lib/hamt.h

   1 /* (Persistent) hash array mapped tries.
   2    Copyright (C) 2021-2024 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Marc Nieper-Wißkirchen <marc@nieper-wisskirchen.de>, 2021. */
  18
  19 /* This module provides a persistent version of hash array mapped
  20    tries (hamts) that can be used in place of hash tables when pure
  21    (functional) operations are needed.
  22
  23    A hash function and an equivalence predicate has to be provided for
  24    the elements that can be inserted, replaced and removed in a hamt.
  25    A hamt cannot contain duplicates that compare equal.
  26
  27    Each non-destructive updating operation returns a new hamt, which
  28    shares structure with the original one.  Destructive updates only
  29    effect the hamt, on which the destructive operation is applied.
  30    For example, given a hamt HAMT1, any non-destructive update
  31    operation (e.g. hamt_insert) will result in a new hamt HAMT2.
  32    Whatever further operations (destructive or not, including freeing
  33    a hamt) are applied to HAMT1 won't change HAMT2 and vice versa.  To
  34    free all the memory, hash_free has therefore to be applied to both
  35    HAMT1 and HAMT2.
  36
  37    If persistence is not needed, transient hash tables are probably
  38    faster.
  39
  40    See also: Phil Bagwell (2000). Ideal Hash Trees (Report). Infoscience
  41    Department, École Polytechnique Fédérale de Lausanne.
  42
  43    http://infoscience.epfl.ch/record/64398/files/idealhashtrees.pdf  */
  44
  45 #ifndef _GL_HAMT_H
  46 #define _GL_HAMT_H
  47
  48 /* This file uses _GL_INLINE_HEADER_BEGIN, _GL_INLINE, _GL_ATTRIBUTE_DEALLOC,
  49    _GL_ATTRIBUTE_NODISCARD.  */
  50 #if !_GL_CONFIG_H_INCLUDED
  51  #error "Please include config.h first."
  52 #endif
  53
  54 _GL_INLINE_HEADER_BEGIN
  55 #ifndef _GL_HAMT_INLINE
  56 # define _GL_HAMT_INLINE _GL_INLINE
  57 #endif
  58
  59 /* The GL_HAMT_THREAD_SAFE flag is set if the implementation of hamts
  60    is thread-safe as long as two threads do not simultaneously access
  61    the same hamt.  This is non-trivial as different hamts may share
  62    some structure.
  63    We can define it only when the compiler supports _Atomic.  For GCC,
  64    it is supported starting with GCC 4.9.  */
  65
  66 #if (__GNUC__ + (__GNUC_MINOR__ >= 9) > 4) \
  67     && __STDC_VERSION__ >= 201112L && !defined __STD_NO_ATOMICS__ \
  68     && !defined __cplusplus
  69 # define GL_HAMT_THREAD_SAFE 1
  70 #else
  71 # define GL_HAMT_THREAD_SAFE 0
  72 #endif
  73
  74 #include <stddef.h>
  75 #include <stdint.h>
  76
  77 /* Hash values are of type size_t.  For each level of the trie, we use
  78    5 bits (corresponding to lg2 of the width of a 32-bit word.  */
  79 #define _GL_HAMT_MAX_DEPTH ((SIZE_WIDTH + 4) / 5)
  80
  81 /************/
  82 /* Elements */
  83 /************/
  84
  85 /* A hamt stores pointers to elements.  Each element has to be a
  86    struct whose initial member is of the type Hamt_entry.  You need to
  87    define this struct yourself.  It will typically contain an
  88    Hamt_entry, a key, and, optionally, a value.
  89
  90    An element is conceptually owned by a hamt as soon as it is
  91    inserted.  It will be automatically freed as soon as the last hamt
  92    containing it is freed.  */
  93 typedef struct
  94 {
  95 #if GL_HAMT_THREAD_SAFE
  96   _Atomic
  97 #endif
  98   size_t ref_count;
  99 } Hamt_entry;
 100
 101 /* Initialize *ELT, which has to point to a structure as described
 102    above and return ELT type-cast.
 103
 104    Before an element is inserted into any hamt, whether once or
 105    multiple times, it has to be initialized exactly once.  */
 106 _GL_HAMT_INLINE Hamt_entry *
 107 hamt_element (void *elt)
 108 {
 109   Hamt_entry *entry = elt;
 110   entry->ref_count = 0;         /* This assumes that element_entry ==
 111                                    0.  */
 112   return entry;
 113 }
 114
 115 /*************************/
 116 /* Opaque Hamt Structure */
 117 /*************************/
 118
 119 /* In user-code, hamts are accessed through pointers to the opaque
 120    Hamt type.  Two hamts are said to be the same if and only if their
 121    pointers are equal. */
 122 typedef struct hamt Hamt;
 123
 124 /*************/
 125 /* Interface */
 126 /*************/
 127
 128 /* A hash function has to be pure, and two elements that compare equal
 129    have to have the same hash value.  For a hash function to be a good
 130    one, it is important that it uses all SIZE_WIDTH bits in
 131    size_t.  */
 132 typedef size_t (Hamt_hasher) (const void *elt);
 133
 134 /* A comparison function has to be pure, and two elements that have
 135    equal pointers have to compare equal.  */
 136 typedef bool (Hamt_comparator) (const void *elt1, const void *elt2);
 137
 138 /* A user-defined function that is called when the last hamt
 139    containing a particular element is freed.  */
 140 typedef void (Hamt_freer) (Hamt_entry *elt);
 141
 142 /****************************/
 143 /* Creation and Destruction */
 144 /****************************/
 145
 146 /* Free the resources solely allocated by HAMT and all elements solely
 147    contained in it.  */
 148 extern void hamt_free (Hamt *hamt);
 149
 150 /* Create and return a new and empty hash array mapped trie.  */
 151 _GL_ATTRIBUTE_NODISCARD
 152 extern Hamt *hamt_create (Hamt_hasher *hasher, Hamt_comparator *comparator,
 153                           Hamt_freer *freer)
 154   _GL_ATTRIBUTE_DEALLOC (hamt_free, 1);
 155
 156 /* Return a copy of HAMT, which is not the same in the sense above.
 157    This procedure can be used, for example, so that two threads can
 158    access the same data independently.  */
 159 _GL_ATTRIBUTE_NODISCARD
 160 extern Hamt *hamt_copy (Hamt *hamt)
 161   _GL_ATTRIBUTE_DEALLOC (hamt_free, 1);
 162
 163 /**********/
 164 /* Lookup */
 165 /**********/
 166
 167 /* If ELT matches an entry in HAMT, return this entry.  Otherwise,
 168    return NULL.  */
 169 extern Hamt_entry *hamt_lookup (const Hamt *hamt, const void *elt);
 170
 171 /**************************/
 172 /* Insertion and Deletion */
 173 /**************************/
 174
 175 /* If *ELT_PTR matches an element already in HAMT, set *ELT_PTR to the
 176    existing element and return the original hamt.  Otherwise, insert
 177    *ELT_PTR into a copy of the hamt and return the copy.  */
 178 _GL_ATTRIBUTE_NODISCARD
 179 extern Hamt *hamt_insert (Hamt *hamt, Hamt_entry **elt_ptr);
 180
 181 /* If *ELT_PTR matches an element already in HAMT, set *ELT_PTR to the
 182 existing element, remove the element from a copy of the hamt and
 183 return the copy.  Otherwise, return the original hamt.  */
 184 _GL_ATTRIBUTE_NODISCARD
 185 extern Hamt *hamt_remove (Hamt *hamt, Hamt_entry **elt_ptr);
 186
 187 /* Insert *ELT_PTR into a copy of HAMT and return the copy.  If an
 188    existing element was replaced, set *ELT_PTR to this element, and to
 189    NULL otherwise.  */
 190 _GL_ATTRIBUTE_NODISCARD
 191 extern Hamt *hamt_replace (Hamt *hamt, Hamt_entry **elt_ptr);
 192
 193 /*************/
 194 /* Iteration */
 195 /*************/
 196
 197 /* A processor function is called during walking of a hamt, which
 198    returns true to continue the walking.  */
 199 typedef bool (Hamt_processor) (Hamt_entry *elt, void *data);
 200
 201 /* Call PROC for every entry of the hamt until it returns false.  The
 202    first argument to the processor is the entry, the second argument
 203    is the value of DATA as received.  Return the number of calls that
 204    returned true.  During processing, the hamt mustn't be
 205    modified.  */
 206 extern size_t hamt_do_while (const Hamt *hamt, Hamt_processor *proc,
 207                              void *data);
 208
 209 /* An alternative interface to iterating through the entry of a hamt
 210    that does not make use of a higher-order function like
 211    hamt_do_while uses the Hamt_iterator type, which can be allocated
 212    through automatic variables on the stack.  As a hamt iterator
 213    operates on a copy of a hamt, the original hamt can modified
 214    (including freeing it) without affecting the iterator.  */
 215 struct hamt_iterator
 216 {
 217   Hamt* hamt;
 218   int depth;
 219   size_t path;
 220   size_t position;
 221   Hamt_entry *entry[_GL_HAMT_MAX_DEPTH + 1];
 222 };
 223 typedef struct hamt_iterator Hamt_iterator;
 224
 225 /* Create of copy of HAMT and return an initialized ITER on the
 226    copy.  */
 227 extern Hamt_iterator hamt_iterator (Hamt *hamt);
 228
 229 /* Free the resources allocated for ITER, including the hamt copy
 230    associated with it.  */
 231 extern void hamt_iterator_free (Hamt_iterator *iter);
 232
 233 /* Return an independent copy of ITER that is initially in the same
 234    state.  Any operation on the original iterator (including freeing
 235    it) doesn't affect the iterator copy and vice versa.  */
 236 extern Hamt_iterator hamt_iterator_copy (Hamt_iterator *iter);
 237
 238 /* Return true if ITER is not at the end, place the current element in
 239    *ELT_PTR and move the iterator forward.  Otherwise, return
 240    false.  */
 241 extern bool hamt_iterator_next (Hamt_iterator *iter,
 242                                 Hamt_entry **elt_ptr);
 243
 244 /***********************/
 245 /* Destructive Updates */
 246 /***********************/
 247
 248 /* If *ELT_PTR matches an element already in HAMT, set *ELT_PTR to the
 249    element from the table and return false.  Otherwise, insert *ELT_PTR
 250    destructively into the hamt and return true.  */
 251 extern bool hamt_insert_x (Hamt *hamt, Hamt_entry **elt_ptr);
 252
 253 /* Insert ELT destructively into HAMT.  If an existing element was
 254    replaced, return true.  Otherwise, return false.  */
 255 extern bool hamt_replace_x (Hamt *hamt, Hamt_entry *elt);
 256
 257 /* If ELT matches an element already in HAMT, remove the element
 258    destructively from the hamt and return true.  Otherwise, return
 259    false.  */
 260 extern bool hamt_remove_x (Hamt *hamt, Hamt_entry *elt);
 261
 262 _GL_INLINE_HEADER_END
 263
 264 #endif /* _GL_HAMT_H */