misc/hsearch_r.c

   1 /* Copyright (C) 1993, 1995, 1996, 1997, 2002 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>, 1993.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the 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    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    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   /* Compute an value for the given string. Perhaps use a better method. */
 147   hval = len;
 148   count = len;
 149   while (count-- > 0)
 150     {
 151       hval <<= 4;
 152       hval += item.key[count];
 153     }
 154
 155   /* First hash function: simply take the modul but prevent zero. */
 156   hval %= htab->size;
 157   if (hval == 0)
 158     ++hval;
 159
 160   /* The first index tried. */
 161   idx = hval;
 162
 163   if (htab->table[idx].used)
 164     {
 165       /* Further action might be required according to the action value. */
 166       unsigned hval2;
 167
 168       if (htab->table[idx].used == hval
 169           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 170         {
 171           *retval = &htab->table[idx].entry;
 172           return 1;
 173         }
 174
 175       /* Second hash function, as suggested in [Knuth] */
 176       hval2 = 1 + hval % (htab->size - 2);
 177
 178       do
 179         {
 180           /* Because SIZE is prime this guarantees to step through all
 181              available indeces.  */
 182           if (idx <= hval2)
 183             idx = htab->size + idx - hval2;
 184           else
 185             idx -= hval2;
 186
 187           /* If we visited all entries leave the loop unsuccessfully.  */
 188           if (idx == hval)
 189             break;
 190
 191             /* If entry is found use it. */
 192           if (htab->table[idx].used == hval
 193               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 194             {
 195               *retval = &htab->table[idx].entry;
 196               return 1;
 197             }
 198         }
 199       while (htab->table[idx].used);
 200     }
 201
 202   /* An empty bucket has been found. */
 203   if (action == ENTER)
 204     {
 205       /* If table is full and another entry should be entered return
 206          with error.  */
 207       if (action == ENTER && htab->filled == htab->size)
 208         {
 209           __set_errno (ENOMEM);
 210           *retval = NULL;
 211           return 0;
 212         }
 213
 214       htab->table[idx].used  = hval;
 215       htab->table[idx].entry = item;
 216
 217       ++htab->filled;
 218
 219       *retval = &htab->table[idx].entry;
 220       return 1;
 221     }
 222
 223   __set_errno (ESRCH);
 224   *retval = NULL;
 225   return 0;
 226 }