misc/hsearch_r.c

   1 /* Copyright (C) 1993,1995-1997,2002,2005,2007,2008,2009
   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, see
  18    <http://www.gnu.org/licenses/>.  */
  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   /* We need a size of at least 3.  Otherwise the hash functions we
  82      use will not work.  */
  83   if (nel < 3)
  84     nel = 3;
  85   /* Change nel to the first prime number not smaller as nel. */
  86   nel |= 1;      /* make odd */
  87   while (!isprime (nel))
  88     nel += 2;
  89
  90   htab->size = nel;
  91   htab->filled = 0;
  92
  93   /* allocate memory and zero out */
  94   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  95   if (htab->table == NULL)
  96     return 0;
  97
  98   /* everything went alright */
  99   return 1;
 100 }
 101 libc_hidden_def (hcreate_r)
 102
 103
 104 /* After using the hash table it has to be destroyed. The used memory can
 105    be freed and the local static variable can be marked as not used.  */
 106 void
 107 hdestroy_r (htab)
 108      struct hsearch_data *htab;
 109 {
 110   /* Test for correct arguments.  */
 111   if (htab == NULL)
 112     {
 113       __set_errno (EINVAL);
 114       return;
 115     }
 116
 117   /* Free used memory.  */
 118   free (htab->table);
 119
 120   /* the sign for an existing table is an value != NULL in htable */
 121   htab->table = NULL;
 122 }
 123 libc_hidden_def (hdestroy_r)
 124
 125
 126 /* This is the search function. It uses double hashing with open addressing.
 127    The argument item.key has to be a pointer to an zero terminated, most
 128    probably strings of chars. The function for generating a number of the
 129    strings is simple but fast. It can be replaced by a more complex function
 130    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 131
 132    We use an trick to speed up the lookup. The table is created by hcreate
 133    with one more element available. This enables us to use the index zero
 134    special. This index will never be used because we store the first hash
 135    index in the field used where zero means not used. Every other value
 136    means used. The used field can be used as a first fast comparison for
 137    equality of the stored and the parameter value. This helps to prevent
 138    unnecessary expensive calls of strcmp.  */
 139 int
 140 hsearch_r (item, action, retval, htab)
 141      ENTRY item;
 142      ACTION action;
 143      ENTRY **retval;
 144      struct hsearch_data *htab;
 145 {
 146   unsigned int hval;
 147   unsigned int count;
 148   unsigned int len = strlen (item.key);
 149   unsigned int idx;
 150
 151   /* Compute an value for the given string. Perhaps use a better method. */
 152   hval = len;
 153   count = len;
 154   while (count-- > 0)
 155     {
 156       hval <<= 4;
 157       hval += item.key[count];
 158     }
 159   if (hval == 0)
 160     ++hval;
 161
 162   /* First hash function: simply take the modul but prevent zero. */
 163   idx = hval % htab->size + 1;
 164
 165   if (htab->table[idx].used)
 166     {
 167       /* Further action might be required according to the action value. */
 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       unsigned int hval2 = 1 + hval % (htab->size - 2);
 177       unsigned int first_idx = idx;
 178
 179       do
 180         {
 181           /* Because SIZE is prime this guarantees to step through all
 182              available indeces.  */
 183           if (idx <= hval2)
 184             idx = htab->size + idx - hval2;
 185           else
 186             idx -= hval2;
 187
 188           /* If we visited all entries leave the loop unsuccessfully.  */
 189           if (idx == first_idx)
 190             break;
 191
 192             /* If entry is found use it. */
 193           if (htab->table[idx].used == hval
 194               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 195             {
 196               *retval = &htab->table[idx].entry;
 197               return 1;
 198             }
 199         }
 200       while (htab->table[idx].used);
 201     }
 202
 203   /* An empty bucket has been found. */
 204   if (action == ENTER)
 205     {
 206       /* If table is full and another entry should be entered return
 207          with error.  */
 208       if (htab->filled == htab->size)
 209         {
 210           __set_errno (ENOMEM);
 211           *retval = NULL;
 212           return 0;
 213         }
 214
 215       htab->table[idx].used  = hval;
 216       htab->table[idx].entry = item;
 217
 218       ++htab->filled;
 219
 220       *retval = &htab->table[idx].entry;
 221       return 1;
 222     }
 223
 224   __set_errno (ESRCH);
 225   *retval = NULL;
 226   return 0;
 227 }
 228 libc_hidden_def (hsearch_r)