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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;
}