macro LFDS700_HASH_A_32BIT_HASH_FUNCTION

Source File

└───liblfds700
    └───inc
        └───liblfds700
                lfds700_hash_addonly.h

Macro

#define LFDS700_HASH_A_32BIT_HASH_FUNCTION( data, data_length_in_bytes, hash )

Parameters

data

A pointer of any kind, to the data to be hashed. This is cast to a char unsigned pointer in the macro, which iterates over every byte pointed to by data.

data_length_in_bytes

The length of the data pointed to by *data, in bytes.

hash

A 32 bit unsigned integer. It is important this is a 32 bit value - the hash constantly overflows the unsigned ranged and this is part of how it works. Using a longer type will affect how the hash behaves, and that means all bets are off. This argument is the output of the hashing function. It is NOT initialized to zero by the macro, so that the user can call the macro many tiems, perhaps on multiple members of a structure, to build up a hash. As such, the user needs to ensure this variable is set to zero before building the hash.

Return Value

No return value.

Example

In this example, we have an example structure which contains data we wish to hash on. The hash is initialized to zero, so we always end up with the same hash for the same data. We the call the macro twice, accumulating the result in hash. At the end of it,

struct test
{
  int long long unsigned
    user_id;

  char
    user_name[64];

  size_t
    user_name_length_in_bytes;
};

int unsigned
  hash = 0;

struct test
  t = { 10, "Paul McKenney", 12 };
 
LFDS700_HASH_A_32BIT_HASH_FUNCTION( &t.user_id, sizeof(int long long unsigned), hash );
LFDS700_HASH_A_32BIT_HASH_FUNCTION( &t.user_name, t.user_name_length_in_bytes, hash );

Notes

This is a convenience macro implemented for users. It implements the [one-at-a-time] hash, which emits to a 32 bit hash. It is provided to give users a high quality hash function to use in their key hash callbacks.

See Also

Hash (add-only)