) const {
    const int p = 16777619;
    int hash = 2166136261;
 
    for (size_t i = 0; i < s; i++)
      hash = (hash ^ buf[i]) * p;
 
    hash += hash << 13;
    hash ^= hash >> 7;
    hash += hash << 3;
    hash ^= hash >> 17;
    hash += hash << 5;
    return static_cast<uint32_t>(hash);
  }
};

Note that the signature of the functor must exactly match the one of the example: uint32_t(const char *buf, size_t s) const. Otherwise, you will get a compilation error.

You can then register your hash function like this:

REGISTER_HASH(hash_ex)

In P4 v1.0.2, hash algorithms are used by field_list_calculation objects

Macro Definition Documentation

REGISTER_HASH

#define REGISTER_HASH

(

hash_name

)

Value:

  bool hash_name##_create_ =                                            \
      bm::CalculationsMap::get_instance()->register_one(                \
          #hash_name,                                                   \
          std::unique_ptr<bm::CalculationsMap::MyC>(                    \
              new bm::RawCalculation<uint64_t, hash_name>()));

When implementing an hash operation for a target, this macro needs to be called to make this module aware of the hash existence. When calling this macro from an anonymous namespace, some compiler may give you a warning.