Hash values

#include <libcork/core.h>

The functions in this section can be used to produce fast, good hash values.


For the curious, libcork currently uses the public-domain MurmurHash3 as its hash implementation.

Hashing in C code

A common pattern would be something along the lines of:

struct my_type {
    int  a;
    long  b;
    double  c;
    size_t  name_length;
    const char  *name;

my_type_hash(const struct my_type *self)
    /* hash of "struct my_type" */
    cork_hash  hash = 0xd4a130d8;
    hash = cork_hash_variable(hash, self->a);
    hash = cork_hash_variable(hash, self->b);
    hash = cork_hash_variable(hash, self->c);
    hash = cork_hash_buffer(hash, self->name, self->name_length);
    return hash;

In this example, the seed value (0xd4a130d8) is the hash of the constant string "struct my_type". You can produce seed values like this using the cork-hash script described below:

$ cork-hash "struct my_type"
uint32_t cork_hash
cork_hash cork_hash_buffer(cork_hash seed, const void *src, size_t len)
cork_hash cork_hash_variable(cork_hash seed, TYPE val)

Incorporate the contents of the given binary buffer or variable into a hash value. For the _variable variant, val must be an lvalue visible in the current scope.

The hash values produces by these functions can change over time, and might not be consistent across different platforms. The only guarantee is that hash values will be consistest for the duration of the current process.

cork_hash cork_stable_hash_buffer(cork_hash seed, const void *src, size_t len)
cork_hash cork_stable_hash_variable(cork_hash seed, TYPE val)

Stable versions of cork_hash_buffer() and cork_hash_variable(). We guarantee that the hash values produced by this function will be consistent across different platforms, and across different versions of the libcork library.

cork_big_hash cork_big_hash_buffer(cork_big_hash seed, const void *src, size_t len)

Incorporate the contents of the given binary buffer into a “big” hash value. A big hash value has a much larger space of possible hash values (128 bits vs 32).

bool cork_big_hash_equal(cork_big_hash hash1, cork_big_hash hash2)

Compare two big hash values for equality.

Hashing from the command line

Several parts of libcork use hash values as identifiers; you use a unique string to identify part of your code, and use the hash of that string as the actual identifier value. We provide a command-line utility that you can use to produce these hash values:

cork-hash <string>

The string to hash. This should be provided as a single argument on the command line, so if your string contains spaces or other shell meta-characters, you must enclose the string in quotes.

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