Revamp to allow fine-tuning to control when and by how

[pspp] / lib / hash.c

/* hash - hashing table processing.
   Copyright (C) 1998, 1999 Free Software Foundation, Inc.
   Written by Jim Meyering, 1992.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software Foundation,
   Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */

/* 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!  */

#if HAVE_CONFIG_H
# include <config.h>
#endif
#if HAVE_STDLIB_H
# include <stdlib.h>
#endif
#if HAVE_STDBOOL_H
# include <stdbool.h>
#else
typedef enum {false = 0, true = 1} bool;
#endif
#include <stdio.h>
#include <assert.h>

#if !HAVE_DECL_FREE
void free ();
#endif
#if !HAVE_DECL_MALLOC
char *malloc ();
#endif

#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

#include "hash.h"

/* An 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 randomisation 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 at constant speed by the `hasher', and the later finding of a precise
   entry is linear in time with the size of the bucket.  Consequently, a
   bigger hash table size (that is, a bigger number of buckets) is prone to
   yielding shorter buckets, *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 at
   least bigger than the actual number of entries.  */

/* If, while adding an to a bucket which was empty, the ratio of used buckets
   over table size goes over the growth threshold (a number between 0.0 and
   1.0), then reorganise the table size bigger 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 0.8
#define DEFAULT_GROWTH_FACTOR 1.414

/* If, while emptying a bucket, the ratio of used buckets over table size
   drops below the shrink threshold (a number between 0.0 and 1.0), then
   reorganise the table size smaller through the usage of a 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 0.0
#define DEFAULT_SHRINK_FACTOR 1.0

/* 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.  */

/* The following few functions provide information about the overall hash
   table organisation: the number of entries, number of buckets and maximum
   length of buckets.  */

/* Return the number of buckets in the hash table.  The table size, the total
   number of buckets (used plus unused), or the maximum number of slots, are
   the same quantity.  */

unsigned
hash_get_n_buckets (const Hash_table *table)
{
  return table->n_buckets;
}

/* Return the number of slots in use (non-empty buckets).  */

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

/* Return the number of active entries.  */

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

/* Return the length of the most lenghty chain (bucket).  */

unsigned
hash_get_max_bucket_length (const Hash_table *table)
{
  struct hash_entry *bucket;
  unsigned max_bucket_length = 0;

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

/* Do a mild validation of an hash table, by traversing it and checking two
   statistics.  */

bool
hash_table_ok (const Hash_table *table)
{
  struct hash_entry *bucket;
  unsigned n_buckets_used = 0;
  unsigned n_entries = 0;

  for (bucket = table->bucket; bucket < table->bucket_limit; bucket++)
    {
      if (bucket->data)
        {
          struct hash_entry *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)
{
  unsigned n_entries = hash_get_n_entries (table);
  unsigned n_buckets = hash_get_n_buckets (table);
  unsigned n_buckets_used = hash_get_n_buckets_used (table);
  unsigned max_bucket_length = hash_get_max_bucket_length (table);

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

/* Return the user entry from the hash table, if some entry in the hash table
   compares equally with ENTRY, or NULL otherwise. */

void *
hash_lookup (const Hash_table *table, const void *entry)
{
  struct hash_entry *bucket
    = table->bucket + table->hasher (entry, table->n_buckets);
  struct hash_entry *cursor;

  assert (bucket < table->bucket_limit);

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

  for (cursor = bucket; cursor; cursor = cursor->next)
    if (table->comparator (entry, cursor->data))
      return cursor->data;

  return NULL;
}

/* Walking.  */

/* The functions in this page traverse the hash table and process the
   contained entries.  For the traversal to work properly, the hash table
   should not be resized nor modified while any particular entry is being
   processed.  In particular, entries should not be added or removed.  */

/* Return the first data in the table, or NULL if the table is empty.  */

void *
hash_get_first (const Hash_table *table)
{
  struct hash_entry *bucket;

  if (table->n_entries == 0)
    return NULL;

  for (bucket = table->bucket; bucket < table->bucket_limit; bucket++)
    if (bucket->data)
      return bucket->data;

  abort ();
}

/* Return the user data for the entry following ENTRY, where ENTRY has been
   returned by a previous call to either `hash_get_first' or `hash_get_next'.
   Return NULL if there is no more entries.  */

void *
hash_get_next (const Hash_table *table, const void *entry)
{
  struct hash_entry *bucket
    = table->bucket + table->hasher (entry, table->n_buckets);
  struct hash_entry *cursor;

  assert (bucket < table->bucket_limit);

  /* Find next entry in the same bucket.  */
  for (cursor = bucket; cursor; cursor = cursor->next)
    if (cursor->data == entry && cursor->next)
      return cursor->next->data;

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

  /* None found.  */
  return NULL;
}

/* Fill BUFFER with pointers to active user entries in the hash table, then
   return the number of pointers copied.  Do not copy more than BUFFER_SIZE
   pointers.  */

unsigned
hash_get_entries (const Hash_table *table, void **buffer,
                  unsigned buffer_size)
{
  unsigned counter = 0;
  struct hash_entry *bucket;
  struct hash_entry *cursor;

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

  return counter;
}

/* Call a PROCESSOR function for each entry of an hash table, and return the
   number of entries for which the processor function returned success.  A
   pointer to some PROCESSOR_DATA which will be made available to each call to
   the processor function.  The PROCESSOR accepts two arguments: the first is
   the user entry being walked into, the second is the value of PROCESSOR_DATA
   as received.  The walking continue for as long as the PROCESSOR function
   returns nonzero.  When it returns zero, the walking is interrupted.  */

unsigned
hash_do_for_each (const Hash_table *table, Hash_processor processor,
                  void *processor_data)
{
  unsigned counter = 0;
  struct hash_entry *bucket;
  struct hash_entry *cursor;

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

  return counter;
}

/* Allocation and clean-up.  */

/* Return a hash index for a NUL-terminated STRING between 0 and N_BUCKETS-1.
   This is a convenience routine for constructing other hashing functions.  */

#if USE_DIFF_HASH

/* About hashings, Paul Eggert writes to me (FP), on 1994-01-01: "Please see
   B. J. McKenzie, R. Harries & T. Bell, Selecting a hashing algorithm,
   Software--practice & experience 20, 2 (Feb 1990), 209-224.  Good hash
   algorithms tend to be domain-specific, so what's good for [diffutils'] io.c
   may not be good for your application."  */

unsigned
hash_string (const char *string, unsigned n_buckets)
{
# ifndef CHAR_BIT
#  define CHAR_BIT 8
# endif
# define ROTATE_LEFT(Value, Shift) \
  ((Value) << (Shift) | (Value) >> ((sizeof (unsigned) * CHAR_BIT) - (Shift)))
# define HASH_ONE_CHAR(Value, Byte) \
  ((Byte) + ROTATE_LEFT (Value, 7))

  unsigned value = 0;

  for (; *string; string++)
    value = HASH_ONE_CHAR (value, *(const unsigned char *) string);
  return value % n_buckets;

# undef ROTATE_LEFT
# undef HASH_ONE_CHAR
}

#else /* not USE_DIFF_HASH */

/* This one comes from `recode', and performs a bit better than the above as
   per a few experiments.  It is inspired from a hashing routine found in the
   very old Cyber `snoop', itself written in typical Greg Mansfield style.
   (By the way, what happened to this excellent man?  Is he still alive?)  */

unsigned
hash_string (const char *string, unsigned n_buckets)
{
  unsigned value = 0;

  while (*string)
    value = ((value * 31 + (int) *(const unsigned char *) string++)
             % n_buckets);
  return value;
}

#endif /* not USE_DIFF_HASH */

/* Return true if CANDIDATE is a prime number.  CANDIDATE should be an odd
   number at least equal to 11.  */

static int
is_prime (unsigned long candidate)
{
  unsigned long divisor = 3;
  unsigned long square = divisor * divisor;

  while (square < candidate && (candidate % divisor))
    {
      divisor++;
      square += 4 * divisor;
      divisor++;
    }

  return candidate % divisor != 0;
}

/* Round a given CANDIDATE number up to the nearest prime, and return that
   prime.  Primes lower than 10 are merely skipped.  */

static unsigned long
next_prime (unsigned long candidate)
{
  /* Skip small primes.  */
  if (candidate < 10)
    candidate = 10;

  /* Make it definitely odd.  */
  candidate |= 1;

  while (!is_prime (candidate))
    candidate += 2;

  return candidate;
}

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

/* 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, definitvely reset the TUNING field to some acceptable default in
   the hash table (that is, the user looses 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->growth_threshold > 0.0
      && tuning->growth_threshold < 1.0
      && tuning->growth_factor > 1.0
      && tuning->shrink_threshold >= 0.0
      && tuning->shrink_threshold < 1.0
      && tuning->shrink_factor > tuning->shrink_threshold
      && tuning->shrink_factor <= 1.0
      && tuning->shrink_threshold < tuning->growth_threshold)
    return true;

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

/* Allocate and return a new hash table, or NULL if an error is met.  The
   initial number of buckets is automatically selected so to _guarantee_ that
   you may insert at least CANDIDATE different user entries before any growth
   of the hash table size occurs.  So, if you happen to know beforehand the
   number of entries you intend to insert in the hash table, you may save some
   table memory and insertion time, by specifying it here.  If the
   IS_N_BUCKETS field of the TUNING structure is true, the CANDIDATE argument
   has its meaning changed to the wanted number of buckets.

   TUNING points to a structure of user-supplied values, in case some fine
   tuning is wanted over the default behaviour of the hasher.  If TUNING is
   NULL, proper defaults are used instead.

   The user-supplied HASHER function should be provided.  It accepts two
   arguments ENTRY and TABLE_SIZE.  It computes, by hasing ENTRY contents, a
   slot number for that entry which should be in the range 0..TABLE_SIZE-1.
   This slot number is then returned.

   The user-supplied COMPARATOR function should be provided.  It accepts two
   arguments pointing to user data, it then returns true for a pair of entries
   that compare equal, or false otherwise.  This function is internally called
   on entries which are already known to hash to the same bucket index.

   The user-supplied DATA_FREER function, when not NULL, may be later called
   with the user data as an argument, just before the entry containing the
   data gets freed.  This happens from within `hash_free' or `hash_clear'.
   You should specify this function only if you want these functions to free
   all of your `data' data.  This is typically the case when your data is
   simply an auxiliary struct that you have malloc'd to aggregate several
   values.  */

Hash_table *
hash_initialize (unsigned candidate, const Hash_tuning *tuning,
                 Hash_hasher hasher, Hash_comparator comparator,
                 Hash_data_freer data_freer)
{
  Hash_table *table;
  struct hash_entry *bucket;

  if (hasher == NULL || comparator == NULL)
    return NULL;

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

  if (!tuning)
    tuning = &default_tuning;
  table->tuning = tuning;
  if (!check_tuning (table))
    {
      /* Abort initialisation if tuning arguments are improper.  This is the
         only occasion when the user gets some feedback about it.  Later on,
         if the user modifies the tuning wrongly, it gets restored to some
         proper default, and the user looses the right of tuning further.  */
      free (table);
      return NULL;
    }

  table->n_buckets
    = next_prime (tuning->is_n_buckets ? candidate
                  : (unsigned) (candidate / tuning->growth_threshold));

  table->bucket = (struct hash_entry *)
    malloc (table->n_buckets * sizeof (struct hash_entry));
  if (table->bucket == NULL)
    {
      free (table);
      return NULL;
    }
  table->bucket_limit = table->bucket + table->n_buckets;

  for (bucket = table->bucket; bucket < table->bucket_limit; bucket++)
    {
      bucket->data = NULL;
      bucket->next = NULL;
    }
  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;
}

/* Make all buckets empty, placing any chained entries on the free list.
   Apply the user-specified function data_freer (if any) to the datas of any
   affected entries.  */

void
hash_clear (Hash_table *table)
{
  struct hash_entry *bucket;
  struct hash_entry *cursor;

  for (bucket = table->bucket; bucket < table->bucket_limit; bucket++)
    {
      if (bucket->data)
        {
          /* Free the bucket overflow.  */
          for (cursor = bucket->next; cursor; cursor = cursor->next)
            {
              if (table->data_freer)
                (*table->data_freer) (cursor->data);
              cursor->data = NULL;

              /* 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;
            }

          /* 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;
}

/* Reclaim all storage associated with an hash table.  If a data_freer
   function has been supplied by the user when the hash table was created,
   this function applies it to the data of each entry before freeing that
   entry.  */

void
hash_free (Hash_table *table)
{
  struct hash_entry *bucket;
  struct hash_entry *cursor;
  struct hash_entry *next;

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

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

#endif

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

/* Insertion and deletion.  */

/* Get a new hash entry for a bucket overflow, possibly by reclying 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 = (struct hash_entry *)
        obstack_alloc (&table->entry_stack, sizeof (struct hash_entry));
#else
      new = (struct hash_entry *) malloc (sizeof (struct hash_entry));
#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 *
hash_find_entry (Hash_table *table, const void *entry,
                 struct hash_entry **bucket_head, bool delete)
{
  struct hash_entry *bucket
    = table->bucket + table->hasher (entry, table->n_buckets);
  struct hash_entry *cursor;

  assert (bucket < table->bucket_limit);
  *bucket_head = bucket;

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

  /* Check if then entry is found as the bucket head.  */
  if ((*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 (cursor = bucket; cursor->next; cursor = cursor->next)
    {
      if ((*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;
}

/* For an already existing hash table, change the number of buckets through
   specifying CANDIDATE.  The contents of the hash table are preserved.  The
   new number of buckets is automatically selected so to _guarantee_ that the
   table may receive at least CANDIDATE different user entries, including
   those already in the table, before any other growth of the hash table size
   occurs.  If the IS_N_BUCKETS field of the TUNING structure is true, the
   CANDIDATE argument has its meaning changed to the wanted number of buckets.
   */

bool
hash_rehash (Hash_table *table, unsigned candidate)
{
  Hash_table *new_table;
  struct hash_entry *bucket;
  struct hash_entry *cursor;
  struct hash_entry *next;

  new_table = hash_initialize (candidate, table->tuning, table->hasher,
                               table->comparator, table->data_freer);
  if (new_table == NULL)
    return false;

  /* Merely reuse the extra old space into the new table.  */
#if USE_OBSTACK
  obstack_free (&new_table->entry_stack, NULL);
  new_table->entry_stack = table->entry_stack;
#endif
  new_table->free_entry_list = table->free_entry_list;

  for (bucket = table->bucket; bucket < table->bucket_limit; bucket++)
    if (bucket->data)
      for (cursor = bucket; cursor; cursor = next)
        {
          void *data = cursor->data;
          struct hash_entry *new_bucket
            = (new_table->bucket
               + new_table->hasher (data, new_table->n_buckets));

          assert (new_bucket < new_table->bucket_limit);
          next = cursor->next;

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

                if (new_entry == NULL)
                  return false;

                new_entry->data = data;
                new_entry->next = new_bucket->next;
                new_bucket->next = new_entry;
              }
            else
              {
                /* 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.  Also take care of the
                 simple case of moving from a bucket header into a bucket
                 header.  */
              new_bucket->data = data;
              new_table->n_buckets_used++;
              if (cursor != bucket)
                free_entry (new_table, cursor);
            }
        }

  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->n_entries already holds its value.  */
#if USE_OBSTACK
  table->entry_stack = new_table->entry_stack;
#endif
  free (new_table);

  return true;
}

/* If ENTRY matches an entry already in the hash table, return the pointer
   to the entry from the table.  Otherwise, insert ENTRY and return ENTRY.
   Return NULL if the storage required for insertion cannot be allocated.  */

void *
hash_insert (Hash_table *table, const void *entry)
{
  void *data;
  struct hash_entry *bucket;

  assert (entry);               /* cannot insert a NULL entry */

  /* If there's a matching entry already in the table, return that.  */
  if ((data = hash_find_entry (table, entry, &bucket, false)) != NULL)
    return data;

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

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

      if (new_entry == NULL)
        return NULL;

      /* 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 (void *) entry;
    }

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

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

  /* If the growth threshold of the buckets in use has been reached, rehash
     the table bigger.  It's no real use checking the number of entries, as
     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 got
         improper, 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;
          unsigned candidate
            = (unsigned) (tuning->is_n_buckets
                          ? (table->n_buckets * tuning->growth_factor)
                          : (table->n_buckets * tuning->growth_factor
                             * tuning->growth_threshold));

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

  return (void *) entry;
}

/* If ENTRY is already in the table, remove it and return the just-deleted
   data (the user may want to deallocate its storage).  If ENTRY is not in the
   table, don't modify the table and return NULL.  */

void *
hash_delete (Hash_table *table, const void *entry)
{
  void *data;
  struct hash_entry *bucket;

  if (data = hash_find_entry (table, entry, &bucket, true), !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 the table smaller.  */

      if (table->n_buckets_used
          < table->tuning->shrink_threshold * table->n_buckets)
        {
          /* Check more fully, before starting real work.  If tuning arguments
             got improper, 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;
              unsigned candidate
                = (unsigned) (tuning->is_n_buckets
                              ? table->n_buckets * tuning->shrink_factor
                              : (table->n_buckets * tuning->shrink_factor
                                 * tuning->growth_threshold));

              hash_rehash (table, candidate);
            }
        }
    }

  return data;
}

/* Testing.  */

#if TESTING

void
hash_print (const Hash_table *table)
{
  struct hash_entry *bucket;

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

      if (bucket)
        printf ("%d:\n", slot);

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

#endif /* TESTING */