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+# 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.
+
+## Overview
+```C
+#include <cx/hash_key.h>
+
+void 	  cx_hash_murmur(CxHashKey *key);
+CxHashKey cx_hash_key(const void *obj, size_t len);
+CxHashKey cx_hash_key_str(const char *str);
+CxHashKey cx_hash_key_bytes(const unsigned char *bytes, size_t len);
+CxHashKey cx_hash_key_cxstr(cxstring str);
+``` 
+
+## Description
+
+The primary function for creating a `CxHashKey` structure is `cx_hash_key()`.
+The other functions do effectively the same, but
+
+* `cx_hash_key_bytes()` is strongly typed if you want to avoid passing `void*` 
+* `cx_hash_key_str()` conveniently takes a C string and computes the length
+* `cx_hash_key_cxstr()` conveniently takes a [UCX string](string.h.md)
+
+In all cases, the hash will be 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);
+```
+
+<seealso>
+<category ref="apidoc">
+<a href="https://ucx.sourceforge.io/api/hash__key_8h.html">hash_key.h</a>
+</category>
+</seealso>
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