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 (size_t nel, struct hsearch_data *htab)
  66 {
  67   /* Test for correct arguments.  */
  68   if (htab == NULL)
  69     {
  70       __set_errno (EINVAL);
  71       return 0;
  72     }
  73
  74   if (nel >= SIZE_MAX / sizeof (_ENTRY))
  75     {
  76       __set_errno (ENOMEM);
  77       return 0;
  78     }
  79
  80
  81   /* There is still another table active. Return with error. */
  82   if (htab->table != NULL)
  83     return 0;
  84
  85   /* We need a size of at least 3.  Otherwise the hash functions we
  86      use will not work.  */
  87   if (nel < 3)
  88     nel = 3;
  89   /* Change nel to the first prime number not smaller as nel. */
  90   nel |= 1;      /* make odd */
  91   while (!isprime (nel))
  92     nel += 2;
  93
  94   htab->size = nel;
  95   htab->filled = 0;
  96
  97   /* allocate memory and zero out */
  98   htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
  99   if (htab->table == NULL)
 100     return 0;
 101
 102   /* everything went alright */
 103   return 1;
 104 }
 105 libc_hidden_def (__hcreate_r)
 106 weak_alias (__hcreate_r, hcreate_r)
 107
 108
 109 /* After using the hash table it has to be destroyed. The used memory can
 110    be freed and the local static variable can be marked as not used.  */
 111 void
 112 __hdestroy_r (struct hsearch_data *htab)
 113 {
 114   /* Test for correct arguments.  */
 115   if (htab == NULL)
 116     {
 117       __set_errno (EINVAL);
 118       return;
 119     }
 120
 121   /* Free used memory.  */
 122   free (htab->table);
 123
 124   /* the sign for an existing table is an value != NULL in htable */
 125   htab->table = NULL;
 126 }
 127 libc_hidden_def (__hdestroy_r)
 128 weak_alias (__hdestroy_r, hdestroy_r)
 129
 130
 131 /* This is the search function. It uses double hashing with open addressing.
 132    The argument item.key has to be a pointer to an zero terminated, most
 133    probably strings of chars. The function for generating a number of the
 134    strings is simple but fast. It can be replaced by a more complex function
 135    like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
 136
 137    We use an trick to speed up the lookup. The table is created by hcreate
 138    with one more element available. This enables us to use the index zero
 139    special. This index will never be used because we store the first hash
 140    index in the field used where zero means not used. Every other value
 141    means used. The used field can be used as a first fast comparison for
 142    equality of the stored and the parameter value. This helps to prevent
 143    unnecessary expensive calls of strcmp.  */
 144 int
 145 __hsearch_r (item, action, retval, htab)
 146      ENTRY item;
 147      ACTION action;
 148      ENTRY **retval;
 149      struct hsearch_data *htab;
 150 {
 151   unsigned int hval;
 152   unsigned int count;
 153   unsigned int len = strlen (item.key);
 154   unsigned int idx;
 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   if (hval == 0)
 165     ++hval;
 166
 167   /* First hash function: simply take the modul but prevent zero. */
 168   idx = hval % htab->size + 1;
 169
 170   if (htab->table[idx].used)
 171     {
 172       /* Further action might be required according to the action value. */
 173       if (htab->table[idx].used == hval
 174           && strcmp (item.key, htab->table[idx].entry.key) == 0)
 175         {
 176           *retval = &htab->table[idx].entry;
 177           return 1;
 178         }
 179
 180       /* Second hash function, as suggested in [Knuth] */
 181       unsigned int hval2 = 1 + hval % (htab->size - 2);
 182       unsigned int first_idx = idx;
 183
 184       do
 185         {
 186           /* Because SIZE is prime this guarantees to step through all
 187              available indeces.  */
 188           if (idx <= hval2)
 189             idx = htab->size + idx - hval2;
 190           else
 191             idx -= hval2;
 192
 193           /* If we visited all entries leave the loop unsuccessfully.  */
 194           if (idx == first_idx)
 195             break;
 196
 197             /* If entry is found use it. */
 198           if (htab->table[idx].used == hval
 199               && strcmp (item.key, htab->table[idx].entry.key) == 0)
 200             {
 201               *retval = &htab->table[idx].entry;
 202               return 1;
 203             }
 204         }
 205       while (htab->table[idx].used);
 206     }
 207
 208   /* An empty bucket has been found. */
 209   if (action == ENTER)
 210     {
 211       /* If table is full and another entry should be entered return
 212          with error.  */
 213       if (htab->filled == htab->size)
 214         {
 215           __set_errno (ENOMEM);
 216           *retval = NULL;
 217           return 0;
 218         }
 219
 220       htab->table[idx].used  = hval;
 221       htab->table[idx].entry = item;
 222
 223       ++htab->filled;
 224
 225       *retval = &htab->table[idx].entry;
 226       return 1;
 227     }
 228
 229   __set_errno (ESRCH);
 230   *retval = NULL;
 231   return 0;
 232 }
 233 libc_hidden_def (__hsearch_r)
 234 weak_alias (__hsearch_r, hsearch_r)