glibc/misc/hash.c

/* hash - hashing table processing.

   Copyright (C) 1998-2004, 2006-2007, 2009-2026 Free Software Foundation, Inc.

   Written by Jim Meyering, 1992.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file 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
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* A generic hash table package.  */

/* Define USE_OBSTACK to 1 if you want the allocator to use obstacks instead
   of malloc.  If you change USE_OBSTACK, you have to recompile!  */

#include <config.h>
#ifdef _LIBC
# define USE_OBSTACK           0
# define TESTING               0
# include <stdbit.h>
# define rotr_sz(__x, __n)     stdc_rotate_right(__x, __n)
#else
# include "bitrotate.h"
#endif

#include "hash.h"

#include "next-prime.h"
#include "xalloc-oversized.h"

#include <errno.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#if USE_OBSTACK
# include "obstack.h"
# ifndef obstack_chunk_alloc
#  define obstack_chunk_alloc malloc
# endif
# ifndef obstack_chunk_free
#  define obstack_chunk_free free
# endif
#endif

struct hash_entry
  {
    void *data;
    struct hash_entry *next;
    /* Invariant: If DATA is NULL, this entry is an unused bucket head,
       and therefore NEXT is NULL as well.  */
  };

struct hash_table
  {
    /* The array of buckets starts at BUCKET and extends to BUCKET_LIMIT-1,
       for a possibility of N_BUCKETS.  Among those, N_BUCKETS_USED buckets
       are not empty, there are N_ENTRIES active entries in the table.  */
    struct hash_entry *bucket;
    struct hash_entry const *bucket_limit;
    size_t n_buckets;
    size_t n_buckets_used;
    size_t n_entries;

    /* Tuning arguments, kept in a physically separate structure.  */
    const Hash_tuning *tuning;

    /* Three functions are given to 'hash_initialize', see the documentation
       block for this function.  In a word, HASHER randomizes a user entry
       into a number up from 0 up to some maximum minus 1; COMPARATOR returns
       true if two user entries compare equally; and DATA_FREER is the cleanup
       function for a user entry.  */
    Hash_hasher hasher;
    Hash_comparator comparator;
    Hash_data_freer data_freer;

    /* A linked list of freed struct hash_entry structs.  */
    struct hash_entry *free_entry_list;

#if USE_OBSTACK
    /* Whenever obstacks are used, it is possible to allocate all overflowed
       entries into a single stack, so they all can be freed in a single
       operation.  It is not clear if the speedup is worth the trouble.  */
    struct obstack entry_stack;
#endif
  };

/* A hash table contains many internal entries, each holding a pointer to
   some user-provided data (also called a user entry).  An entry indistinctly
   refers to both the internal entry and its associated user entry.  A user
   entry contents may be hashed by a randomization function (the hashing
   function, or just "hasher" for short) into a number (or "slot") between 0
   and the current table size.  At each slot position in the hash table,
   starts a linked chain of entries for which the user data all hash to this
   slot.  A bucket is the collection of all entries hashing to the same slot.

   A good "hasher" function will distribute entries rather evenly in buckets.
   In the ideal case, the length of each bucket is roughly the number of
   entries divided by the table size.  Finding the slot for a data is usually
   done in constant time by the "hasher", and the later finding of a precise
   entry is linear in time with the size of the bucket.  Consequently, a
   larger hash table size (that is, a larger number of buckets) is prone to
   yielding shorter chains, *given* the "hasher" function behaves properly.

   Long buckets slow down the lookup algorithm.  One might use big hash table
   sizes in hope to reduce the average length of buckets, but this might
   become inordinate, as unused slots in the hash table take some space.  The
   best bet is to make sure you are using a good "hasher" function (beware
   that those are not that easy to write! :-), and to use a table size
   larger than the actual number of entries.  */

/* If an insertion makes the ratio of nonempty buckets to table size larger
   than the growth threshold (a number between 0.0 and 1.0), then increase
   the table size by multiplying by the growth factor (a number greater than
   1.0).  The growth threshold defaults to 0.8, and the growth factor
   defaults to 1.414, meaning that the table will have doubled its size
   every second time 80% of the buckets get used.  */
#define DEFAULT_GROWTH_THRESHOLD 0.8f
#define DEFAULT_GROWTH_FACTOR 1.414f

/* If a deletion empties a bucket and causes the ratio of used buckets to
   table size to become smaller than the shrink threshold (a number between
   0.0 and 1.0), then shrink the table by multiplying by the shrink factor (a
   number greater than the shrink threshold but smaller than 1.0).  The shrink
   threshold and factor default to 0.0 and 1.0, meaning that the table never
   shrinks.  */
#define DEFAULT_SHRINK_THRESHOLD 0.0f
#define DEFAULT_SHRINK_FACTOR 1.0f

/* Use this to initialize or reset a TUNING structure to
   some sensible values. */
static const Hash_tuning default_tuning =
  {
    DEFAULT_SHRINK_THRESHOLD,
    DEFAULT_SHRINK_FACTOR,
    DEFAULT_GROWTH_THRESHOLD,
    DEFAULT_GROWTH_FACTOR,
    false
  };

/* Information and lookup.  */

#if !_LIBC
size_t
hash_get_n_buckets (const Hash_table *table)
{
  return table->n_buckets;
}

size_t
hash_get_n_buckets_used (const Hash_table *table)
{
  return table->n_buckets_used;
}

size_t
hash_get_n_entries (const Hash_table *table)
{
  return table->n_entries;
}

size_t
hash_get_max_bucket_length (const Hash_table *table)
{
  size_t max_bucket_length = 0;

  for (struct hash_entry const *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket->data)
        {
          struct hash_entry const *cursor = bucket;
          size_t bucket_length = 1;

          while (cursor = cursor->next, cursor)
            bucket_length++;

          if (bucket_length > max_bucket_length)
            max_bucket_length = bucket_length;
        }
    }

  return max_bucket_length;
}

bool
hash_table_ok (const Hash_table *table)
{
  size_t n_buckets_used = 0;
  size_t n_entries = 0;

  for (struct hash_entry const *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket->data)
        {
          struct hash_entry const *cursor = bucket;

          /* Count bucket head.  */
          n_buckets_used++;
          n_entries++;

          /* Count bucket overflow.  */
          while (cursor = cursor->next, cursor)
            n_entries++;
        }
    }

  if (n_buckets_used == table->n_buckets_used && n_entries == table->n_entries)
    return true;

  return false;
}

void
hash_print_statistics (const Hash_table *table, FILE *stream)
{
  size_t n_entries = hash_get_n_entries (table);
  size_t n_buckets = hash_get_n_buckets (table);
  size_t n_buckets_used = hash_get_n_buckets_used (table);
  size_t max_bucket_length = hash_get_max_bucket_length (table);

  fprintf (stream, "# entries:         %lu\n", (unsigned long int) n_entries);
  fprintf (stream, "# buckets:         %lu\n", (unsigned long int) n_buckets);
  fprintf (stream, "# buckets used:    %lu (%.2f%%)\n",
           (unsigned long int) n_buckets_used,
           (100.0 * n_buckets_used) / n_buckets);
  fprintf (stream, "max bucket length: %lu\n",
           (unsigned long int) max_bucket_length);
}
#endif

/* Hash KEY and return a pointer to the selected bucket.
   If TABLE->hasher misbehaves, abort.  */
static struct hash_entry *
safe_hasher (const Hash_table *table, const void *key)
{
  size_t n = table->hasher (key, table->n_buckets);
  if (! (n < table->n_buckets))
    abort ();
  return table->bucket + n;
}

void *
hash_lookup (const Hash_table *table, const void *entry)
{
  struct hash_entry const *bucket = safe_hasher (table, entry);

  if (bucket->data == NULL)
    return NULL;

  for (struct hash_entry const *cursor = bucket; cursor; cursor = cursor->next)
    if (entry == cursor->data || table->comparator (entry, cursor->data))
      return cursor->data;

  return NULL;
}

#if !_LIBC
/* Walking.  */

void *
hash_get_first (const Hash_table *table)
{
  if (table->n_entries == 0)
    return NULL;

  for (struct hash_entry const *bucket = table->bucket; ; bucket++)
    if (! (bucket < table->bucket_limit))
      abort ();
    else if (bucket->data)
      return bucket->data;
}

void *
hash_get_next (const Hash_table *table, const void *entry)
{
  struct hash_entry const *bucket = safe_hasher (table, entry);

  /* Find next entry in the same bucket.  */
  {
    struct hash_entry const *cursor = bucket;
    do
      {
        if (cursor->data == entry && cursor->next)
          return cursor->next->data;
        cursor = cursor->next;
      }
    while (cursor != NULL);
  }

  /* Find first entry in any subsequent bucket.  */
  while (++bucket < table->bucket_limit)
    if (bucket->data)
      return bucket->data;

  /* None found.  */
  return NULL;
}

size_t
hash_get_entries (const Hash_table *table, void **buffer,
                  size_t buffer_size)
{
  size_t counter = 0;

  for (struct hash_entry const *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket->data)
        {
          for (struct hash_entry const *cursor = bucket; cursor; cursor = cursor->next)
            {
              if (counter >= buffer_size)
                return counter;
              buffer[counter++] = cursor->data;
            }
        }
    }

  return counter;
}

size_t
hash_do_for_each (const Hash_table *table, Hash_processor processor,
                  void *processor_data)
{
  size_t counter = 0;

  for (struct hash_entry const *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket->data)
        {
          for (struct hash_entry const *cursor = bucket; cursor; cursor = cursor->next)
            {
              if (! processor (cursor->data, processor_data))
                return counter;
              counter++;
            }
        }
    }

  return counter;
}
#endif


/* If the user passes a NULL hasher, we hash the raw pointer.  */
static size_t
raw_hasher (const void *data, size_t n)
{
  /* When hashing unique pointers, it is often the case that they were
     generated by malloc and thus have the property that the low-order
     bits are 0.  As this tends to give poorer performance with small
     tables, we rotate the pointer value before performing division,
     in an attempt to improve hash quality.  */
  size_t val = rotr_sz ((size_t) data, 3);
  return val % n;
}

/* If the user passes a NULL comparator, we use pointer comparison.  */
static bool
raw_comparator (const void *a, const void *b)
{
  return a == b;
}


#if !_LIBC
/* Allocation and clean-up.  */

void
hash_reset_tuning (Hash_tuning *tuning)
{
  *tuning = default_tuning;
}
#endif

/* For the given hash TABLE, check the user supplied tuning structure for
   reasonable values, and return true if there is no gross error with it.
   Otherwise, definitively reset the TUNING field to some acceptable default
   in the hash table (that is, the user loses the right of further modifying
   tuning arguments), and return false.  */

static bool
check_tuning (Hash_table *table)
{
  const Hash_tuning *tuning = table->tuning;
  if (tuning == &default_tuning)
    return true;

  /* Be a bit stricter than mathematics would require, so that
     rounding errors in size calculations do not cause allocations to
     fail to grow or shrink as they should.  The smallest allocation
     is 11 (due to next_prime's algorithm), so an epsilon of 0.1
     should be good enough.  */
  float epsilon = 0.1f;

  if (epsilon < tuning->growth_threshold
      && tuning->growth_threshold < 1 - epsilon
      && 1 + epsilon < tuning->growth_factor
      && 0 <= tuning->shrink_threshold
      && tuning->shrink_threshold + epsilon < tuning->shrink_factor
      && tuning->shrink_factor <= 1
      && tuning->shrink_threshold + epsilon < tuning->growth_threshold)
    return true;

  table->tuning = &default_tuning;
  return false;
}

/* Compute the size of the bucket array for the given CANDIDATE and
   TUNING, or return 0 if there is no possible way to allocate that
   many entries.  */

static size_t _GL_ATTRIBUTE_PURE
compute_bucket_size (size_t candidate, const Hash_tuning *tuning)
{
  if (!tuning->is_n_buckets)
    {
      float new_candidate = candidate / tuning->growth_threshold;
      if ((float) SIZE_MAX <= new_candidate)
        goto nomem;
      candidate = new_candidate;
    }
  candidate = next_prime (candidate);
  if (xalloc_oversized (candidate, sizeof (struct hash_entry *)))
    goto nomem;
  return candidate;

 nomem:
  errno = ENOMEM;
  return 0;
}

Hash_table *
hash_initialize (size_t candidate, const Hash_tuning *tuning,
                 Hash_hasher hasher, Hash_comparator comparator,
                 Hash_data_freer data_freer)
{
  if (hasher == NULL)
    hasher = raw_hasher;
  if (comparator == NULL)
    comparator = raw_comparator;

  Hash_table *table = malloc (sizeof *table);
  if (table == NULL)
    return NULL;

  if (!tuning)
    tuning = &default_tuning;
  table->tuning = tuning;
  if (!check_tuning (table))
    {
      /* Fail if the tuning options are invalid.  This is the only occasion
         when the user gets some feedback about it.  Once the table is created,
         if the user provides invalid tuning options, we silently revert to
         using the defaults, and ignore further request to change the tuning
         options.  */
      errno = EINVAL;
      goto fail;
    }

  table->n_buckets = compute_bucket_size (candidate, tuning);
  if (!table->n_buckets)
    goto fail;

  table->bucket = calloc (table->n_buckets, sizeof *table->bucket);
  if (table->bucket == NULL)
    goto fail;
  table->bucket_limit = table->bucket + table->n_buckets;
  table->n_buckets_used = 0;
  table->n_entries = 0;

  table->hasher = hasher;
  table->comparator = comparator;
  table->data_freer = data_freer;

  table->free_entry_list = NULL;
#if USE_OBSTACK
  obstack_init (&table->entry_stack);
#endif
  return table;

 fail:
  free (table);
  return NULL;
}

#if !_LIBC
void
hash_clear (Hash_table *table)
{
  for (struct hash_entry *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket->data)
        {
          /* Free the bucket overflow.  */
          for (struct hash_entry *cursor = bucket->next; cursor; )
            {
              if (table->data_freer)
                table->data_freer (cursor->data);
              cursor->data = NULL;

              struct hash_entry *next = cursor->next;
              /* Relinking is done one entry at a time, as it is to be expected
                 that overflows are either rare or short.  */
              cursor->next = table->free_entry_list;
              table->free_entry_list = cursor;

              cursor = next;
            }

          /* Free the bucket head.  */
          if (table->data_freer)
            table->data_freer (bucket->data);
          bucket->data = NULL;
          bucket->next = NULL;
        }
    }

  table->n_buckets_used = 0;
  table->n_entries = 0;
}
#endif

void
hash_free (Hash_table *table)
{
  int saved_errno = errno;

  /* Call the user data_freer function.  */
  if (table->data_freer && table->n_entries)
    {
      for (struct hash_entry *bucket = table->bucket;
           bucket < table->bucket_limit;
           bucket++)
        {
          if (bucket->data)
            {
              for (struct hash_entry *cursor = bucket; cursor; cursor = cursor->next)
                table->data_freer (cursor->data);
            }
        }
    }

#if USE_OBSTACK

  obstack_free (&table->entry_stack, NULL);

#else

  /* Free all bucket overflowed entries.  */
  for (struct hash_entry *bucket = table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      for (struct hash_entry *cursor = bucket->next; cursor; )
        {
          struct hash_entry *next = cursor->next;
          free (cursor);
          cursor = next;
        }
    }

  /* Also reclaim the internal list of previously freed entries.  */
  for (struct hash_entry *cursor = table->free_entry_list; cursor; )
    {
      struct hash_entry *next = cursor->next;
      free (cursor);
      cursor = next;
    }

#endif

  /* Free the remainder of the hash table structure.  */
  free (table->bucket);
  free (table);

  errno = saved_errno;
}

/* Insertion and deletion.  */

/* Get a new hash entry for a bucket overflow, possibly by recycling a
   previously freed one.  If this is not possible, allocate a new one.  */

static struct hash_entry *
allocate_entry (Hash_table *table)
{
  struct hash_entry *new;

  if (table->free_entry_list)
    {
      new = table->free_entry_list;
      table->free_entry_list = new->next;
    }
  else
    {
#if USE_OBSTACK
      new = obstack_alloc (&table->entry_stack, sizeof *new);
#else
      new = malloc (sizeof *new);
#endif
    }

  return new;
}

/* Free a hash entry which was part of some bucket overflow,
   saving it for later recycling.  */

static void
free_entry (Hash_table *table, struct hash_entry *entry)
{
  entry->data = NULL;
  entry->next = table->free_entry_list;
  table->free_entry_list = entry;
}

/* This private function is used to help with insertion and deletion.  When
   ENTRY matches an entry in the table, return a pointer to the corresponding
   user data and set *BUCKET_HEAD to the head of the selected bucket.
   Otherwise, return NULL.  When DELETE is true and ENTRY matches an entry in
   the table, unlink the matching entry.  */

static void *
find_entry (Hash_table *table, const void *entry,
            struct hash_entry **bucket_head, bool delete)
{
  struct hash_entry *bucket = safe_hasher (table, entry);

  *bucket_head = bucket;

  /* Test for empty bucket.  */
  if (bucket->data == NULL)
    return NULL;

  /* See if the entry is the first in the bucket.  */
  if (entry == bucket->data || table->comparator (entry, bucket->data))
    {
      void *data = bucket->data;

      if (delete)
        {
          if (bucket->next)
            {
              struct hash_entry *next = bucket->next;

              /* Bump the first overflow entry into the bucket head, then save
                 the previous first overflow entry for later recycling.  */
              *bucket = *next;
              free_entry (table, next);
            }
          else
            {
              bucket->data = NULL;
            }
        }

      return data;
    }

  /* Scan the bucket overflow.  */
  for (struct hash_entry *cursor = bucket; cursor->next; cursor = cursor->next)
    {
      if (entry == cursor->next->data
          || table->comparator (entry, cursor->next->data))
        {
          void *data = cursor->next->data;

          if (delete)
            {
              struct hash_entry *next = cursor->next;

              /* Unlink the entry to delete, then save the freed entry for later
                 recycling.  */
              cursor->next = next->next;
              free_entry (table, next);
            }

          return data;
        }
    }

  /* No entry found.  */
  return NULL;
}

/* Internal helper, to move entries from SRC to DST.  Both tables must
   share the same free entry list.  If SAFE, only move overflow
   entries, saving bucket heads for later, so that no allocations will
   occur.  Return false (setting errno) if the free entry list is
   exhausted and an allocation fails.  */

static bool
transfer_entries (Hash_table *dst, Hash_table *src, bool safe)
{
  for (struct hash_entry *bucket = src->bucket; bucket < src->bucket_limit; bucket++)
    if (bucket->data)
      {
        void *data;
        struct hash_entry *new_bucket;

        /* Within each bucket, transfer overflow entries first and
           then the bucket head, to minimize memory pressure.  After
           all, the only time we might allocate is when moving the
           bucket head, but moving overflow entries first may create
           free entries that can be recycled by the time we finally
           get to the bucket head.  */
        for (struct hash_entry *cursor = bucket->next; cursor; )
          {
            data = cursor->data;
            new_bucket = safe_hasher (dst, data);

            struct hash_entry *next = cursor->next;

            if (new_bucket->data)
              {
                /* Merely relink an existing entry, when moving from a
                   bucket overflow into a bucket overflow.  */
                cursor->next = new_bucket->next;
                new_bucket->next = cursor;
              }
            else
              {
                /* Free an existing entry, when moving from a bucket
                   overflow into a bucket header.  */
                new_bucket->data = data;
                dst->n_buckets_used++;
                free_entry (dst, cursor);
              }

            cursor = next;
          }
        /* Now move the bucket head.  Be sure that if we fail due to
           allocation failure that the src table is in a consistent
           state.  */
        data = bucket->data;
        bucket->next = NULL;
        if (!safe)
          {
            new_bucket = safe_hasher (dst, data);

            if (new_bucket->data)
              {
                /* Allocate or recycle an entry, when moving from a bucket
                   header into a bucket overflow.  */
                struct hash_entry *new_entry = allocate_entry (dst);

                if (new_entry == NULL)
                  return false;

                new_entry->data = data;
                new_entry->next = new_bucket->next;
                new_bucket->next = new_entry;
              }
            else
              {
                /* Move from one bucket header to another.  */
                new_bucket->data = data;
                dst->n_buckets_used++;
              }
            bucket->data = NULL;
            src->n_buckets_used--;
          }
      }
  return true;
}

bool
hash_rehash (Hash_table *table, size_t candidate)
{
  size_t new_size = compute_bucket_size (candidate, table->tuning);

  if (!new_size)
    return false;
  if (new_size == table->n_buckets)
    return true;

  Hash_table storage;
  Hash_table *new_table = &storage;
  new_table->bucket = calloc (new_size, sizeof *new_table->bucket);
  if (new_table->bucket == NULL)
    return false;
  new_table->n_buckets = new_size;
  new_table->bucket_limit = new_table->bucket + new_size;
  new_table->n_buckets_used = 0;
  new_table->n_entries = 0;
  new_table->tuning = table->tuning;
  new_table->hasher = table->hasher;
  new_table->comparator = table->comparator;
  new_table->data_freer = table->data_freer;

  /* In order for the transfer to successfully complete, we need
     additional overflow entries when distinct buckets in the old
     table collide into a common bucket in the new table.  The worst
     case possible is a hasher that gives a good spread with the old
     size, but returns a constant with the new size; if we were to
     guarantee table->n_buckets_used-1 free entries in advance, then
     the transfer would be guaranteed to not allocate memory.
     However, for large tables, a guarantee of no further allocation
     introduces a lot of extra memory pressure, all for an unlikely
     corner case (most rehashes reduce, rather than increase, the
     number of overflow entries needed).  So, we instead ensure that
     the transfer process can be reversed if we hit a memory
     allocation failure mid-transfer.  */

  /* Merely reuse the extra old space into the new table.  */
#if USE_OBSTACK
  new_table->entry_stack = table->entry_stack;
#endif
  new_table->free_entry_list = table->free_entry_list;

  if (transfer_entries (new_table, table, false))
    {
      /* Entries transferred successfully; tie up the loose ends.  */
      free (table->bucket);
      table->bucket = new_table->bucket;
      table->bucket_limit = new_table->bucket_limit;
      table->n_buckets = new_table->n_buckets;
      table->n_buckets_used = new_table->n_buckets_used;
      table->free_entry_list = new_table->free_entry_list;
      /* table->n_entries and table->entry_stack already hold their value.  */
      return true;
    }

  /* We've allocated new_table->bucket (and possibly some entries),
     exhausted the free list, and moved some but not all entries into
     new_table.  We must undo the partial move before returning
     failure.  The only way to get into this situation is if new_table
     uses fewer buckets than the old table, so we will reclaim some
     free entries as overflows in the new table are put back into
     distinct buckets in the old table.

     There are some pathological cases where a single pass through the
     table requires more intermediate overflow entries than using two
     passes.  Two passes give worse cache performance and takes
     longer, but at this point, we're already out of memory, so slow
     and safe is better than failure.  */
  int saved_errno = errno;
  table->free_entry_list = new_table->free_entry_list;
  if (! (transfer_entries (table, new_table, true)
         && transfer_entries (table, new_table, false)))
    abort ();
  /* table->n_entries already holds its value.  */
  free (new_table->bucket);
  errno = saved_errno;
  return false;
}

int
hash_insert_if_absent (Hash_table *table, void const *entry,
                       void const **matched_ent)
{

  /* The caller cannot insert a NULL entry, since hash_lookup returns NULL
     to indicate "not found", and find_entry uses "bucket->data == NULL"
     to indicate an empty bucket.  */
  if (! entry)
    abort ();

  /* If there's a matching entry already in the table, return that.  */
  struct hash_entry *bucket;
  void *data = find_entry (table, entry, &bucket, false);
  if (data != NULL)
    {
      if (matched_ent)
        *matched_ent = data;
      return 0;
    }

  /* If the growth threshold of the buckets in use has been reached, increase
     the table size and rehash.  There's no point in checking the number of
     entries:  if the hashing function is ill-conditioned, rehashing is not
     likely to improve it.  */

  if (table->n_buckets_used
      > table->tuning->growth_threshold * table->n_buckets)
    {
      /* Check more fully, before starting real work.  If tuning arguments
         became invalid, the second check will rely on proper defaults.  */
      check_tuning (table);
      if (table->n_buckets_used
          > table->tuning->growth_threshold * table->n_buckets)
        {
          const Hash_tuning *tuning = table->tuning;
          float candidate =
            (tuning->is_n_buckets
             ? (table->n_buckets * tuning->growth_factor)
             : (table->n_buckets * tuning->growth_factor
                * tuning->growth_threshold));

          if ((float) SIZE_MAX <= candidate)
            {
              errno = ENOMEM;
              return -1;
            }

          /* If the rehash fails, arrange to return NULL.  */
          if (!hash_rehash (table, candidate))
            return -1;

          /* Update the bucket we are interested in.  */
          if (find_entry (table, entry, &bucket, false) != NULL)
            abort ();
        }
    }

  /* ENTRY is not matched, it should be inserted.  */

  if (bucket->data)
    {
      struct hash_entry *new_entry = allocate_entry (table);

      if (new_entry == NULL)
        return -1;

      /* Add ENTRY in the overflow of the bucket.  */

      new_entry->data = (void *) entry;
      new_entry->next = bucket->next;
      bucket->next = new_entry;
      table->n_entries++;
      return 1;
    }

  /* Add ENTRY right in the bucket head.  */

  bucket->data = (void *) entry;
  table->n_entries++;
  table->n_buckets_used++;

  return 1;
}

void *
hash_insert (Hash_table *table, void const *entry)
{
  void const *matched_ent;
  int err = hash_insert_if_absent (table, entry, &matched_ent);
  return (err == -1
          ? NULL
          : (void *) (err == 0 ? matched_ent : entry));
}

void *
hash_remove (Hash_table *table, const void *entry)
{
  struct hash_entry *bucket;
  void *data = find_entry (table, entry, &bucket, true);
  if (!data)
    return NULL;

  table->n_entries--;
  if (!bucket->data)
    {
      table->n_buckets_used--;

      /* If the shrink threshold of the buckets in use has been reached,
         rehash into a smaller table.  */

      if (table->n_buckets_used
          < table->tuning->shrink_threshold * table->n_buckets)
        {
          /* Check more fully, before starting real work.  If tuning arguments
             became invalid, the second check will rely on proper defaults.  */
          check_tuning (table);
          if (table->n_buckets_used
              < table->tuning->shrink_threshold * table->n_buckets)
            {
              const Hash_tuning *tuning = table->tuning;
              size_t candidate =
                (tuning->is_n_buckets
                 ? table->n_buckets * tuning->shrink_factor
                 : (table->n_buckets * tuning->shrink_factor
                    * tuning->growth_threshold));

              if (!hash_rehash (table, candidate))
                {
                  /* Failure to allocate memory in an attempt to
                     shrink the table is not fatal.  But since memory
                     is low, we can at least be kind and free any
                     spare entries, rather than keeping them tied up
                     in the free entry list.  */
#if ! USE_OBSTACK
                  struct hash_entry *cursor = table->free_entry_list;
                  struct hash_entry *next;
                  while (cursor)
                    {
                      next = cursor->next;
                      free (cursor);
                      cursor = next;
                    }
                  table->free_entry_list = NULL;
#endif
                }
            }
        }
    }

  return data;
}

/* Testing.  */

#if TESTING

void
hash_print (const Hash_table *table)
{
  for (struct hash_entry *bucket = (struct hash_entry *) table->bucket;
       bucket < table->bucket_limit;
       bucket++)
    {
      if (bucket)
        printf ("%lu:\n", (unsigned long int) (bucket - table->bucket));

      for (struct hash_entry *cursor = bucket; cursor; cursor = cursor->next)
        {
          char const *s = cursor->data;
          /* FIXME */
          if (s)
            printf ("  %s\n", s);
        }
    }
}

#endif /* TESTING */