Replace implementation of hsearch() by one that scales.

Traditionally the hcreate() function creates a hash table that uses
chaining, using a fixed user-provided size. The problem with this
approach is that this often either wastes memory (table too big) or
yields bad performance (table too small). For applications it may not
always be easy to estimate the right hash table size. A fixed number
only increases performance compared to a linked list by a constant
factor.

This problem can be solved easily by dynamically resizing the hash
table. If the size of the hash table is at least doubled, this has no
negative on the running time complexity. If a dynamically sized hash
table is used, we can also switch to using open addressing instead of
chaining, which has the advantage of just using a single allocation for
the entire table, instead of allocating many small objects.

Finally, a problem with the existing implementation is that its
deterministic algorithm for hashing makes it possible to come up with
fixed patterns to trigger an excessive number of collisions. We can
easily solve this by using FNV-1a as a hashing algorithm in combination
with a randomly generated offset basis.

Measurements have shown that this implementation is about 20-25% faster
than the existing implementation (even if the existing implementation is
given an excessive number of buckets). Though it allocates more memory
through malloc() than the old implementation (between 4-8 pointers per
used entry instead of 3), process memory use is similar to the old
implementation as if the estimated size was underestimated by a factor
10. This is due to the fact that malloc() needs to perform less
bookkeeping.

Reviewed by:	jilles, pfg
Obtained from:	https://github.com/NuxiNL/cloudlibc
Differential Revision:	https://reviews.freebsd.org/D4644

8 files changed, 336 insertions, 189 deletions

diff --git a/include/search.h b/include/search.h
index 8ddf416..4c1f534 100644
--- a/include/search.h
+++ b/include/search.h
@@ -47,11 +47,8 @@ struct que_elem {
 #endif
 
 #if __BSD_VISIBLE
-struct _ENTRY;
 struct hsearch_data {
-	struct _ENTRY *table;
-	size_t size;
-	size_t filled;
+	struct __hsearch *__hsearch;
 };
 #endif
 
diff --git a/lib/libc/stdlib/Makefile.inc b/lib/libc/stdlib/Makefile.inc
index 7cee03a..3c6ba17 100644
--- a/lib/libc/stdlib/Makefile.inc
+++ b/lib/libc/stdlib/Makefile.inc
@@ -6,7 +6,8 @@
 
 MISRCS+=_Exit.c a64l.c abort.c abs.c atexit.c atof.c atoi.c atol.c atoll.c \
 	bsearch.c div.c exit.c getenv.c getopt.c getopt_long.c \
-	getsubopt.c hcreate.c heapsort.c heapsort_b.c imaxabs.c imaxdiv.c \
+	getsubopt.c hcreate.c hcreate_r.c hdestroy_r.c heapsort.c heapsort_b.c \
+	hsearch_r.c imaxabs.c imaxdiv.c \
 	insque.c l64a.c labs.c ldiv.c llabs.c lldiv.c lsearch.c \
 	merge.c mergesort_b.c ptsname.c qsort.c qsort_r.c quick_exit.c \
 	radixsort.c rand.c \
diff --git a/lib/libc/stdlib/hcreate.3 b/lib/libc/stdlib/hcreate.3
index 2161f92..5709157 100644
--- a/lib/libc/stdlib/hcreate.3
+++ b/lib/libc/stdlib/hcreate.3
@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 21, 2014
+.Dd December 26, 2015
 .Dt HCREATE 3
 .Os
 .Sh NAME
@@ -76,8 +76,8 @@ The
 .Fa nel
 argument is an estimate of the maximum
 number of entries that the table should contain.
-This number may be adjusted upward by the
-algorithm in order to obtain certain mathematically favorable circumstances.
+As this implementation resizes the hash table dynamically,
+this argument is ignored.
 .Pp
 The
 .Fn hdestroy
@@ -274,8 +274,6 @@ functions will fail if:
 .Bl -tag -width Er
 .It Bq Er ENOMEM
 Insufficient memory is available.
-.It Bq Er EINVAL
-A table already exists.
 .El
 .Pp
 The
diff --git a/lib/libc/stdlib/hcreate.c b/lib/libc/stdlib/hcreate.c
index b3be9b4..2d5d943 100644
--- a/lib/libc/stdlib/hcreate.c
+++ b/lib/libc/stdlib/hcreate.c
@@ -1,9 +1,6 @@
-/* $NetBSD: hcreate.c,v 1.7 2011/09/14 23:33:51 christos Exp $ */
-
-/*
- * Copyright (c) 2001 Christopher G. Demetriou
- * All rights reserved.
- * 
+/*-
+ * 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:
@@ -12,207 +9,65 @@
  * 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.
- * 3. The name of the author may not be used to endorse or promote products
- *    derived from this software without specific prior written permission.
- * 
- * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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.
- * 
- * <<Id: LICENSE,v 1.2 2000/06/14 15:57:33 cgd Exp>>
- */
-
-/*
- * hcreate() / hsearch() / hdestroy()
  *
- * SysV/XPG4 hash table functions.
- *
- * Implementation done based on NetBSD manual page and Solaris manual page,
- * plus my own personal experience about how they're supposed to work.
- *
- * I tried to look at Knuth (as cited by the Solaris manual page), but
- * nobody had a copy in the office, so...
+ * 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 <sys/cdefs.h>
-#if 0
-#if defined(LIBC_SCCS) && !defined(lint)
-__RCSID("$NetBSD: hcreate.c,v 1.8 2011/09/17 16:54:39 christos Exp $");
-#endif /* LIBC_SCCS and not lint */
-#endif
 __FBSDID("$FreeBSD$");
 
-#include <sys/types.h>
-#include <sys/queue.h>
-#include <errno.h>
 #include <search.h>
-#include <stdlib.h>
-#include <string.h>
+#include <stdbool.h>
+#include <stddef.h>
 
 /*
- * DO NOT MAKE THIS STRUCTURE LARGER THAN 32 BYTES (4 ptrs on 64-bit
- * ptr machine) without adjusting MAX_BUCKETS_LG2 below.
+ * Thread unsafe interface: use a single process-wide hash table and
+ * forward calls to *_r() functions.
  */
-struct internal_entry {
-	SLIST_ENTRY(internal_entry) link;
-	ENTRY ent;
-};
-SLIST_HEAD(internal_head, internal_entry);
-
-#define	MIN_BUCKETS_LG2	4
-#define	MIN_BUCKETS	(1 << MIN_BUCKETS_LG2)
 
-/*
- * max * sizeof internal_entry must fit into size_t.
- * assumes internal_entry is <= 32 (2^5) bytes.
- */
-#define	MAX_BUCKETS_LG2	(sizeof (size_t) * 8 - 1 - 5)
-#define	MAX_BUCKETS	((size_t)1 << MAX_BUCKETS_LG2)
-
-/* Default hash function, from db/hash/hash_func.c */
-extern u_int32_t (*__default_hash)(const void *, size_t);
-
-static struct hsearch_data htable;
+static struct hsearch_data global_hashtable;
+static bool global_hashtable_initialized = false;
 
 int
 hcreate(size_t nel)
 {
 
-	/* Make sure this is not called when a table already exists. */
-	if (htable.table != NULL) {
-		errno = EINVAL;
-		return 0;
-	}
-	return hcreate_r(nel, &htable);
-}
-
-int
-hcreate_r(size_t nel, struct hsearch_data *head)
-{
-	struct internal_head *table;
-	size_t idx;
-	unsigned int p2;
-	void *p;
-
-	/* If nel is too small, make it min sized. */
-	if (nel < MIN_BUCKETS)
-		nel = MIN_BUCKETS;
-
-	/* If it is too large, cap it. */
-	if (nel > MAX_BUCKETS)
-		nel = MAX_BUCKETS;
-
-	/* If it is not a power of two in size, round up. */
-	if ((nel & (nel - 1)) != 0) {
-		for (p2 = 0; nel != 0; p2++)
-			nel >>= 1;
-		nel = 1 << p2;
-	}
-	
-	/* Allocate the table. */
-	head->size = nel;
-	head->filled = 0;
-	p = malloc(nel * sizeof table[0]);
-	if (p == NULL) {
-		errno = ENOMEM;
-		return 0;
-	}
-	head->table = p;
-	table = p;
-
-	/* Initialize it. */
-	for (idx = 0; idx < nel; idx++)
-		SLIST_INIT(&table[idx]);
-
-	return 1;
+	return (1);
 }
 
 void
 hdestroy(void)
 {
-	hdestroy_r(&htable);
-}
-
-void
-hdestroy_r(struct hsearch_data *head)
-{
-	struct internal_entry *ie;
-	size_t idx;
-	void *p;
-	struct internal_head *table;
-
-	if (head == NULL)
-		return;
-
-	p = head->table;
-	head->table = NULL;
-	table = p;
 
-	for (idx = 0; idx < head->size; idx++) {
-		while (!SLIST_EMPTY(&table[idx])) {
-			ie = SLIST_FIRST(&table[idx]);
-			SLIST_REMOVE_HEAD(&table[idx], link);
-			free(ie);
-		}
+	/* Destroy global hash table if present. */
+	if (global_hashtable_initialized) {
+		hdestroy_r(&global_hashtable);
+		global_hashtable_initialized = false;
 	}
-	free(table);
 }
 
 ENTRY *
 hsearch(ENTRY item, ACTION action)
 {
-	ENTRY *ep;
-	(void)hsearch_r(item, action, &ep, &htable);
-	return ep;
-}
-
-int
-hsearch_r(ENTRY item, ACTION action, ENTRY **itemp, struct hsearch_data *head)
-{
-	struct internal_head *table, *chain;
-	struct internal_entry *ie;
-	uint32_t hashval;
-	size_t len;
-	void *p;
-
-	p = head->table;
-	table = p;
+	ENTRY *retval;
 
-	len = strlen(item.key);
-	hashval = (*__default_hash)(item.key, len);
-
-	chain = &table[hashval & (head->size - 1)];
-	ie = SLIST_FIRST(chain);
-	while (ie != NULL) {
-		if (strcmp(ie->ent.key, item.key) == 0)
-			break;
-		ie = SLIST_NEXT(ie, link);
-	}
-
-	if (ie != NULL) {
-		*itemp = &ie->ent;
-		return 1;
-	} else if (action == FIND) {
-		*itemp = NULL;
-		errno = ESRCH;
-		return 1;
+	/* Create global hash table if needed. */
+	if (!global_hashtable_initialized) {
+		if (hcreate_r(0, &global_hashtable) == 0)
+			return (NULL);
+		global_hashtable_initialized = true;
 	}
-
-	ie = malloc(sizeof *ie);
-	if (ie == NULL)
-		return 0;
-	ie->ent.key = item.key;
-	ie->ent.data = item.data;
-
-	SLIST_INSERT_HEAD(chain, ie, link);
-	*itemp = &ie->ent;
-	head->filled++;
-	return 1;
+	if (hsearch_r(item, action, &retval, &global_hashtable) == 0)
+		return (NULL);
+	return (retval);
 }
diff --git a/lib/libc/stdlib/hcreate_r.c b/lib/libc/stdlib/hcreate_r.c
new file mode 100644
index 0000000..83e322a
--- /dev/null
+++ b/lib/libc/stdlib/hcreate_r.c
@@ -0,0 +1,63 @@
+/*-
+ * 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 <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <search.h>
+#include <stdlib.h>
+
+#include "hsearch.h"
+
+int
+hcreate_r(size_t nel, struct hsearch_data *htab)
+{
+	struct __hsearch *hsearch;
+
+	/*
+	 * Allocate a hash table object. Ignore the provided hint and start
+	 * off with a table of sixteen entries. In most cases this hint is
+	 * just a wild guess. Resizing the table dynamically if the use
+	 * increases a threshold does not affect the worst-case running time.
+	 */
+	hsearch = malloc(sizeof(*hsearch));
+	if (hsearch == NULL)
+		return 0;
+	hsearch->entries = calloc(16, sizeof(ENTRY));
+	if (hsearch->entries == NULL) {
+		free(hsearch);
+		return 0;
+	}
+
+	/*
+	 * Pick a random initialization for the FNV-1a hashing. This makes it
+	 * hard to come up with a fixed set of keys to force hash collisions.
+	 */
+	arc4random_buf(&hsearch->offset_basis, sizeof(hsearch->offset_basis));
+	hsearch->index_mask = 0xf;
+	hsearch->entries_used = 0;
+	htab->__hsearch = hsearch;
+	return 1;
+}
diff --git a/lib/libc/stdlib/hdestroy_r.c b/lib/libc/stdlib/hdestroy_r.c
new file mode 100644
index 0000000..890bd085
--- /dev/null
+++ b/lib/libc/stdlib/hdestroy_r.c
@@ -0,0 +1,43 @@
+/*-
+ * 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 <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <search.h>
+#include <stdlib.h>
+
+#include "hsearch.h"
+
+void
+hdestroy_r(struct hsearch_data *htab)
+{
+	struct __hsearch *hsearch;
+
+	/* Free hash table object and its entries. */
+	hsearch = htab->__hsearch;
+	free(hsearch->entries);
+	free(hsearch);
+}
diff --git a/lib/libc/stdlib/hsearch.h b/lib/libc/stdlib/hsearch.h
new file mode 100644
index 0000000..649933d
--- /dev/null
+++ b/lib/libc/stdlib/hsearch.h
@@ -0,0 +1,40 @@
+/*-
+ * 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 HSEARCH_H
+#define HSEARCH_H
+
+#include <search.h>
+
+struct __hsearch {
+	size_t offset_basis;	/* Initial value for FNV-1a hashing. */
+	size_t index_mask;	/* Bitmask for indexing the table. */
+	size_t entries_used;	/* Number of entries currently used. */
+	ENTRY *entries;		/* Hash table entries. */
+};
+
+#endif
diff --git a/lib/libc/stdlib/hsearch_r.c b/lib/libc/stdlib/hsearch_r.c
new file mode 100644
index 0000000..2fb5991
--- /dev/null
+++ b/lib/libc/stdlib/hsearch_r.c
@@ -0,0 +1,150 @@
+/*-
+ * 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 <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#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);
+}