src/basic/prioq.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 /*
   4  * Priority Queue
   5  * The prioq object implements a priority queue. That is, it orders objects by
   6  * their priority and allows O(1) access to the object with the highest
   7  * priority. Insertion and removal are Θ(log n). Optionally, the caller can
   8  * provide a pointer to an index which will be kept up-to-date by the prioq.
   9  *
  10  * The underlying algorithm used in this implementation is a Heap.
  11  */
  12
  13 #include <errno.h>
  14 #include <stdlib.h>
  15
  16 #include "alloc-util.h"
  17 #include "hashmap.h"
  18 #include "prioq.h"
  19
  20 struct prioq_item {
  21         void *data;
  22         unsigned *idx;
  23 };
  24
  25 struct Prioq {
  26         compare_func_t compare_func;
  27         unsigned n_items, n_allocated;
  28
  29         struct prioq_item *items;
  30 };
  31
  32 Prioq *prioq_new(compare_func_t compare_func) {
  33         Prioq *q;
  34
  35         q = new0(Prioq, 1);
  36         if (!q)
  37                 return q;
  38
  39         q->compare_func = compare_func;
  40         return q;
  41 }
  42
  43 Prioq* prioq_free(Prioq *q) {
  44         if (!q)
  45                 return NULL;
  46
  47         free(q->items);
  48         return mfree(q);
  49 }
  50
  51 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {
  52         assert(q);
  53
  54         if (*q)
  55                 return 0;
  56
  57         *q = prioq_new(compare_func);
  58         if (!*q)
  59                 return -ENOMEM;
  60
  61         return 0;
  62 }
  63
  64 static void swap(Prioq *q, unsigned j, unsigned k) {
  65         void *saved_data;
  66         unsigned *saved_idx;
  67
  68         assert(q);
  69         assert(j < q->n_items);
  70         assert(k < q->n_items);
  71
  72         assert(!q->items[j].idx || *(q->items[j].idx) == j);
  73         assert(!q->items[k].idx || *(q->items[k].idx) == k);
  74
  75         saved_data = q->items[j].data;
  76         saved_idx = q->items[j].idx;
  77         q->items[j].data = q->items[k].data;
  78         q->items[j].idx = q->items[k].idx;
  79         q->items[k].data = saved_data;
  80         q->items[k].idx = saved_idx;
  81
  82         if (q->items[j].idx)
  83                 *q->items[j].idx = j;
  84
  85         if (q->items[k].idx)
  86                 *q->items[k].idx = k;
  87 }
  88
  89 static unsigned shuffle_up(Prioq *q, unsigned idx) {
  90         assert(q);
  91
  92         while (idx > 0) {
  93                 unsigned k;
  94
  95                 k = (idx-1)/2;
  96
  97                 if (q->compare_func(q->items[k].data, q->items[idx].data) <= 0)
  98                         break;
  99
 100                 swap(q, idx, k);
 101                 idx = k;
 102         }
 103
 104         return idx;
 105 }
 106
 107 static unsigned shuffle_down(Prioq *q, unsigned idx) {
 108         assert(q);
 109
 110         for (;;) {
 111                 unsigned j, k, s;
 112
 113                 k = (idx+1)*2; /* right child */
 114                 j = k-1;       /* left child */
 115
 116                 if (j >= q->n_items)
 117                         break;
 118
 119                 if (q->compare_func(q->items[j].data, q->items[idx].data) < 0)
 120
 121                         /* So our left child is smaller than we are, let's
 122                          * remember this fact */
 123                         s = j;
 124                 else
 125                         s = idx;
 126
 127                 if (k < q->n_items &&
 128                     q->compare_func(q->items[k].data, q->items[s].data) < 0)
 129
 130                         /* So our right child is smaller than we are, let's
 131                          * remember this fact */
 132                         s = k;
 133
 134                 /* s now points to the smallest of the three items */
 135
 136                 if (s == idx)
 137                         /* No swap necessary, we're done */
 138                         break;
 139
 140                 swap(q, idx, s);
 141                 idx = s;
 142         }
 143
 144         return idx;
 145 }
 146
 147 int prioq_put(Prioq *q, void *data, unsigned *idx) {
 148         struct prioq_item *i;
 149         unsigned k;
 150
 151         assert(q);
 152
 153         if (q->n_items >= q->n_allocated) {
 154                 unsigned n;
 155                 struct prioq_item *j;
 156
 157                 n = MAX((q->n_items+1) * 2, 16u);
 158                 j = reallocarray(q->items, n, sizeof(struct prioq_item));
 159                 if (!j)
 160                         return -ENOMEM;
 161
 162                 q->items = j;
 163                 q->n_allocated = n;
 164         }
 165
 166         k = q->n_items++;
 167         i = q->items + k;
 168         i->data = data;
 169         i->idx = idx;
 170
 171         if (idx)
 172                 *idx = k;
 173
 174         shuffle_up(q, k);
 175
 176         return 0;
 177 }
 178
 179 static void remove_item(Prioq *q, struct prioq_item *i) {
 180         struct prioq_item *l;
 181
 182         assert(q);
 183         assert(i);
 184
 185         l = q->items + q->n_items - 1;
 186
 187         if (i == l)
 188                 /* Last entry, let's just remove it */
 189                 q->n_items--;
 190         else {
 191                 unsigned k;
 192
 193                 /* Not last entry, let's replace the last entry with
 194                  * this one, and reshuffle */
 195
 196                 k = i - q->items;
 197
 198                 i->data = l->data;
 199                 i->idx = l->idx;
 200                 if (i->idx)
 201                         *i->idx = k;
 202                 q->n_items--;
 203
 204                 k = shuffle_down(q, k);
 205                 shuffle_up(q, k);
 206         }
 207 }
 208
 209 _pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx) {
 210         struct prioq_item *i;
 211
 212         assert(q);
 213
 214         if (idx) {
 215                 if (*idx == PRIOQ_IDX_NULL ||
 216                     *idx > q->n_items)
 217                         return NULL;
 218
 219                 i = q->items + *idx;
 220                 if (i->data != data)
 221                         return NULL;
 222
 223                 return i;
 224         } else {
 225                 for (i = q->items; i < q->items + q->n_items; i++)
 226                         if (i->data == data)
 227                                 return i;
 228                 return NULL;
 229         }
 230 }
 231
 232 int prioq_remove(Prioq *q, void *data, unsigned *idx) {
 233         struct prioq_item *i;
 234
 235         if (!q)
 236                 return 0;
 237
 238         i = find_item(q, data, idx);
 239         if (!i)
 240                 return 0;
 241
 242         remove_item(q, i);
 243         return 1;
 244 }
 245
 246 int prioq_reshuffle(Prioq *q, void *data, unsigned *idx) {
 247         struct prioq_item *i;
 248         unsigned k;
 249
 250         assert(q);
 251
 252         i = find_item(q, data, idx);
 253         if (!i)
 254                 return 0;
 255
 256         k = i - q->items;
 257         k = shuffle_down(q, k);
 258         shuffle_up(q, k);
 259         return 1;
 260 }
 261
 262 void *prioq_peek(Prioq *q) {
 263
 264         if (!q)
 265                 return NULL;
 266
 267         if (q->n_items <= 0)
 268                 return NULL;
 269
 270         return q->items[0].data;
 271 }
 272
 273 void *prioq_pop(Prioq *q) {
 274         void *data;
 275
 276         if (!q)
 277                 return NULL;
 278
 279         if (q->n_items <= 0)
 280                 return NULL;
 281
 282         data = q->items[0].data;
 283         remove_item(q, q->items);
 284         return data;
 285 }
 286
 287 unsigned prioq_size(Prioq *q) {
 288
 289         if (!q)
 290                 return 0;
 291
 292         return q->n_items;
 293 }
 294
 295 bool prioq_isempty(Prioq *q) {
 296
 297         if (!q)
 298                 return true;
 299
 300         return q->n_items <= 0;
 301 }