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         free(q);
  66
  67         return NULL;
  68 }
  69
  70 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {
  71         assert(q);
  72
  73         if (*q)
  74                 return 0;
  75
  76         *q = prioq_new(compare_func);
  77         if (!*q)
  78                 return -ENOMEM;
  79
  80         return 0;
  81 }
  82
  83 static void swap(Prioq *q, unsigned j, unsigned k) {
  84         void *saved_data;
  85         unsigned *saved_idx;
  86
  87         assert(q);
  88         assert(j < q->n_items);
  89         assert(k < q->n_items);
  90
  91         assert(!q->items[j].idx || *(q->items[j].idx) == j);
  92         assert(!q->items[k].idx || *(q->items[k].idx) == k);
  93
  94         saved_data = q->items[j].data;
  95         saved_idx = q->items[j].idx;
  96         q->items[j].data = q->items[k].data;
  97         q->items[j].idx = q->items[k].idx;
  98         q->items[k].data = saved_data;
  99         q->items[k].idx = saved_idx;
 100
 101         if (q->items[j].idx)
 102                 *q->items[j].idx = j;
 103
 104         if (q->items[k].idx)
 105                 *q->items[k].idx = k;
 106 }
 107
 108 static unsigned shuffle_up(Prioq *q, unsigned idx) {
 109         assert(q);
 110
 111         while (idx > 0) {
 112                 unsigned k;
 113
 114                 k = (idx-1)/2;
 115
 116                 if (q->compare_func(q->items[k].data, q->items[idx].data) <= 0)
 117                         break;
 118
 119                 swap(q, idx, k);
 120                 idx = k;
 121         }
 122
 123         return idx;
 124 }
 125
 126 static unsigned shuffle_down(Prioq *q, unsigned idx) {
 127         assert(q);
 128
 129         for (;;) {
 130                 unsigned j, k, s;
 131
 132                 k = (idx+1)*2; /* right child */
 133                 j = k-1;       /* left child */
 134
 135                 if (j >= q->n_items)
 136                         break;
 137
 138                 if (q->compare_func(q->items[j].data, q->items[idx].data) < 0)
 139
 140                         /* So our left child is smaller than we are, let's
 141                          * remember this fact */
 142                         s = j;
 143                 else
 144                         s = idx;
 145
 146                 if (k < q->n_items &&
 147                     q->compare_func(q->items[k].data, q->items[s].data) < 0)
 148
 149                         /* So our right child is smaller than we are, let's
 150                          * remember this fact */
 151                         s = k;
 152
 153                 /* s now points to the smallest of the three items */
 154
 155                 if (s == idx)
 156                         /* No swap necessary, we're done */
 157                         break;
 158
 159                 swap(q, idx, s);
 160                 idx = s;
 161         }
 162
 163         return idx;
 164 }
 165
 166 int prioq_put(Prioq *q, void *data, unsigned *idx) {
 167         struct prioq_item *i;
 168         unsigned k;
 169
 170         assert(q);
 171
 172         if (q->n_items >= q->n_allocated) {
 173                 unsigned n;
 174                 struct prioq_item *j;
 175
 176                 n = MAX((q->n_items+1) * 2, 16u);
 177                 j = realloc(q->items, sizeof(struct prioq_item) * n);
 178                 if (!j)
 179                         return -ENOMEM;
 180
 181                 q->items = j;
 182                 q->n_allocated = n;
 183         }
 184
 185         k = q->n_items++;
 186         i = q->items + k;
 187         i->data = data;
 188         i->idx = idx;
 189
 190         if (idx)
 191                 *idx = k;
 192
 193         shuffle_up(q, k);
 194
 195         return 0;
 196 }
 197
 198 static void remove_item(Prioq *q, struct prioq_item *i) {
 199         struct prioq_item *l;
 200
 201         assert(q);
 202         assert(i);
 203
 204         l = q->items + q->n_items - 1;
 205
 206         if (i == l)
 207                 /* Last entry, let's just remove it */
 208                 q->n_items--;
 209         else {
 210                 unsigned k;
 211
 212                 /* Not last entry, let's replace the last entry with
 213                  * this one, and reshuffle */
 214
 215                 k = i - q->items;
 216
 217                 i->data = l->data;
 218                 i->idx = l->idx;
 219                 if (i->idx)
 220                         *i->idx = k;
 221                 q->n_items--;
 222
 223                 k = shuffle_down(q, k);
 224                 shuffle_up(q, k);
 225         }
 226 }
 227
 228 _pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx) {
 229         struct prioq_item *i;
 230
 231         assert(q);
 232
 233         if (idx) {
 234                 if (*idx == PRIOQ_IDX_NULL ||
 235                     *idx > q->n_items)
 236                         return NULL;
 237
 238                 i = q->items + *idx;
 239                 if (i->data != data)
 240                         return NULL;
 241
 242                 return i;
 243         } else {
 244                 for (i = q->items; i < q->items + q->n_items; i++)
 245                         if (i->data == data)
 246                                 return i;
 247                 return NULL;
 248         }
 249 }
 250
 251 int prioq_remove(Prioq *q, void *data, unsigned *idx) {
 252         struct prioq_item *i;
 253
 254         if (!q)
 255                 return 0;
 256
 257         i = find_item(q, data, idx);
 258         if (!i)
 259                 return 0;
 260
 261         remove_item(q, i);
 262         return 1;
 263 }
 264
 265 int prioq_reshuffle(Prioq *q, void *data, unsigned *idx) {
 266         struct prioq_item *i;
 267         unsigned k;
 268
 269         assert(q);
 270
 271         i = find_item(q, data, idx);
 272         if (!i)
 273                 return 0;
 274
 275         k = i - q->items;
 276         k = shuffle_down(q, k);
 277         shuffle_up(q, k);
 278         return 1;
 279 }
 280
 281 void *prioq_peek(Prioq *q) {
 282
 283         if (!q)
 284                 return NULL;
 285
 286         if (q->n_items <= 0)
 287                 return NULL;
 288
 289         return q->items[0].data;
 290 }
 291
 292 void *prioq_pop(Prioq *q) {
 293         void *data;
 294
 295         if (!q)
 296                 return NULL;
 297
 298         if (q->n_items <= 0)
 299                 return NULL;
 300
 301         data = q->items[0].data;
 302         remove_item(q, q->items);
 303         return data;
 304 }
 305
 306 unsigned prioq_size(Prioq *q) {
 307
 308         if (!q)
 309                 return 0;
 310
 311         return q->n_items;
 312 }
 313
 314 bool prioq_isempty(Prioq *q) {
 315
 316         if (!q)
 317                 return true;
 318
 319         return q->n_items <= 0;
 320 }