misc/hsearch_r.c

   1 /* Copyright (C) 1993-2023 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <malloc.h>
  20 #include <string.h>
  21 #include <stdint.h>
  22 #include <search.h>
  23 #include <limits.h>
  24
  25 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
  26    [Knuth]            The Art of Computer Programming, part 3 (6.4)  */
  27
  28
  29 /* The reentrant version has no static variables to maintain the state.
  30    Instead the interface of all functions is extended to take an argument
  31    which describes the current status.  */
  32 typedef struct _ENTRY
  33 {
  34   unsigned int used;
  35   ENTRY entry;
  36 }
  37 _ENTRY;
  38
  39
  40 /* For the used double hash method the table size has to be a prime. To
  41    correct the user given table size we need a prime test.  This trivial
  42    algorithm is adequate because
  43    a)  the code is (most probably) called a few times per program run and
  44    b)  the number is small because the table must fit in the core  */
  45 static int
  46 isprime (unsigned int number)
  47 {
  48   /* no even number will be passed */
  49   for (unsigned int div = 3; div <= number / div; div += 2)
  50     if (number % div == 0)
  51       return 0;
  52   return 1;
  53 }
  54
  55 /* Before using the hash table we must allocate memory for it.
  56    Test for an existing table are done. We allocate one element
  57    more as the found prime number says. This is done for more effective
  58    indexing as explained in the comment for the hsearch function.
  59    The contents of the table is zeroed, especially the field used
  60    becomes zero.  */
  61 int
  62 __hcreate_r (size_t nel, struct hsearch_data *htab)
  63 {
  64   /* Test for correct arguments.  */
  65   if (htab == NULL)
  66     {
  67       __set_errno (EINVAL);
  68       return 0;
  69     }
  70
  71   /* There is still another table active. Return with error. */
  72   if (htab->table != NULL)
  73     return 0;
  74
  75   /* We need a size of at least 3.  Otherwise the hash functions we
  76      use will not work.  */
  77   if (nel < 3)
  78     nel = 3;
  79
  80   /* Change nel to the first prime number in the range [nel, UINT_MAX - 2],
  81      The '- 2' means 'nel += 2' cannot overflow.  */
  82   for (nel |= 1; ; nel += 2)
  83     {
  84       if (UINT_MAX - 2 < nel)
  85         {
  86           __set_errno (ENOMEM);
  87           return 0;
  88         }
  89       if (isprime (nel))
  90         break;
  91     }
  92
  93   htab->size = nel;
  94   htab->filled = 0;
  95
  96   /* allocate memory and zero out */
  97   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  98   if (htab->table == NULL)
  99     return 0;
 100
 101   /* everything went alright */
 102   return 1;
 103 }
 104 libc_hidden_def (__hcreate_r)
 105 weak_alias (__hcreate_r, hcreate_r)
 106
 107
 108 /* After using the hash table it has to be destroyed. The used memory can
 109    be freed and the local static variable can be marked as not used.  */
 110 void
 111 __hdestroy_r (struct hsearch_data *htab)
 112 {
 113   /* Test for correct arguments.  */
 114   if (htab == NULL)
 115     {
 116       __set_errno (EINVAL);
 117       return;
 118     }
 119
 120   /* Free used memory.  */
 121   free (htab->table);
 122
 123   /* the sign for an existing table is an value != NULL in htable */
 124   htab->table = NULL;
 125 }
 126 libc_hidden_def (__hdestroy_r)
 127 weak_alias (__hdestroy_r, hdestroy_r)
 128
 129
 130 /* This is the search function. It uses double hashing with open addressing.
 131    The argument item.key has to be a pointer to an zero terminated, most
 132    probably strings of chars. The function for generating a number of the
 133    strings is simple but fast. It can be replaced by a more complex function
 134    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 135
 136    We use an trick to speed up the lookup. The table is created by hcreate
 137    with one more element available. This enables us to use the index zero
 138    special. This index will never be used because we store the first hash
 139    index in the field used where zero means not used. Every other value
 140    means used. The used field can be used as a first fast comparison for
 141    equality of the stored and the parameter value. This helps to prevent
 142    unnecessary expensive calls of strcmp.  */
 143 int
 144 __hsearch_r (ENTRY item, ACTION action, ENTRY **retval,
 145              struct hsearch_data *htab)
 146 {
 147   unsigned int hval;
 148   unsigned int count;
 149   unsigned int len = strlen (item.key);
 150   unsigned int idx;
 151
 152   /* Compute an value for the given string. Perhaps use a better method. */
 153   hval = len;
 154   count = len;
 155   while (count-- > 0)
 156     {
 157       hval <<= 4;
 158       hval += item.key[count];
 159     }
 160   if (hval == 0)
 161     ++hval;
 162
 163   /* First hash function: simply take the modulo but prevent zero. */
 164   idx = hval % htab->size + 1;
 165
 166   if (htab->table[idx].used)
 167     {
 168       /* Further action might be required according to the action value. */
 169       if (htab->table[idx].used == hval
 170           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 171         {
 172           *retval = &htab->table[idx].entry;
 173           return 1;
 174         }
 175
 176       /* Second hash function, as suggested in [Knuth] */
 177       unsigned int hval2 = 1 + hval % (htab->size - 2);
 178       unsigned int first_idx = idx;
 179
 180       do
 181         {
 182           /* Because SIZE is prime this guarantees to step through all
 183              available indices.  */
 184           if (idx <= hval2)
 185             idx = htab->size + idx - hval2;
 186           else
 187             idx -= hval2;
 188
 189           /* If we visited all entries leave the loop unsuccessfully.  */
 190           if (idx == first_idx)
 191             break;
 192
 193             /* If entry is found use it. */
 194           if (htab->table[idx].used == hval
 195               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 196             {
 197               *retval = &htab->table[idx].entry;
 198               return 1;
 199             }
 200         }
 201       while (htab->table[idx].used);
 202     }
 203
 204   /* An empty bucket has been found. */
 205   if (action == ENTER)
 206     {
 207       /* If table is full and another entry should be entered return
 208          with error.  */
 209       if (htab->filled == htab->size)
 210         {
 211           __set_errno (ENOMEM);
 212           *retval = NULL;
 213           return 0;
 214         }
 215
 216       htab->table[idx].used  = hval;
 217       htab->table[idx].entry = item;
 218
 219       ++htab->filled;
 220
 221       *retval = &htab->table[idx].entry;
 222       return 1;
 223     }
 224
 225   __set_errno (ESRCH);
 226   *retval = NULL;
 227   return 0;
 228 }
 229 libc_hidden_def (__hsearch_r)
 230 weak_alias (__hsearch_r, hsearch_r)