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sourceware.org Git - glibc.git/blob - misc/hsearch_r.c
1 /* Copyright (C) 1993, 1995, 1996, 1997 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.
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.
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.
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, write to the Free
17 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
26 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
27 [Knuth] The Art of Computer Programming, part 3 (6.4) */
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. */
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 */
47 isprime (unsigned int number
)
49 /* no even number will be passed */
52 while (div
* div
< number
&& number
% div
!= 0)
55 return number
% div
!= 0;
59 /* Before using the hash table we must allocate memory for it.
60 Test for an existing table are done. We allocate one element
61 more as the found prime number says. This is done for more effective
62 indexing as explained in the comment for the hsearch function.
63 The contents of the table is zeroed, especially the field used
68 struct hsearch_data
*htab
;
70 /* Test for correct arguments. */
77 /* There is still another table active. Return with error. */
78 if (htab
->table
!= NULL
)
81 /* Change nel to the first prime number not smaller as nel. */
82 nel
|= 1; /* make odd */
83 while (!isprime (nel
))
89 /* allocate memory and zero out */
90 htab
->table
= (_ENTRY
*) calloc (htab
->size
+ 1, sizeof (_ENTRY
));
91 if (htab
->table
== NULL
)
94 /* everything went alright */
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. */
103 struct hsearch_data
*htab
;
105 /* Test for correct arguments. */
108 __set_errno (EINVAL
);
112 if (htab
->table
!= NULL
)
113 /* free used memory */
116 /* the sign for an existing table is an value != NULL in htable */
121 /* This is the search function. It uses double hashing with open addressing.
122 The argument item.key has to be a pointer to an zero terminated, most
123 probably strings of chars. The function for generating a number of the
124 strings is simple but fast. It can be replaced by a more complex function
125 like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
127 We use an trick to speed up the lookup. The table is created by hcreate
128 with one more element available. This enables us to use the index zero
129 special. This index will never be used because we store the first hash
130 index in the field used where zero means not used. Every other value
131 means used. The used field can be used as a first fast comparison for
132 equality of the stored and the parameter value. This helps to prevent
133 unnecessary expensive calls of strcmp. */
135 hsearch_r (item
, action
, retval
, htab
)
139 struct hsearch_data
*htab
;
143 unsigned int len
= strlen (item
.key
);
146 /* Compute an value for the given string. Perhaps use a better method. */
152 hval
+= item
.key
[count
];
155 /* First hash function: simply take the modul but prevent zero. */
160 /* The first index tried. */
163 if (htab
->table
[idx
].used
)
165 /* Further action might be required according to the action value. */
168 if (htab
->table
[idx
].used
== hval
169 && strcmp (item
.key
, htab
->table
[idx
].entry
.key
) == 0)
172 htab
->table
[idx
].entry
.data
= item
.data
;
174 *retval
= &htab
->table
[idx
].entry
;
178 /* Second hash function, as suggested in [Knuth] */
179 hval2
= 1 + hval
% (htab
->size
- 2);
183 /* Because SIZE is prime this guarantees to step through all
184 available indeces. */
186 idx
= htab
->size
+ idx
- hval2
;
190 /* If we visited all entries leave the loop unsuccessfully. */
194 /* If entry is found use it. */
195 if (htab
->table
[idx
].used
== hval
196 && strcmp (item
.key
, htab
->table
[idx
].entry
.key
) == 0)
199 htab
->table
[idx
].entry
.data
= item
.data
;
201 *retval
= &htab
->table
[idx
].entry
;
205 while (htab
->table
[idx
].used
);
208 /* An empty bucket has been found. */
211 /* If table is full and another entry should be entered return
213 if (action
== ENTER
&& htab
->filled
== htab
->size
)
215 __set_errno (ENOMEM
);
220 htab
->table
[idx
].used
= hval
;
221 htab
->table
[idx
].entry
= item
;
225 *retval
= &htab
->table
[idx
].entry
;
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