misc/hsearch_r.c

   1 /* Copyright (C) 1993,1995,1996,1997,2002,2005 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 libc_hidden_def (hcreate_r)
  98
  99
 100 /* After using the hash table it has to be destroyed. The used memory can
 101    be freed and the local static variable can be marked as not used.  */
 102 void
 103 hdestroy_r (htab)
 104      struct hsearch_data *htab;
 105 {
 106   /* Test for correct arguments.  */
 107   if (htab == NULL)
 108     {
 109       __set_errno (EINVAL);
 110       return;
 111     }
 112
 113   if (htab->table != NULL)
 114     /* free used memory */
 115     free (htab->table);
 116
 117   /* the sign for an existing table is an value != NULL in htable */
 118   htab->table = NULL;
 119 }
 120 libc_hidden_def (hdestroy_r)
 121
 122
 123 /* This is the search function. It uses double hashing with open addressing.
 124    The argument item.key has to be a pointer to an zero terminated, most
 125    probably strings of chars. The function for generating a number of the
 126    strings is simple but fast. It can be replaced by a more complex function
 127    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 128
 129    We use an trick to speed up the lookup. The table is created by hcreate
 130    with one more element available. This enables us to use the index zero
 131    special. This index will never be used because we store the first hash
 132    index in the field used where zero means not used. Every other value
 133    means used. The used field can be used as a first fast comparison for
 134    equality of the stored and the parameter value. This helps to prevent
 135    unnecessary expensive calls of strcmp.  */
 136 int
 137 hsearch_r (item, action, retval, htab)
 138      ENTRY item;
 139      ACTION action;
 140      ENTRY **retval;
 141      struct hsearch_data *htab;
 142 {
 143   unsigned int hval;
 144   unsigned int count;
 145   unsigned int len = strlen (item.key);
 146   unsigned int idx;
 147
 148   /* Compute an value for the given string. Perhaps use a better method. */
 149   hval = len;
 150   count = len;
 151   while (count-- > 0)
 152     {
 153       hval <<= 4;
 154       hval += item.key[count];
 155     }
 156
 157   /* First hash function: simply take the modul but prevent zero. */
 158   hval %= htab->size;
 159   if (hval == 0)
 160     ++hval;
 161
 162   /* The first index tried. */
 163   idx = hval;
 164
 165   if (htab->table[idx].used)
 166     {
 167       /* Further action might be required according to the action value. */
 168       unsigned hval2;
 169
 170       if (htab->table[idx].used == hval
 171           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 172         {
 173           *retval = &htab->table[idx].entry;
 174           return 1;
 175         }
 176
 177       /* Second hash function, as suggested in [Knuth] */
 178       hval2 = 1 + hval % (htab->size - 2);
 179
 180       do
 181         {
 182           /* Because SIZE is prime this guarantees to step through all
 183              available indeces.  */
 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 == hval)
 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)