libc/misc/search/_hsearch_r.c

   1 /* Copyright (C) 1993, 1995, 1996, 1997, 2002 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 <stdlib.h>
  21 #include <string.h>
  22
  23 #include <search.h>
  24
  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 #ifdef L_hcreate_r
  42
  43 /* For the used double hash method the table size has to be a prime. To
  44    correct the user given table size we need a prime test.  This trivial
  45    algorithm is adequate because
  46    a)  the code is (most probably) called a few times per program run and
  47    b)  the number is small because the table must fit in the core  */
  48 static int isprime (unsigned int number)
  49 {
  50   /* no even number will be passed */
  51   unsigned int divisor = 3;
  52
  53   while (divisor * divisor < number && number % divisor != 0)
  54     divisor += 2;
  55
  56   return number % divisor != 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 hcreate_r (size_t nel, struct hsearch_data *htab)
  67 {
  68   /* Test for correct arguments.  */
  69   if (htab == NULL)
  70     {
  71       __set_errno (EINVAL);
  72       return 0;
  73     }
  74
  75   /* There is still another table active. Return with error. */
  76   if (htab->table != NULL)
  77     return 0;
  78
  79   /* Change nel to the first prime number not smaller as nel. */
  80   nel |= 1;      /* make odd */
  81   while (!isprime (nel))
  82     nel += 2;
  83
  84   htab->size = nel;
  85   htab->filled = 0;
  86
  87   /* allocate memory and zero out */
  88   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  89   if (htab->table == NULL)
  90     return 0;
  91
  92   /* everything went alright */
  93   return 1;
  94 }
  95 libc_hidden_def(hcreate_r)
  96 #endif
  97
  98 #ifdef L_hdestroy_r
  99 /* After using the hash table it has to be destroyed. The used memory can
 100    be freed and the local static variable can be marked as not used.  */
 101 void hdestroy_r (struct hsearch_data *htab)
 102 {
 103   /* Test for correct arguments.  */
 104   if (htab == NULL)
 105     {
 106       __set_errno (EINVAL);
 107       return;
 108     }
 109
 110   /* free used memory */
 111   free (htab->table);
 112
 113   /* the sign for an existing table is an value != NULL in htable */
 114   htab->table = NULL;
 115 }
 116 libc_hidden_def(hdestroy_r)
 117 #endif
 118
 119 #ifdef L_hsearch_r
 120 /* This is the search function. It uses double hashing with open addressing.
 121    The argument item.key has to be a pointer to an zero terminated, most
 122    probably strings of chars. The function for generating a number of the
 123    strings is simple but fast. It can be replaced by a more complex function
 124    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 125
 126    We use an trick to speed up the lookup. The table is created by hcreate
 127    with one more element available. This enables us to use the index zero
 128    special. This index will never be used because we store the first hash
 129    index in the field used where zero means not used. Every other value
 130    means used. The used field can be used as a first fast comparison for
 131    equality of the stored and the parameter value. This helps to prevent
 132    unnecessary expensive calls of strcmp.  */
 133
 134
 135 int hsearch_r (ENTRY item, ACTION action, ENTRY **retval,
 136                struct hsearch_data *htab)
 137 {
 138   unsigned int hval;
 139   unsigned int count;
 140   unsigned int len = strlen (item.key);
 141   unsigned int idx;
 142
 143   /* Compute an value for the given string. Perhaps use a better method. */
 144   hval = len;
 145   count = len;
 146   while (count-- > 0)
 147     {
 148       hval <<= 4;
 149       hval += item.key[count];
 150     }
 151
 152   /* First hash function: simply take the modul but prevent zero. */
 153   hval %= htab->size;
 154   if (hval == 0)
 155     ++hval;
 156
 157   /* The first index tried. */
 158   idx = hval;
 159
 160   if (htab->table[idx].used)
 161     {
 162       /* Further action might be required according to the action value. */
 163       unsigned hval2;
 164
 165       if (htab->table[idx].used == hval
 166           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 167         {
 168           *retval = &htab->table[idx].entry;
 169           return 1;
 170         }
 171
 172       /* Second hash function, as suggested in [Knuth] */
 173       hval2 = 1 + hval % (htab->size - 2);
 174
 175       do
 176         {
 177           /* Because SIZE is prime this guarantees to step through all
 178              available indeces.  */
 179           if (idx <= hval2)
 180             idx = htab->size + idx - hval2;
 181           else
 182             idx -= hval2;
 183
 184           /* If we visited all entries leave the loop unsuccessfully.  */
 185           if (idx == hval)
 186             break;
 187
 188             /* If entry is found use it. */
 189           if (htab->table[idx].used == hval
 190               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 191             {
 192               *retval = &htab->table[idx].entry;
 193               return 1;
 194             }
 195         }
 196       while (htab->table[idx].used);
 197     }
 198
 199   /* An empty bucket has been found. */
 200   if (action == ENTER)
 201     {
 202       /* If table is full and another entry should be entered return
 203          with error.  */
 204       if (htab->filled == htab->size)
 205         {
 206           __set_errno (ENOMEM);
 207           *retval = NULL;
 208           return 0;
 209         }
 210
 211       htab->table[idx].used  = hval;
 212       htab->table[idx].entry = item;
 213
 214       ++htab->filled;
 215
 216       *retval = &htab->table[idx].entry;
 217       return 1;
 218     }
 219
 220   __set_errno (ESRCH);
 221   *retval = NULL;
 222   return 0;
 223 }
 224 libc_hidden_def(hsearch_r)
 225 #endif