misc/hsearch_r.c

   1 /* Copyright (C) 1993-2015 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 #include <stdint.h>
  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   if (nel >= SIZE_MAX / sizeof (_ENTRY))
  77     {
  78       __set_errno (ENOMEM);
  79       return 0;
  80     }
  81
  82
  83   /* There is still another table active. Return with error. */
  84   if (htab->table != NULL)
  85     return 0;
  86
  87   /* We need a size of at least 3.  Otherwise the hash functions we
  88      use will not work.  */
  89   if (nel < 3)
  90     nel = 3;
  91   /* Change nel to the first prime number not smaller as nel. */
  92   nel |= 1;      /* make odd */
  93   while (!isprime (nel))
  94     nel += 2;
  95
  96   htab->size = nel;
  97   htab->filled = 0;
  98
  99   /* allocate memory and zero out */
 100   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
 101   if (htab->table == NULL)
 102     return 0;
 103
 104   /* everything went alright */
 105   return 1;
 106 }
 107 libc_hidden_def (__hcreate_r)
 108 weak_alias (__hcreate_r, hcreate_r)
 109
 110
 111 /* After using the hash table it has to be destroyed. The used memory can
 112    be freed and the local static variable can be marked as not used.  */
 113 void
 114 __hdestroy_r (htab)
 115      struct hsearch_data *htab;
 116 {
 117   /* Test for correct arguments.  */
 118   if (htab == NULL)
 119     {
 120       __set_errno (EINVAL);
 121       return;
 122     }
 123
 124   /* Free used memory.  */
 125   free (htab->table);
 126
 127   /* the sign for an existing table is an value != NULL in htable */
 128   htab->table = NULL;
 129 }
 130 libc_hidden_def (__hdestroy_r)
 131 weak_alias (__hdestroy_r, hdestroy_r)
 132
 133
 134 /* This is the search function. It uses double hashing with open addressing.
 135    The argument item.key has to be a pointer to an zero terminated, most
 136    probably strings of chars. The function for generating a number of the
 137    strings is simple but fast. It can be replaced by a more complex function
 138    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 139
 140    We use an trick to speed up the lookup. The table is created by hcreate
 141    with one more element available. This enables us to use the index zero
 142    special. This index will never be used because we store the first hash
 143    index in the field used where zero means not used. Every other value
 144    means used. The used field can be used as a first fast comparison for
 145    equality of the stored and the parameter value. This helps to prevent
 146    unnecessary expensive calls of strcmp.  */
 147 int
 148 __hsearch_r (item, action, retval, htab)
 149      ENTRY item;
 150      ACTION action;
 151      ENTRY **retval;
 152      struct hsearch_data *htab;
 153 {
 154   unsigned int hval;
 155   unsigned int count;
 156   unsigned int len = strlen (item.key);
 157   unsigned int idx;
 158
 159   /* Compute an value for the given string. Perhaps use a better method. */
 160   hval = len;
 161   count = len;
 162   while (count-- > 0)
 163     {
 164       hval <<= 4;
 165       hval += item.key[count];
 166     }
 167   if (hval == 0)
 168     ++hval;
 169
 170   /* First hash function: simply take the modul but prevent zero. */
 171   idx = hval % htab->size + 1;
 172
 173   if (htab->table[idx].used)
 174     {
 175       /* Further action might be required according to the action value. */
 176       if (htab->table[idx].used == hval
 177           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 178         {
 179           *retval = &htab->table[idx].entry;
 180           return 1;
 181         }
 182
 183       /* Second hash function, as suggested in [Knuth] */
 184       unsigned int hval2 = 1 + hval % (htab->size - 2);
 185       unsigned int first_idx = idx;
 186
 187       do
 188         {
 189           /* Because SIZE is prime this guarantees to step through all
 190              available indeces.  */
 191           if (idx <= hval2)
 192             idx = htab->size + idx - hval2;
 193           else
 194             idx -= hval2;
 195
 196           /* If we visited all entries leave the loop unsuccessfully.  */
 197           if (idx == first_idx)
 198             break;
 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               *retval = &htab->table[idx].entry;
 205               return 1;
 206             }
 207         }
 208       while (htab->table[idx].used);
 209     }
 210
 211   /* An empty bucket has been found. */
 212   if (action == ENTER)
 213     {
 214       /* If table is full and another entry should be entered return
 215          with error.  */
 216       if (htab->filled == htab->size)
 217         {
 218           __set_errno (ENOMEM);
 219           *retval = NULL;
 220           return 0;
 221         }
 222
 223       htab->table[idx].used  = hval;
 224       htab->table[idx].entry = item;
 225
 226       ++htab->filled;
 227
 228       *retval = &htab->table[idx].entry;
 229       return 1;
 230     }
 231
 232   __set_errno (ESRCH);
 233   *retval = NULL;
 234   return 0;
 235 }
 236 libc_hidden_def (__hsearch_r)
 237 weak_alias (__hsearch_r, hsearch_r)