misc/hsearch_r.c

   1 /* Copyright (C) 1993-2013 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, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include <malloc.h>
  21 #include <string.h>
  22
  23 #include <search.h>
  24
  25 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
  26    [Knuth]            The Art of Computer Programming, part 3 (6.4)  */
  27
  28
  29 /* The reentrant version has no static variables to maintain the state.
  30    Instead the interface of all functions is extended to take an argument
  31    which describes the current status.  */
  32 typedef struct _ENTRY
  33 {
  34   unsigned int used;
  35   ENTRY entry;
  36 }
  37 _ENTRY;
  38
  39
  40 /* For the used double hash method the table size has to be a prime. To
  41    correct the user given table size we need a prime test.  This trivial
  42    algorithm is adequate because
  43    a)  the code is (most probably) called a few times per program run and
  44    b)  the number is small because the table must fit in the core  */
  45 static int
  46 isprime (unsigned int number)
  47 {
  48   /* no even number will be passed */
  49   unsigned int div = 3;
  50
  51   while (div * div < number && number % div != 0)
  52     div += 2;
  53
  54   return number % div != 0;
  55 }
  56
  57
  58 /* Before using the hash table we must allocate memory for it.
  59    Test for an existing table are done. We allocate one element
  60    more as the found prime number says. This is done for more effective
  61    indexing as explained in the comment for the hsearch function.
  62    The contents of the table is zeroed, especially the field used
  63    becomes zero.  */
  64 int
  65 hcreate_r (nel, htab)
  66      size_t nel;
  67      struct hsearch_data *htab;
  68 {
  69   /* Test for correct arguments.  */
  70   if (htab == NULL)
  71     {
  72       __set_errno (EINVAL);
  73       return 0;
  74     }
  75
  76   /* There is still another table active. Return with error. */
  77   if (htab->table != NULL)
  78     return 0;
  79
  80   /* We need a size of at least 3.  Otherwise the hash functions we
  81      use will not work.  */
  82   if (nel < 3)
  83     nel = 3;
  84   /* Change nel to the first prime number not smaller as nel. */
  85   nel |= 1;      /* make odd */
  86   while (!isprime (nel))
  87     nel += 2;
  88
  89   htab->size = nel;
  90   htab->filled = 0;
  91
  92   /* allocate memory and zero out */
  93   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  94   if (htab->table == NULL)
  95     return 0;
  96
  97   /* everything went alright */
  98   return 1;
  99 }
 100 libc_hidden_def (hcreate_r)
 101
 102
 103 /* After using the hash table it has to be destroyed. The used memory can
 104    be freed and the local static variable can be marked as not used.  */
 105 void
 106 hdestroy_r (htab)
 107      struct hsearch_data *htab;
 108 {
 109   /* Test for correct arguments.  */
 110   if (htab == NULL)
 111     {
 112       __set_errno (EINVAL);
 113       return;
 114     }
 115
 116   /* Free used memory.  */
 117   free (htab->table);
 118
 119   /* the sign for an existing table is an value != NULL in htable */
 120   htab->table = NULL;
 121 }
 122 libc_hidden_def (hdestroy_r)
 123
 124
 125 /* This is the search function. It uses double hashing with open addressing.
 126    The argument item.key has to be a pointer to an zero terminated, most
 127    probably strings of chars. The function for generating a number of the
 128    strings is simple but fast. It can be replaced by a more complex function
 129    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 130
 131    We use an trick to speed up the lookup. The table is created by hcreate
 132    with one more element available. This enables us to use the index zero
 133    special. This index will never be used because we store the first hash
 134    index in the field used where zero means not used. Every other value
 135    means used. The used field can be used as a first fast comparison for
 136    equality of the stored and the parameter value. This helps to prevent
 137    unnecessary expensive calls of strcmp.  */
 138 int
 139 hsearch_r (item, action, retval, htab)
 140      ENTRY item;
 141      ACTION action;
 142      ENTRY **retval;
 143      struct hsearch_data *htab;
 144 {
 145   unsigned int hval;
 146   unsigned int count;
 147   unsigned int len = strlen (item.key);
 148   unsigned int idx;
 149
 150   /* Compute an value for the given string. Perhaps use a better method. */
 151   hval = len;
 152   count = len;
 153   while (count-- > 0)
 154     {
 155       hval <<= 4;
 156       hval += item.key[count];
 157     }
 158   if (hval == 0)
 159     ++hval;
 160
 161   /* First hash function: simply take the modul but prevent zero. */
 162   idx = hval % htab->size + 1;
 163
 164   if (htab->table[idx].used)
 165     {
 166       /* Further action might be required according to the action value. */
 167       if (htab->table[idx].used == hval
 168           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 169         {
 170           *retval = &htab->table[idx].entry;
 171           return 1;
 172         }
 173
 174       /* Second hash function, as suggested in [Knuth] */
 175       unsigned int hval2 = 1 + hval % (htab->size - 2);
 176       unsigned int first_idx = idx;
 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 == first_idx)
 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 (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 }
 227 libc_hidden_def (hsearch_r)