source: vendor/python/2.5/Modules/rotatingtree.c

#include "rotatingtree.h"

#define KEY_LOWER_THAN(key1, key2)  ((char*)(key1) < (char*)(key2))

/* The randombits() function below is a fast-and-dirty generator that
 * is probably irregular enough for our purposes.  Note that it's biased:
 * I think that ones are slightly more probable than zeroes.  It's not
 * important here, though.
 */

static unsigned int random_value = 1;
static unsigned int random_stream = 0;

static int
randombits(int bits)
{
        int result;
        if (random_stream < (1U << bits)) {
                random_value *= 1082527;
                random_stream = random_value;
        }
        result = random_stream & ((1<<bits)-1);
        random_stream >>= bits;
        return result;
}


/* Insert a new node into the tree.
   (*root) is modified to point to the new root. */
void
RotatingTree_Add(rotating_node_t **root, rotating_node_t *node)
{
        while (*root != NULL) {
                if (KEY_LOWER_THAN(node->key, (*root)->key))
                        root = &((*root)->left);
                else
                        root = &((*root)->right);
        }
        node->left = NULL;
        node->right = NULL;
        *root = node;
}

/* Locate the node with the given key.  This is the most complicated
   function because it occasionally rebalances the tree to move the
   resulting node closer to the root. */
rotating_node_t *
RotatingTree_Get(rotating_node_t **root, void *key)
{
        if (randombits(3) != 4) {
                /* Fast path, no rebalancing */
                rotating_node_t *node = *root;
                while (node != NULL) {
                        if (node->key == key)
                                return node;
                        if (KEY_LOWER_THAN(key, node->key))
                                node = node->left;
                        else
                                node = node->right;
                }
                return NULL;
        }
        else {
                rotating_node_t **pnode = root;
                rotating_node_t *node = *pnode;
                rotating_node_t *next;
                int rotate;
                if (node == NULL)
                        return NULL;
                while (1) {
                        if (node->key == key)
                                return node;
                        rotate = !randombits(1);
                        if (KEY_LOWER_THAN(key, node->key)) {
                                next = node->left;
                                if (next == NULL)
                                        return NULL;
                                if (rotate) {
                                        node->left = next->right;
                                        next->right = node;
                                        *pnode = next;
                                }
                                else
                                        pnode = &(node->left);
                        }
                        else {
                                next = node->right;
                                if (next == NULL)
                                        return NULL;
                                if (rotate) {
                                        node->right = next->left;
                                        next->left = node;
                                        *pnode = next;
                                }
                                else
                                        pnode = &(node->right);
                        }
                        node = next;
                }
        }
}

/* Enumerate all nodes in the tree.  The callback enumfn() should return
   zero to continue the enumeration, or non-zero to interrupt it.
   A non-zero value is directly returned by RotatingTree_Enum(). */
int
RotatingTree_Enum(rotating_node_t *root, rotating_tree_enum_fn enumfn,
                  void *arg)
{
        int result;
        rotating_node_t *node;
        while (root != NULL) {
                result = RotatingTree_Enum(root->left, enumfn, arg);
                if (result != 0) return result;
                node = root->right;
                result = enumfn(root, arg);
                if (result != 0) return result;
                root = node;
        }
        return 0;
}