xref: /freebsd/lib/libc/stdlib/tsearch_path.h (revision 459d04a5eecbe8ea330ef9c6cbba6392b16d317e)

/*-
 * Copyright (c) 2015 Nuxi, https://nuxi.nl/
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD$
 */

#ifndef TSEARCH_PATH_H
#define	TSEARCH_PATH_H

#include <limits.h>
#include <stdbool.h>
#include <stdint.h>

/*
 * Bookkeeping for storing a path in a balanced binary search tree from
 * the root to a leaf node.
 *
 * For an AVL tree we know that its maximum height of a tree is bounded
 * by approximately 1.44 * log2(n) - 0.328. Given that the number of
 * entries of the tree is constrained by the size of the address space,
 * two uintptr_t's provide sufficient space to store the path from the
 * root to any leaf.
 */
struct path {
	uintptr_t steps[2];
	unsigned int nsteps;
};

/* Initializes the path structure with a zero-length path. */
static inline void
path_init(struct path *p)
{

	p->nsteps = 0;
}

#define	STEPS_BIT (sizeof(uintptr_t) * CHAR_BIT)

/* Pushes a step to the left to the end of the path. */
static inline void
path_taking_left(struct path *p)
{

	p->steps[p->nsteps / STEPS_BIT] |=
	    (uintptr_t)1 << (p->nsteps % STEPS_BIT);
	++p->nsteps;
}

/* Pushes a step to the right to the end of the path. */
static inline void
path_taking_right(struct path *p)
{

	p->steps[p->nsteps / STEPS_BIT] &=
	    ~((uintptr_t)1 << (p->nsteps % STEPS_BIT));
	++p->nsteps;
}

/*
 * Pops the first step from the path and returns whether it was a step
 * to the left.
 */
static inline bool
path_took_left(struct path *p)
{
	bool result;

	result = p->steps[0] & 0x1;
	p->steps[0] = (p->steps[0] >> 1) | (p->steps[1] << (STEPS_BIT - 1));
	p->steps[1] >>= 1;
	return (result);
}

#undef STEPS_BIT

#endif