src/basic/prioq.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2013 Lennart Poettering
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18 ***/
  19
  20 /*
  21  * Priority Queue
  22  * The prioq object implements a priority queue. That is, it orders objects by
  23  * their priority and allows O(1) access to the object with the highest
  24  * priority. Insertion and removal are Θ(log n). Optionally, the caller can
  25  * provide a pointer to an index which will be kept up-to-date by the prioq.
  26  *
  27  * The underlying algorithm used in this implementation is a Heap.
  28  */
  29
  30 #include <errno.h>
  31 #include <stdlib.h>
  32
  33 #include "alloc-util.h"
  34 #include "hashmap.h"
  35 #include "prioq.h"
  36
  37 struct prioq_item {
  38         void *data;
  39         unsigned *idx;
  40 };
  41
  42 struct Prioq {
  43         compare_func_t compare_func;
  44         unsigned n_items, n_allocated;
  45
  46         struct prioq_item *items;
  47 };
  48
  49 Prioq *prioq_new(compare_func_t compare_func) {
  50         Prioq *q;
  51
  52         q = new0(Prioq, 1);
  53         if (!q)
  54                 return q;
  55
  56         q->compare_func = compare_func;
  57         return q;
  58 }
  59
  60 Prioq* prioq_free(Prioq *q) {
  61         if (!q)
  62                 return NULL;
  63
  64         free(q->items);
  65         return mfree(q);
  66 }
  67
  68 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {
  69         assert(q);
  70
  71         if (*q)
  72                 return 0;
  73
  74         *q = prioq_new(compare_func);
  75         if (!*q)
  76                 return -ENOMEM;
  77
  78         return 0;
  79 }
  80
  81 static void swap(Prioq *q, unsigned j, unsigned k) {
  82         void *saved_data;
  83         unsigned *saved_idx;
  84
  85         assert(q);
  86         assert(j < q->n_items);
  87         assert(k < q->n_items);
  88
  89         assert(!q->items[j].idx || *(q->items[j].idx) == j);
  90         assert(!q->items[k].idx || *(q->items[k].idx) == k);
  91
  92         saved_data = q->items[j].data;
  93         saved_idx = q->items[j].idx;
  94         q->items[j].data = q->items[k].data;
  95         q->items[j].idx = q->items[k].idx;
  96         q->items[k].data = saved_data;
  97         q->items[k].idx = saved_idx;
  98
  99         if (q->items[j].idx)
 100                 *q->items[j].idx = j;
 101
 102         if (q->items[k].idx)
 103                 *q->items[k].idx = k;
 104 }
 105
 106 static unsigned shuffle_up(Prioq *q, unsigned idx) {
 107         assert(q);
 108
 109         while (idx > 0) {
 110                 unsigned k;
 111
 112                 k = (idx-1)/2;
 113
 114                 if (q->compare_func(q->items[k].data, q->items[idx].data) <= 0)
 115                         break;
 116
 117                 swap(q, idx, k);
 118                 idx = k;
 119         }
 120
 121         return idx;
 122 }
 123
 124 static unsigned shuffle_down(Prioq *q, unsigned idx) {
 125         assert(q);
 126
 127         for (;;) {
 128                 unsigned j, k, s;
 129
 130                 k = (idx+1)*2; /* right child */
 131                 j = k-1;       /* left child */
 132
 133                 if (j >= q->n_items)
 134                         break;
 135
 136                 if (q->compare_func(q->items[j].data, q->items[idx].data) < 0)
 137
 138                         /* So our left child is smaller than we are, let's
 139                          * remember this fact */
 140                         s = j;
 141                 else
 142                         s = idx;
 143
 144                 if (k < q->n_items &&
 145                     q->compare_func(q->items[k].data, q->items[s].data) < 0)
 146
 147                         /* So our right child is smaller than we are, let's
 148                          * remember this fact */
 149                         s = k;
 150
 151                 /* s now points to the smallest of the three items */
 152
 153                 if (s == idx)
 154                         /* No swap necessary, we're done */
 155                         break;
 156
 157                 swap(q, idx, s);
 158                 idx = s;
 159         }
 160
 161         return idx;
 162 }
 163
 164 int prioq_put(Prioq *q, void *data, unsigned *idx) {
 165         struct prioq_item *i;
 166         unsigned k;
 167
 168         assert(q);
 169
 170         if (q->n_items >= q->n_allocated) {
 171                 unsigned n;
 172                 struct prioq_item *j;
 173
 174                 n = MAX((q->n_items+1) * 2, 16u);
 175                 j = realloc(q->items, sizeof(struct prioq_item) * n);
 176                 if (!j)
 177                         return -ENOMEM;
 178
 179                 q->items = j;
 180                 q->n_allocated = n;
 181         }
 182
 183         k = q->n_items++;
 184         i = q->items + k;
 185         i->data = data;
 186         i->idx = idx;
 187
 188         if (idx)
 189                 *idx = k;
 190
 191         shuffle_up(q, k);
 192
 193         return 0;
 194 }
 195
 196 static void remove_item(Prioq *q, struct prioq_item *i) {
 197         struct prioq_item *l;
 198
 199         assert(q);
 200         assert(i);
 201
 202         l = q->items + q->n_items - 1;
 203
 204         if (i == l)
 205                 /* Last entry, let's just remove it */
 206                 q->n_items--;
 207         else {
 208                 unsigned k;
 209
 210                 /* Not last entry, let's replace the last entry with
 211                  * this one, and reshuffle */
 212
 213                 k = i - q->items;
 214
 215                 i->data = l->data;
 216                 i->idx = l->idx;
 217                 if (i->idx)
 218                         *i->idx = k;
 219                 q->n_items--;
 220
 221                 k = shuffle_down(q, k);
 222                 shuffle_up(q, k);
 223         }
 224 }
 225
 226 _pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx) {
 227         struct prioq_item *i;
 228
 229         assert(q);
 230
 231         if (idx) {
 232                 if (*idx == PRIOQ_IDX_NULL ||
 233                     *idx > q->n_items)
 234                         return NULL;
 235
 236                 i = q->items + *idx;
 237                 if (i->data != data)
 238                         return NULL;
 239
 240                 return i;
 241         } else {
 242                 for (i = q->items; i < q->items + q->n_items; i++)
 243                         if (i->data == data)
 244                                 return i;
 245                 return NULL;
 246         }
 247 }
 248
 249 int prioq_remove(Prioq *q, void *data, unsigned *idx) {
 250         struct prioq_item *i;
 251
 252         if (!q)
 253                 return 0;
 254
 255         i = find_item(q, data, idx);
 256         if (!i)
 257                 return 0;
 258
 259         remove_item(q, i);
 260         return 1;
 261 }
 262
 263 int prioq_reshuffle(Prioq *q, void *data, unsigned *idx) {
 264         struct prioq_item *i;
 265         unsigned k;
 266
 267         assert(q);
 268
 269         i = find_item(q, data, idx);
 270         if (!i)
 271                 return 0;
 272
 273         k = i - q->items;
 274         k = shuffle_down(q, k);
 275         shuffle_up(q, k);
 276         return 1;
 277 }
 278
 279 void *prioq_peek(Prioq *q) {
 280
 281         if (!q)
 282                 return NULL;
 283
 284         if (q->n_items <= 0)
 285                 return NULL;
 286
 287         return q->items[0].data;
 288 }
 289
 290 void *prioq_pop(Prioq *q) {
 291         void *data;
 292
 293         if (!q)
 294                 return NULL;
 295
 296         if (q->n_items <= 0)
 297                 return NULL;
 298
 299         data = q->items[0].data;
 300         remove_item(q, q->items);
 301         return data;
 302 }
 303
 304 unsigned prioq_size(Prioq *q) {
 305
 306         if (!q)
 307                 return 0;
 308
 309         return q->n_items;
 310 }
 311
 312 bool prioq_isempty(Prioq *q) {
 313
 314         if (!q)
 315                 return true;
 316
 317         return q->n_items <= 0;
 318 }