misc/hsearch_r.c

   1 /* Copyright (C) 1993, 1995, 1996 Free Software Foundation, Inc.
   2    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Library General Public License as
   7    published by the Free Software Foundation; either version 2 of the
   8    License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Library General Public License for more details.
  14
  15    You should have received a copy of the GNU Library General Public
  16    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  17    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18    Boston, MA 02111-1307, USA.  */
  19
  20 #include <errno.h>
  21 #include <malloc.h>
  22 #include <string.h>
  23
  24 #include <search.h>
  25
  26 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
  27    [Knuth]            The Art of Computer Programming, part 3 (6.4)  */
  28
  29
  30 /* The reentrant version has no static variables to maintain the state.
  31    Instead the interface of all functions is extended to take an argument
  32    which describes the current status.  */
  33 typedef struct _ENTRY
  34 {
  35   unsigned int used;
  36   ENTRY entry;
  37 }
  38 _ENTRY;
  39
  40
  41 /* For the used double hash method the table size has to be a prime. To
  42    correct the user given table size we need a prime test.  This trivial
  43    algorithm is adequate because
  44    a)  the code is (most probably) called a few times per program run and
  45    b)  the number is small because the table must fit in the core  */
  46 static int
  47 isprime (unsigned int number)
  48 {
  49   /* no even number will be passed */
  50   unsigned int div = 3;
  51
  52   while (div * div < number && number % div != 0)
  53     div += 2;
  54
  55   return number % div != 0;
  56 }
  57
  58
  59 /* Before using the hash table we must allocate memory for it.
  60    Test for an existing table are done. We allocate one element
  61    more as the found prime number says. This is done for more effective
  62    indexing as explained in the comment for the hsearch function.
  63    The contents of the table is zeroed, especially the field used
  64    becomes zero.  */
  65 int
  66 hcreate_r (nel, htab)
  67      size_t nel;
  68      struct hsearch_data *htab;
  69 {
  70   /* Test for correct arguments.  */
  71   if (htab == NULL)
  72     {
  73       __set_errno (EINVAL);
  74       return 0;
  75     }
  76
  77   /* There is still another table active. Return with error. */
  78   if (htab->table != NULL)
  79     return 0;
  80
  81   /* Change nel to the first prime number not smaller as nel. */
  82   nel |= 1;      /* make odd */
  83   while (!isprime (nel))
  84     nel += 2;
  85
  86   htab->size = nel;
  87   htab->filled = 0;
  88
  89   /* allocate memory and zero out */
  90   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  91   if (htab->table == NULL)
  92     return 0;
  93
  94   /* everything went alright */
  95   return 1;
  96 }
  97
  98
  99 /* After using the hash table it has to be destroyed. The used memory can
 100    be freed and the local static variable can be marked as not used.  */
 101 void
 102 hdestroy_r (htab)
 103      struct hsearch_data *htab;
 104 {
 105   /* Test for correct arguments.  */
 106   if (htab == NULL)
 107     {
 108       __set_errno (EINVAL);
 109       return;
 110     }
 111
 112   if (htab->table != NULL)
 113     /* free used memory */
 114     free (htab->table);
 115
 116   /* the sign for an existing table is an value != NULL in htable */
 117   htab->table = NULL;
 118 }
 119
 120
 121 /* This is the search function. It uses double hashing with open addressing.
 122    The argument item.key has to be a pointer to an zero terminated, most
 123    probably strings of chars. The function for generating a number of the
 124    strings is simple but fast. It can be replaced by a more complex function
 125    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 126
 127    We use an trick to speed up the lookup. The table is created by hcreate
 128    with one more element available. This enables us to use the index zero
 129    special. This index will never be used because we store the first hash
 130    index in the field used where zero means not used. Every other value
 131    means used. The used field can be used as a first fast comparison for
 132    equality of the stored and the parameter value. This helps to prevent
 133    unnecessary expensive calls of strcmp.  */
 134 int
 135 hsearch_r (item, action, retval, htab)
 136      ENTRY item;
 137      ACTION action;
 138      ENTRY **retval;
 139      struct hsearch_data *htab;
 140 {
 141   unsigned int hval;
 142   unsigned int count;
 143   unsigned int len = strlen (item.key);
 144   unsigned int idx;
 145
 146   /* If table is full and another entry should be entered return with
 147      error.  */
 148   if (action == ENTER && htab->filled == htab->size)
 149     {
 150       __set_errno (ENOMEM);
 151       *retval = NULL;
 152       return 0;
 153     }
 154
 155   /* Compute an value for the given string. Perhaps use a better method. */
 156   hval = len;
 157   count = len;
 158   while (count-- > 0)
 159     {
 160       hval <<= 4;
 161       hval += item.key[count];
 162     }
 163
 164   /* First hash function: simply take the modul but prevent zero. */
 165   hval %= htab->size;
 166   if (hval == 0)
 167     ++hval;
 168
 169   /* The first index tried. */
 170   idx = hval;
 171
 172   if (htab->table[idx].used)
 173     {
 174       /* Further action might be required according to the action value. */
 175       unsigned hval2;
 176
 177       if (htab->table[idx].used == hval
 178           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 179         {
 180           if (action == ENTER)
 181             htab->table[idx].entry.data = item.data;
 182
 183           *retval = &htab->table[idx].entry;
 184           return 1;
 185         }
 186
 187       /* Second hash function, as suggested in [Knuth] */
 188       hval2 = 1 + hval % (htab->size - 2);
 189
 190       do
 191         {
 192           /* Because SIZE is prime this guarantees to step through all
 193              available indeces.  */
 194           if (idx <= hval2)
 195             idx = htab->size + idx - hval2;
 196           else
 197             idx -= hval2;
 198
 199             /* If entry is found use it. */
 200           if (htab->table[idx].used == hval
 201               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 202             {
 203               if (action == ENTER)
 204                 htab->table[idx].entry.data = item.data;
 205
 206               *retval = &htab->table[idx].entry;
 207               return 1;
 208             }
 209         }
 210       while (htab->table[idx].used);
 211     }
 212
 213   /* An empty bucket has been found. */
 214   if (action == ENTER)
 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 }