The Finest Radix Tree this side of the Linux Kernel
Josh Allmann <joshua.allmann@gmail.com>

Implementation of a Radix Tree. This is well suited to 'dense' sets
of keys, eg those that have many leading bits in common (a prefix).
For more information on the Radix Tree and its variations, see:

  * http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Tree/PATRICIA/
  * http://cr.yp.to/critbit.html
  * https://secure.wikimedia.org/wikipedia/en/wiki/Radix_tree

Keys are character strings, and must be null terminated.

API:

    rxt_node *rxt_init();

Initializes a radix tree node. This will allocate memory for the
root node and zero it out. It should be freed with rxt_free. Returns
a pointer to the root node, or NULL if malloc fails.

    void* rxt_get(char *key, rxt_node *root);

Returns the value associated with the key, starting the search from
the node at root. If the key is not found, NULL is returned.

    int rxt_put(char *key, void *value, rxt_node *root);

Inserts the key in the tree, starting from the root node. The key must
be null terminated.

Returns 0 for success, -1 for error. This could indicate a malloc error,
or an already existing key (values will not be overwritten).

    void* rxt_delete(char *key, rxt_node *root);

Removes the key from the tree, and returns its value. If the key
is not found, NULL is returned.

    void rxt_free(char *key);

Empties and frees the nodes in the tree. Since this excludes keys and
values, this might not be very useful unless the K/Vs are managed out-of-
tree.

Extras:

   -DLSB_FIRST

This compiler flag will start the bit comparision at the LSB (least
significant bit), rather than the MSB as is the default.

This will result in fewer compares when inserting and deleting, and
thus slightly faster performance. However, lexical ordering is lost
after the first differing byte*, and results may be unpredictable if
a strict ordering is needed. Eg, a LSB-first comparison would rank
"foob" before "fooa" because 'a' is ASCII 97 and 'b' is ASCII 98,
and the even final bit of 'b' is first, before the odd bit of 'a'.

* LSB (or MSB)-first searches are only done at the first differing byte
between two keys. Bytes are still searched in order; it is the bits
within those bytes that are compared differently.

For example, given two keys "fooa" and "foob", "foo" would be skipped
over, then the LSB search would kick in when comparing "a" and "b".
Sort of a reverse endianness.

LICENSE:

Simplified BSD. See LICENSE file.