hsearch_r.c

/*-
 * 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.
 */

#include <errno.h>
#include <limits.h>
#include <search.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "hsearch.h"

/*
 * Look up an unused entry in the hash table for a given hash. For this
 * implementation we use quadratic probing. Quadratic probing has the
 * advantage of preventing primary clustering.
 */
static ENTRY *
hsearch_lookup_free(struct __hsearch *hsearch, size_t hash)
{
	size_t index, i;

	for (index = hash, i = 0;; index += ++i) {
		ENTRY *entry = &hsearch->entries[index & hsearch->index_mask];
		if (entry->key == NULL)
			return (entry);
	}
}

/*
 * Computes an FNV-1a hash of the key. Depending on the pointer size, this
 * either uses the 32- or 64-bit FNV prime.
 */
static size_t
hsearch_hash(size_t offset_basis, const char *str)
{
	size_t hash;

	hash = offset_basis;
	while (*str != '\0') {
		hash ^= (uint8_t)*str++;
		if (sizeof(size_t) * CHAR_BIT <= 32)
			hash *= UINT32_C(16777619);
		else
			hash *= UINT64_C(1099511628211);
	}
	return (hash);
}

int
hsearch_r(ENTRY item, ACTION action, ENTRY **retval, struct hsearch_data *htab)
{
	struct __hsearch *hsearch;
	ENTRY *entry, *old_entries, *new_entries;
	size_t hash, index, i, old_hash, old_count, new_count;

	hsearch = htab->__hsearch;
	hash = hsearch_hash(hsearch->offset_basis, item.key);

	/*
	 * Search the hash table for an existing entry for this key.
	 * Stop searching if we run into an unused hash table entry.
	 */
	for (index = hash, i = 0;; index += ++i) {
		entry = &hsearch->entries[index & hsearch->index_mask];
		if (entry->key == NULL)
			break;
		if (strcmp(entry->key, item.key) == 0) {
			*retval = entry;
			return (1);
		}
	}

	/* Only perform the insertion if action is set to ENTER. */
	if (action == FIND) {
		errno = ESRCH;
		return (0);
	}

	if (hsearch->entries_used * 2 >= hsearch->index_mask) {
		/* Preserve the old hash table entries. */
		old_count = hsearch->index_mask + 1;
		old_entries = hsearch->entries;

		/*
		 * Allocate and install a new table if insertion would
		 * yield a hash table that is more than 50% used. By
		 * using 50% as a threshold, a lookup will only take up
		 * to two steps on average.
		 */
		new_count = (hsearch->index_mask + 1) * 2;
		new_entries = calloc(new_count, sizeof(ENTRY));
		if (new_entries == NULL)
			return (0);
		hsearch->entries = new_entries;
		hsearch->index_mask = new_count - 1;

		/* Copy over the entries from the old table to the new table. */
		for (i = 0; i < old_count; ++i) {
			entry = &old_entries[i];
			if (entry->key != NULL) {
				old_hash = hsearch_hash(hsearch->offset_basis,
				    entry->key);
				*hsearch_lookup_free(hsearch, old_hash) =
				    *entry;
			}
		}

		/* Destroy the old hash table entries. */
		free(old_entries);

		/*
		 * Perform a new lookup for a free table entry, so that
		 * we insert the entry into the new hash table.
		 */
		hsearch = htab->__hsearch;
		entry = hsearch_lookup_free(hsearch, hash);
	}

	/* Insert the new entry into the hash table. */
	*entry = item;
	++hsearch->entries_used;
	*retval = entry;
	return (1);
}