misc/hsearch_r.c

   1 /* Copyright (C) 1993,1995-1997,2002,2005,2007,2008
   2    Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>, 1993.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, write to the Free
  18    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19    02111-1307 USA.  */
  20
  21 #include <errno.h>
  22 #include <malloc.h>
  23 #include <string.h>
  24
  25 #include <search.h>
  26
  27 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
  28    [Knuth]            The Art of Computer Programming, part 3 (6.4)  */
  29
  30
  31 /* The reentrant version has no static variables to maintain the state.
  32    Instead the interface of all functions is extended to take an argument
  33    which describes the current status.  */
  34 typedef struct _ENTRY
  35 {
  36   unsigned int used;
  37   ENTRY entry;
  38 }
  39 _ENTRY;
  40
  41
  42 /* For the used double hash method the table size has to be a prime. To
  43    correct the user given table size we need a prime test.  This trivial
  44    algorithm is adequate because
  45    a)  the code is (most probably) called a few times per program run and
  46    b)  the number is small because the table must fit in the core  */
  47 static int
  48 isprime (unsigned int number)
  49 {
  50   /* no even number will be passed */
  51   unsigned int div = 3;
  52
  53   while (div * div < number && number % div != 0)
  54     div += 2;
  55
  56   return number % div != 0;
  57 }
  58
  59
  60 /* Before using the hash table we must allocate memory for it.
  61    Test for an existing table are done. We allocate one element
  62    more as the found prime number says. This is done for more effective
  63    indexing as explained in the comment for the hsearch function.
  64    The contents of the table is zeroed, especially the field used
  65    becomes zero.  */
  66 int
  67 hcreate_r (nel, htab)
  68      size_t nel;
  69      struct hsearch_data *htab;
  70 {
  71   /* Test for correct arguments.  */
  72   if (htab == NULL)
  73     {
  74       __set_errno (EINVAL);
  75       return 0;
  76     }
  77
  78   /* There is still another table active. Return with error. */
  79   if (htab->table != NULL)
  80     return 0;
  81
  82   /* Change nel to the first prime number not smaller as nel. */
  83   nel |= 1;      /* make odd */
  84   while (!isprime (nel))
  85     nel += 2;
  86
  87   htab->size = nel;
  88   htab->filled = 0;
  89
  90   /* allocate memory and zero out */
  91   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  92   if (htab->table == NULL)
  93     return 0;
  94
  95   /* everything went alright */
  96   return 1;
  97 }
  98 libc_hidden_def (hcreate_r)
  99
 100
 101 /* After using the hash table it has to be destroyed. The used memory can
 102    be freed and the local static variable can be marked as not used.  */
 103 void
 104 hdestroy_r (htab)
 105      struct hsearch_data *htab;
 106 {
 107   /* Test for correct arguments.  */
 108   if (htab == NULL)
 109     {
 110       __set_errno (EINVAL);
 111       return;
 112     }
 113
 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   idx = hval % htab->size + 1;
 159
 160   if (htab->table[idx].used)
 161     {
 162       /* Further action might be required according to the action value. */
 163       if (htab->table[idx].used == hval
 164           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 165         {
 166           *retval = &htab->table[idx].entry;
 167           return 1;
 168         }
 169
 170       /* Second hash function, as suggested in [Knuth] */
 171       unsigned int hval2 = 1 + hval % (htab->size - 2);
 172       unsigned int first_idx = idx;
 173
 174       do
 175         {
 176           /* Because SIZE is prime this guarantees to step through all
 177              available indeces.  */
 178           if (idx <= hval2)
 179             idx = htab->size + idx - hval2;
 180           else
 181             idx -= hval2;
 182
 183           /* If we visited all entries leave the loop unsuccessfully.  */
 184           if (idx == first_idx)
 185             break;
 186
 187             /* If entry is found use it. */
 188           if (htab->table[idx].used == hval
 189               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 190             {
 191               *retval = &htab->table[idx].entry;
 192               return 1;
 193             }
 194         }
 195       while (htab->table[idx].used);
 196     }
 197
 198   /* An empty bucket has been found. */
 199   if (action == ENTER)
 200     {
 201       /* If table is full and another entry should be entered return
 202          with error.  */
 203       if (htab->filled == htab->size)
 204         {
 205           __set_errno (ENOMEM);
 206           *retval = NULL;
 207           return 0;
 208         }
 209
 210       htab->table[idx].used  = hval;
 211       htab->table[idx].entry = item;
 212
 213       ++htab->filled;
 214
 215       *retval = &htab->table[idx].entry;
 216       return 1;
 217     }
 218
 219   __set_errno (ESRCH);
 220   *retval = NULL;
 221   return 0;
 222 }
 223 libc_hidden_def (hsearch_r)