misc/hsearch_r.c

   1 /* Copyright (C) 1993, 1995 Free Software Foundation, Inc.
   2 Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
   3 This file is part of the GNU C Library.
   4
   5 The GNU C Library is free software; you can redistribute it and/or
   6 modify it under the terms of the GNU Library General Public License as
   7 published by the Free Software Foundation; either version 2 of the
   8 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 Library General Public License for more details.
  14
  15 You should have received a copy of the GNU Library General Public
  16 License along with the GNU C Library; see the file COPYING.LIB.  If
  17 not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18 Boston, MA 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 reentrent 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   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 (number)
  48      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      unsigned int nel;
  69      struct hsearch_data *htab;
  70 {
  71   /* Test for correct arguments.  */
  72   if (htab == NULL)
  73     {
  74       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
  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       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
 121
 122 /* This is the search function. It uses double hashing with open adressing.
 123    The argument item.key has to be a pointer to an zero terminated, most
 124    probably strings of chars. The function for generating a number of the
 125    strings is simple but fast. It can be replaced by a more complex function
 126    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 127
 128    We use an trick to speed up the lookup. The table is created by hcreate
 129    with one more element available. This enables us to use the index zero
 130    special. This index will never be used because we store the first hash
 131    index in the field used where zero means not used. Every other value
 132    means used. The used field can be used as a first fast comparison for
 133    equality of the stored and the parameter value. This helps to prevent
 134    unnecessary expensive calls of strcmp.  */
 135 int
 136 hsearch_r (item, action, retval, htab)
 137      ENTRY item;
 138      ACTION action;
 139      ENTRY **retval;
 140      struct hsearch_data *htab;
 141 {
 142   unsigned int hval;
 143   unsigned int count;
 144   unsigned int len = strlen (item.key);
 145   unsigned int idx;
 146
 147   /* If table is full and another entry should be entered return with
 148      error.  */
 149   if (action == ENTER && htab->filled == htab->size)
 150     {
 151       errno = ENOMEM;
 152       *retval = NULL;
 153       return 0;
 154     }
 155
 156   /* Compute an value for the given string. Perhaps use a better method. */
 157   hval = len;
 158   count = len;
 159   while (count-- > 0)
 160     {
 161       hval <<= 4;
 162       hval += item.key[count];
 163     }
 164
 165   /* First hash function: simply take the modul but prevent zero. */
 166   hval %= htab->size;
 167   if (hval == 0)
 168     ++hval;
 169
 170   /* The first index tried. */
 171   idx = hval;
 172
 173   if (htab->table[idx].used)
 174     {
 175       /* Further action might be required according to the action value. */
 176       unsigned hval2;
 177
 178       if (htab->table[idx].used == hval
 179           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 180         {
 181           if (action == ENTER)
 182             htab->table[idx].entry.data = item.data;
 183
 184           *retval = &htab->table[idx].entry;
 185           return 1;
 186         }
 187
 188       /* Second hash function, as suggested in [Knuth] */
 189       hval2 = 1 + hval % (htab->size - 2);
 190
 191       do
 192         {
 193           /* Because SIZE is prime this guarantees to step through all
 194              available indeces.  */
 195           if (idx <= hval2)
 196             idx = htab->size + idx - hval2;
 197           else
 198             idx -= hval2;
 199
 200             /* If entry is found use it. */
 201           if (htab->table[idx].used == hval
 202               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 203             {
 204               if (action == ENTER)
 205                 htab->table[idx].entry.data = item.data;
 206
 207               *retval = &htab->table[idx].entry;
 208               return 1;
 209             }
 210         }
 211       while (htab->table[idx].used);
 212     }
 213
 214   /* An empty bucket has been found. */
 215   if (action == ENTER)
 216     {
 217       htab->table[idx].used  = hval;
 218       htab->table[idx].entry = item;
 219
 220       ++htab->filled;
 221
 222       *retval = &htab->table[idx].entry;
 223       return 1;
 224     }
 225
 226   errno = ESRCH;
 227   *retval = NULL;
 228   return 0;
 229 }