docs/Writerside/topics/hash_key.h.md

# Hash Function

UCX implements the MurmurHash2 algorithm for computing hashes that are primarily used for [CxMap](map.h.md).
But it can be used for arbitrary custom scenarios, too.

There are four functions which all generate a `CxHashKey` structure for given data.

* `cx_hash_key()` takes an arbitrary pointer and a length
* `cx_hash_key_bytes()` takes an `unsigned char*` and a length
* `cx_hash_key_str()` takes a usual C string
* `cx_hash_key_cxstr()` takes a [UCX string](string.h.md)

The hash is then available in the `hash` field of the returned structure.

<note>
UCX assigns the hash value <code>1574210520</code> to the <code>NULL</code> pointer.
This is a careful choice which is not standard MurmurHash2 and an extension to support <code>NULL</code> pointers.
</note>

If you want to create a hash completely manually,
you can initialize the `data` and `len` members of `CxHashKey`
and call `cx_hash_murmur()`.
It is _not_ recommended to do so.

Example that is equivalent to `CxHashKey key = cx_hash_str(mystring)`
```C
CxHashKey key;
key.data = mystring;
key.len = strlen(mystring);
cx_hash_murmur(&key);
```