mwparserfromhell/src/mwparserfromhell/parser/ctokenizer/avl_tree.c

/*
 * avl_tree.c - intrusive, nonrecursive AVL tree data structure (self-balancing
 *              binary search tree), implementation file
 *
 * Written in 2014-2016 by Eric Biggers <ebiggers3@gmail.com>
 * Slight changes for compatibility by Ben Kurtovic <ben.kurtovic@gmail.com>
 *
 * To the extent possible under law, the author(s) have dedicated all copyright
 * and related and neighboring rights to this software to the public domain
 * worldwide via the Creative Commons Zero 1.0 Universal Public Domain
 * Dedication (the "CC0").
 *
 * This software is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the CC0 for more details.
 *
 * You should have received a copy of the CC0 along with this software; if not
 * see <http://creativecommons.org/publicdomain/zero/1.0/>.
 */

#define false 0
#define true 1

typedef int bool;

#include "avl_tree.h"

/* Returns the left child (sign < 0) or the right child (sign > 0) of the
 * specified AVL tree node.
 * Note: for all calls of this, 'sign' is constant at compilation time,
 * so the compiler can remove the conditional.  */
static AVL_INLINE struct avl_tree_node *
avl_get_child(const struct avl_tree_node *parent, int sign)
{
    if (sign < 0) {
        return parent->left;
    } else {
        return parent->right;
    }
}

static AVL_INLINE struct avl_tree_node *
avl_tree_first_or_last_in_order(const struct avl_tree_node *root, int sign)
{
    const struct avl_tree_node *first = root;

    if (first) {
        while (avl_get_child(first, +sign)) {
            first = avl_get_child(first, +sign);
        }
    }
    return (struct avl_tree_node *) first;
}

/* Starts an in-order traversal of the tree: returns the least-valued node, or
 * NULL if the tree is empty.  */
struct avl_tree_node *
avl_tree_first_in_order(const struct avl_tree_node *root)
{
    return avl_tree_first_or_last_in_order(root, -1);
}

/* Starts a *reverse* in-order traversal of the tree: returns the
 * greatest-valued node, or NULL if the tree is empty.  */
struct avl_tree_node *
avl_tree_last_in_order(const struct avl_tree_node *root)
{
    return avl_tree_first_or_last_in_order(root, 1);
}

static AVL_INLINE struct avl_tree_node *
avl_tree_next_or_prev_in_order(const struct avl_tree_node *node, int sign)
{
    const struct avl_tree_node *next;

    if (avl_get_child(node, +sign)) {
        for (next = avl_get_child(node, +sign); avl_get_child(next, -sign);
             next = avl_get_child(next, -sign)) {
        }
    } else {
        for (next = avl_get_parent(node); next && node == avl_get_child(next, +sign);
             node = next, next = avl_get_parent(next)) {
        }
    }
    return (struct avl_tree_node *) next;
}

/* Continues an in-order traversal of the tree: returns the next-greatest-valued
 * node, or NULL if there is none.  */
struct avl_tree_node *
avl_tree_next_in_order(const struct avl_tree_node *node)
{
    return avl_tree_next_or_prev_in_order(node, 1);
}

/* Continues a *reverse* in-order traversal of the tree: returns the
 * previous-greatest-valued node, or NULL if there is none.  */
struct avl_tree_node *
avl_tree_prev_in_order(const struct avl_tree_node *node)
{
    return avl_tree_next_or_prev_in_order(node, -1);
}

/* Starts a postorder traversal of the tree.  */
struct avl_tree_node *
avl_tree_first_in_postorder(const struct avl_tree_node *root)
{
    const struct avl_tree_node *first = root;

    if (first) {
        while (first->left || first->right) {
            first = first->left ? first->left : first->right;
        }
    }

    return (struct avl_tree_node *) first;
}

/* Continues a postorder traversal of the tree.  @prev will not be deferenced as
 * it's allowed that its memory has been freed; @prev_parent must be its saved
 * parent node.  Returns NULL if there are no more nodes (i.e. @prev was the
 * root of the tree).  */
struct avl_tree_node *
avl_tree_next_in_postorder(const struct avl_tree_node *prev,
                           const struct avl_tree_node *prev_parent)
{
    const struct avl_tree_node *next = prev_parent;

    if (next && prev == next->left && next->right) {
        for (next = next->right; next->left || next->right;
             next = next->left ? next->left : next->right) {
        }
    }
    return (struct avl_tree_node *) next;
}

/* Sets the left child (sign < 0) or the right child (sign > 0) of the
 * specified AVL tree node.
 * Note: for all calls of this, 'sign' is constant at compilation time,
 * so the compiler can remove the conditional.  */
static AVL_INLINE void
avl_set_child(struct avl_tree_node *parent, int sign, struct avl_tree_node *child)
{
    if (sign < 0) {
        parent->left = child;
    } else {
        parent->right = child;
    }
}

/* Sets the parent and balance factor of the specified AVL tree node.  */
static AVL_INLINE void
avl_set_parent_balance(struct avl_tree_node *node,
                       struct avl_tree_node *parent,
                       int balance_factor)
{
    node->parent_balance = (uintptr_t) parent | (balance_factor + 1);
}

/* Sets the parent of the specified AVL tree node.  */
static AVL_INLINE void
avl_set_parent(struct avl_tree_node *node, struct avl_tree_node *parent)
{
    node->parent_balance = (uintptr_t) parent | (node->parent_balance & 3);
}

/* Returns the balance factor of the specified AVL tree node --- that is, the
 * height of its right subtree minus the height of its left subtree.  */
static AVL_INLINE int
avl_get_balance_factor(const struct avl_tree_node *node)
{
    return (int) (node->parent_balance & 3) - 1;
}

/* Adds @amount to the balance factor of the specified AVL tree node.
 * The caller must ensure this still results in a valid balance factor
 * (-1, 0, or 1).  */
static AVL_INLINE void
avl_adjust_balance_factor(struct avl_tree_node *node, int amount)
{
    node->parent_balance += amount;
}

static AVL_INLINE void
avl_replace_child(struct avl_tree_node **root_ptr,
                  struct avl_tree_node *parent,
                  struct avl_tree_node *old_child,
                  struct avl_tree_node *new_child)
{
    if (parent) {
        if (old_child == parent->left) {
            parent->left = new_child;
        } else {
            parent->right = new_child;
        }
    } else {
        *root_ptr = new_child;
    }
}

/*
 * Template for performing a single rotation ---
 *
 * sign > 0:  Rotate clockwise (right) rooted at A:
 *
 *           P?            P?
 *           |             |
 *           A             B
 *          / \           / \
 *         B   C?  =>    D?  A
 *        / \               / \
 *       D?  E?            E?  C?
 *
 * (nodes marked with ? may not exist)
 *
 * sign < 0:  Rotate counterclockwise (left) rooted at A:
 *
 *           P?            P?
 *           |             |
 *           A             B
 *          / \           / \
 *         C?  B   =>    A   D?
 *            / \       / \
 *           E?  D?    C?  E?
 *
 * This updates pointers but not balance factors!
 */
static AVL_INLINE void
avl_rotate(struct avl_tree_node **const root_ptr,
           struct avl_tree_node *const A,
           const int sign)
{
    struct avl_tree_node *const B = avl_get_child(A, -sign);
    struct avl_tree_node *const E = avl_get_child(B, +sign);
    struct avl_tree_node *const P = avl_get_parent(A);

    avl_set_child(A, -sign, E);
    avl_set_parent(A, B);

    avl_set_child(B, +sign, A);
    avl_set_parent(B, P);

    if (E) {
        avl_set_parent(E, A);
    }

    avl_replace_child(root_ptr, P, A, B);
}

/*
 * Template for performing a double rotation ---
 *
 * sign > 0:  Rotate counterclockwise (left) rooted at B, then
 *                   clockwise (right) rooted at A:
 *
 *           P?            P?          P?
 *           |             |           |
 *           A             A           E
 *          / \           / \        /   \
 *         B   C?  =>    E   C? =>  B     A
 *        / \           / \        / \   / \
 *       D?  E         B   G?     D?  F?G?  C?
 *          / \       / \
 *         F?  G?    D?  F?
 *
 * (nodes marked with ? may not exist)
 *
 * sign < 0:  Rotate clockwise (right) rooted at B, then
 *                   counterclockwise (left) rooted at A:
 *
 *         P?          P?              P?
 *         |           |               |
 *         A           A               E
 *        / \         / \            /   \
 *       C?  B   =>  C?  E    =>    A     B
 *          / \         / \        / \   / \
 *         E   D?      G?  B      C?  G?F?  D?
 *        / \             / \
 *       G?  F?          F?  D?
 *
 * Returns a pointer to E and updates balance factors.  Except for those
 * two things, this function is equivalent to:
 *      avl_rotate(root_ptr, B, -sign);
 *      avl_rotate(root_ptr, A, +sign);
 *
 * See comment in avl_handle_subtree_growth() for explanation of balance
 * factor updates.
 */
static AVL_INLINE struct avl_tree_node *
avl_do_double_rotate(struct avl_tree_node **const root_ptr,
                     struct avl_tree_node *const B,
                     struct avl_tree_node *const A,
                     const int sign)
{
    struct avl_tree_node *const E = avl_get_child(B, +sign);
    struct avl_tree_node *const F = avl_get_child(E, -sign);
    struct avl_tree_node *const G = avl_get_child(E, +sign);
    struct avl_tree_node *const P = avl_get_parent(A);
    const int e = avl_get_balance_factor(E);

    avl_set_child(A, -sign, G);
    avl_set_parent_balance(A, E, ((sign * e >= 0) ? 0 : -e));

    avl_set_child(B, +sign, F);
    avl_set_parent_balance(B, E, ((sign * e <= 0) ? 0 : -e));

    avl_set_child(E, +sign, A);
    avl_set_child(E, -sign, B);
    avl_set_parent_balance(E, P, 0);

    if (G) {
        avl_set_parent(G, A);
    }

    if (F) {
        avl_set_parent(F, B);
    }

    avl_replace_child(root_ptr, P, A, E);

    return E;
}

/*
 * This function handles the growth of a subtree due to an insertion.
 *
 * @root_ptr
 *      Location of the tree's root pointer.
 *
 * @node
 *      A subtree that has increased in height by 1 due to an insertion.
 *
 * @parent
 *      Parent of @node; must not be NULL.
 *
 * @sign
 *      -1 if @node is the left child of @parent;
 *      +1 if @node is the right child of @parent.
 *
 * This function will adjust @parent's balance factor, then do a (single
 * or double) rotation if necessary.  The return value will be %true if
 * the full AVL tree is now adequately balanced, or %false if the subtree
 * rooted at @parent is now adequately balanced but has increased in
 * height by 1, so the caller should continue up the tree.
 *
 * Note that if %false is returned, no rotation will have been done.
 * Indeed, a single node insertion cannot require that more than one
 * (single or double) rotation be done.
 */
static AVL_INLINE bool
avl_handle_subtree_growth(struct avl_tree_node **const root_ptr,
                          struct avl_tree_node *const node,
                          struct avl_tree_node *const parent,
                          const int sign)
{
    int old_balance_factor, new_balance_factor;

    old_balance_factor = avl_get_balance_factor(parent);

    if (old_balance_factor == 0) {
        avl_adjust_balance_factor(parent, sign);
        /* @parent is still sufficiently balanced (-1 or +1
         * balance factor), but must have increased in height.
         * Continue up the tree.  */
        return false;
    }

    new_balance_factor = old_balance_factor + sign;

    if (new_balance_factor == 0) {
        avl_adjust_balance_factor(parent, sign);
        /* @parent is now perfectly balanced (0 balance factor).
         * It cannot have increased in height, so there is
         * nothing more to do.  */
        return true;
    }

    /* @parent is too left-heavy (new_balance_factor == -2) or
     * too right-heavy (new_balance_factor == +2).  */

    /* Test whether @node is left-heavy (-1 balance factor) or
     * right-heavy (+1 balance factor).
     * Note that it cannot be perfectly balanced (0 balance factor)
     * because here we are under the invariant that @node has
     * increased in height due to the insertion.  */
    if (sign * avl_get_balance_factor(node) > 0) {

        /* @node (B below) is heavy in the same direction @parent
         * (A below) is heavy.
         *
         * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
         * The comment, diagram, and equations below assume sign < 0.
         * The other case is symmetric!
         * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
         *
         * Do a clockwise rotation rooted at @parent (A below):
         *
         *           A              B
         *          / \           /   \
         *         B   C?  =>    D     A
         *        / \           / \   / \
         *       D   E?        F?  G?E?  C?
         *      / \
         *     F?  G?
         *
         * Before the rotation:
         *      balance(A) = -2
         *      balance(B) = -1
         * Let x = height(C).  Then:
         *      height(B) = x + 2
         *      height(D) = x + 1
         *      height(E) = x
         *      max(height(F), height(G)) = x.
         *
         * After the rotation:
         *      height(D) = max(height(F), height(G)) + 1
         *                = x + 1
         *      height(A) = max(height(E), height(C)) + 1
         *                = max(x, x) + 1 = x + 1
         *      balance(B) = 0
         *      balance(A) = 0
         */
        avl_rotate(root_ptr, parent, -sign);

        /* Equivalent to setting @parent's balance factor to 0.  */
        avl_adjust_balance_factor(parent, -sign); /* A */

        /* Equivalent to setting @node's balance factor to 0.  */
        avl_adjust_balance_factor(node, -sign); /* B */
    } else {
        /* @node (B below) is heavy in the direction opposite
         * from the direction @parent (A below) is heavy.
         *
         * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
         * The comment, diagram, and equations below assume sign < 0.
         * The other case is symmetric!
         * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
         *
         * Do a counterblockwise rotation rooted at @node (B below),
         * then a clockwise rotation rooted at @parent (A below):
         *
         *           A             A           E
         *          / \           / \        /   \
         *         B   C?  =>    E   C? =>  B     A
         *        / \           / \        / \   / \
         *       D?  E         B   G?     D?  F?G?  C?
         *          / \       / \
         *         F?  G?    D?  F?
         *
         * Before the rotation:
         *      balance(A) = -2
         *      balance(B) = +1
         * Let x = height(C).  Then:
         *      height(B) = x + 2
         *      height(E) = x + 1
         *      height(D) = x
         *      max(height(F), height(G)) = x
         *
         * After both rotations:
         *      height(A) = max(height(G), height(C)) + 1
         *                = x + 1
         *      balance(A) = balance(E{orig}) >= 0 ? 0 : -balance(E{orig})
         *      height(B) = max(height(D), height(F)) + 1
         *                = x + 1
         *      balance(B) = balance(E{orig} <= 0) ? 0 : -balance(E{orig})
         *
         *      height(E) = x + 2
         *      balance(E) = 0
         */
        avl_do_double_rotate(root_ptr, node, parent, -sign);
    }

    /* Height after rotation is unchanged; nothing more to do.  */
    return true;
}

/* Rebalance the tree after insertion of the specified node.  */
void
avl_tree_rebalance_after_insert(struct avl_tree_node **root_ptr,
                                struct avl_tree_node *inserted)
{
    struct avl_tree_node *node, *parent;
    bool done;

    inserted->left = NULL;
    inserted->right = NULL;

    node = inserted;

    /* Adjust balance factor of new node's parent.
     * No rotation will need to be done at this level.  */

    parent = avl_get_parent(node);
    if (!parent) {
        return;
    }

    if (node == parent->left) {
        avl_adjust_balance_factor(parent, -1);
    } else {
        avl_adjust_balance_factor(parent, +1);
    }

    if (avl_get_balance_factor(parent) == 0) {
        /* @parent did not change in height.  Nothing more to do.  */
        return;
    }

    /* The subtree rooted at @parent increased in height by 1.  */

    do {
        /* Adjust balance factor of next ancestor.  */

        node = parent;
        parent = avl_get_parent(node);
        if (!parent) {
            return;
        }

        /* The subtree rooted at @node has increased in height by 1.  */
        if (node == parent->left) {
            done = avl_handle_subtree_growth(root_ptr, node, parent, -1);
        } else {
            done = avl_handle_subtree_growth(root_ptr, node, parent, +1);
        }
    } while (!done);
}

/*
 * This function handles the shrinkage of a subtree due to a deletion.
 *
 * @root_ptr
 *      Location of the tree's root pointer.
 *
 * @parent
 *      A node in the tree, exactly one of whose subtrees has decreased
 *      in height by 1 due to a deletion.  (This includes the case where
 *      one of the child pointers has become NULL, since we can consider
 *      the "NULL" subtree to have a height of 0.)
 *
 * @sign
 *      +1 if the left subtree of @parent has decreased in height by 1;
 *      -1 if the right subtree of @parent has decreased in height by 1.
 *
 * @left_deleted_ret
 *      If the return value is not NULL, this will be set to %true if the
 *      left subtree of the returned node has decreased in height by 1,
 *      or %false if the right subtree of the returned node has decreased
 *      in height by 1.
 *
 * This function will adjust @parent's balance factor, then do a (single
 * or double) rotation if necessary.  The return value will be NULL if
 * the full AVL tree is now adequately balanced, or a pointer to the
 * parent of @parent if @parent is now adequately balanced but has
 * decreased in height by 1.  Also in the latter case, *left_deleted_ret
 * will be set.
 */
static AVL_INLINE struct avl_tree_node *
avl_handle_subtree_shrink(struct avl_tree_node **const root_ptr,
                          struct avl_tree_node *parent,
                          const int sign,
                          bool *const left_deleted_ret)
{
    struct avl_tree_node *node;
    int old_balance_factor, new_balance_factor;

    old_balance_factor = avl_get_balance_factor(parent);

    if (old_balance_factor == 0) {
        /* Prior to the deletion, the subtree rooted at
         * @parent was perfectly balanced.  It's now
         * unbalanced by 1, but that's okay and its height
         * hasn't changed.  Nothing more to do.  */
        avl_adjust_balance_factor(parent, sign);
        return NULL;
    }

    new_balance_factor = old_balance_factor + sign;

    if (new_balance_factor == 0) {
        /* The subtree rooted at @parent is now perfectly
         * balanced, whereas before the deletion it was
         * unbalanced by 1.  Its height must have decreased
         * by 1.  No rotation is needed at this location,
         * but continue up the tree.  */
        avl_adjust_balance_factor(parent, sign);
        node = parent;
    } else {
        /* @parent is too left-heavy (new_balance_factor == -2) or
         * too right-heavy (new_balance_factor == +2).  */

        node = avl_get_child(parent, sign);

        /* The rotations below are similar to those done during
         * insertion (see avl_handle_subtree_growth()), so full
         * comments are not provided.  The only new case is the
         * one where @node has a balance factor of 0, and that is
         * commented.  */

        if (sign * avl_get_balance_factor(node) >= 0) {

            avl_rotate(root_ptr, parent, -sign);

            if (avl_get_balance_factor(node) == 0) {
                /*
                 * @node (B below) is perfectly balanced.
                 *
                 * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
                 * The comment, diagram, and equations
                 * below assume sign < 0.  The other case
                 * is symmetric!
                 * @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
                 *
                 * Do a clockwise rotation rooted at
                 * @parent (A below):
                 *
                 *           A              B
                 *          / \           /   \
                 *         B   C?  =>    D     A
                 *        / \           / \   / \
                 *       D   E         F?  G?E   C?
                 *      / \
                 *     F?  G?
                 *
                 * Before the rotation:
                 *      balance(A) = -2
                 *      balance(B) =  0
                 * Let x = height(C).  Then:
                 *      height(B) = x + 2
                 *      height(D) = x + 1
                 *      height(E) = x + 1
                 *      max(height(F), height(G)) = x.
                 *
                 * After the rotation:
                 *      height(D) = max(height(F), height(G)) + 1
                 *                = x + 1
                 *      height(A) = max(height(E), height(C)) + 1
                 *                = max(x + 1, x) + 1 = x + 2
                 *      balance(A) = -1
                 *      balance(B) = +1
                 */

                /* A: -2 => -1 (sign < 0)
                 * or +2 => +1 (sign > 0)
                 * No change needed --- that's the same as
                 * old_balance_factor.  */

                /* B: 0 => +1 (sign < 0)
                 * or 0 => -1 (sign > 0)  */
                avl_adjust_balance_factor(node, -sign);

                /* Height is unchanged; nothing more to do.  */
                return NULL;
            } else {
                avl_adjust_balance_factor(parent, -sign);
                avl_adjust_balance_factor(node, -sign);
            }
        } else {
            node = avl_do_double_rotate(root_ptr, node, parent, -sign);
        }
    }
    parent = avl_get_parent(node);
    if (parent) {
        *left_deleted_ret = (node == parent->left);
    }
    return parent;
}

/* Swaps node X, which must have 2 children, with its in-order successor, then
 * unlinks node X.  Returns the parent of X just before unlinking, without its
 * balance factor having been updated to account for the unlink.  */
static AVL_INLINE struct avl_tree_node *
avl_tree_swap_with_successor(struct avl_tree_node **root_ptr,
                             struct avl_tree_node *X,
                             bool *left_deleted_ret)
{
    struct avl_tree_node *Y, *ret;

    Y = X->right;
    if (!Y->left) {
        /*
         *     P?           P?           P?
         *     |            |            |
         *     X            Y            Y
         *    / \          / \          / \
         *   A   Y    =>  A   X    =>  A   B?
         *      / \          / \
         *    (0)  B?      (0)  B?
         *
         * [ X unlinked, Y returned ]
         */
        ret = Y;
        *left_deleted_ret = false;
    } else {
        struct avl_tree_node *Q;

        do {
            Q = Y;
            Y = Y->left;
        } while (Y->left);

        /*
         *     P?           P?           P?
         *     |            |            |
         *     X            Y            Y
         *    / \          / \          / \
         *   A   ...  =>  A  ...   =>  A  ...
         *       |            |            |
         *       Q            Q            Q
         *      /            /            /
         *     Y            X            B?
         *    / \          / \
         *  (0)  B?      (0)  B?
         *
         *
         * [ X unlinked, Q returned ]
         */

        Q->left = Y->right;
        if (Q->left) {
            avl_set_parent(Q->left, Q);
        }
        Y->right = X->right;
        avl_set_parent(X->right, Y);
        ret = Q;
        *left_deleted_ret = true;
    }

    Y->left = X->left;
    avl_set_parent(X->left, Y);

    Y->parent_balance = X->parent_balance;
    avl_replace_child(root_ptr, avl_get_parent(X), X, Y);

    return ret;
}

/*
 * Removes an item from the specified AVL tree.
 *
 * @root_ptr
 *      Location of the AVL tree's root pointer.  Indirection is needed
 *      because the root node may change if the tree needed to be rebalanced
 *      because of the deletion or if @node was the root node.
 *
 * @node
 *      Pointer to the `struct avl_tree_node' embedded in the item to
 *      remove from the tree.
 *
 * Note: This function *only* removes the node and rebalances the tree.
 * It does not free any memory, nor does it do the equivalent of
 * avl_tree_node_set_unlinked().
 */
void
avl_tree_remove(struct avl_tree_node **root_ptr, struct avl_tree_node *node)
{
    struct avl_tree_node *parent;
    bool left_deleted = false;

    if (node->left && node->right) {
        /* @node is fully internal, with two children.  Swap it
         * with its in-order successor (which must exist in the
         * right subtree of @node and can have, at most, a right
         * child), then unlink @node.  */
        parent = avl_tree_swap_with_successor(root_ptr, node, &left_deleted);
        /* @parent is now the parent of what was @node's in-order
         * successor.  It cannot be NULL, since @node itself was
         * an ancestor of its in-order successor.
         * @left_deleted has been set to %true if @node's
         * in-order successor was the left child of @parent,
         * otherwise %false.  */
    } else {
        struct avl_tree_node *child;

        /* @node is missing at least one child.  Unlink it.  Set
         * @parent to @node's parent, and set @left_deleted to
         * reflect which child of @parent @node was.  Or, if
         * @node was the root node, simply update the root node
         * and return.  */
        child = node->left ? node->left : node->right;
        parent = avl_get_parent(node);
        if (parent) {
            if (node == parent->left) {
                parent->left = child;
                left_deleted = true;
            } else {
                parent->right = child;
                left_deleted = false;
            }
            if (child) {
                avl_set_parent(child, parent);
            }
        } else {
            if (child) {
                avl_set_parent(child, parent);
            }
            *root_ptr = child;
            return;
        }
    }

    /* Rebalance the tree.  */
    do {
        if (left_deleted) {
            parent = avl_handle_subtree_shrink(root_ptr, parent, +1, &left_deleted);
        } else {
            parent = avl_handle_subtree_shrink(root_ptr, parent, -1, &left_deleted);
        }
    } while (parent);
}