Intrusive, single file, C89+ imperative AA tree that works on freestanding
systems.

The implementation is about 300 lines.

BSD 2-Clause license.

------------------
What are AA Trees?
------------------

`AA Trees <https://user.it.uu.se/~arneande/ps/simp.pdf>`_ are a type of
self-balancing binary tree. Unlike AVL trees or red-black trees, which
store a constant amount of information per node, AA trees store the
"level" of a node (which is approximately the height of the node from
the bottom of the tree). This makes rebalancing operations much simpler:
most of the code in this library is for handling standard binary tree
operations.

Theoretically, this limits the size of the binary tree to
``2**(2**16-1)`` nodes, which is no problem in practice.

This implementation is "intrusive", meaning that the node data structure
does not contain the key or value stored in it. You need to define your
own structure and search function like ::

	struct node {
		char *key;
		void *val;
		struct aatree node;
	};

	int node_cmp(struct aatree *a1, struct aatree *a2)
	{
		struct node *n1 = (void *)((char *)a1 - offsetof(struct node, node));
		struct node *n2 = (void *)((char *)a2 - offsetof(struct node, node));

		return strcmp(n1->key, n2->key);
	}

This is standard, portable C89. Intrusive code has the advantage of
requiring only one allocation instead of two, allowing for easier
memory management (i.e. arena memory allocation).

-----------------
Using the Library
-----------------

In *one* C file, do the following ::

	/* Optional declarations: */
	#define AATREE_PREFIX /* prefix of each function: put "declspec" or
	                         "static" here */
	#define AATREE_ASSERT /* Define this if you want the code to have
	                         asserts. This library uses assert() from the
	                         standard library. */

	/* Mandatory declarations */
	#define AATREE_IMPL
	#include "aatree.h"

This will include the implementation of the functions into the C file.
If your project consists of more than one C file, then all you need to
do is ``#include aatree.h``. The linker will take care of everything else.

The library also includes it own test code for convienence. Define
``AATREE_TEST`` to turn the header into a self-contained C file with a
``main()`` function. See ``Makefile`` for an example of compiling the
tests.