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 = new(Prioq, 1);
  36         if (!q)
  37                 return q;
  38
  39         *q = (Prioq) {
  40                 .compare_func = compare_func,
  41         };
  42
  43         return q;
  44 }
  45
  46 Prioq* prioq_free(Prioq *q) {
  47         if (!q)
  48                 return NULL;
  49
  50         free(q->items);
  51         return mfree(q);
  52 }
  53
  54 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {
  55         assert(q);
  56
  57         if (*q)
  58                 return 0;
  59
  60         *q = prioq_new(compare_func);
  61         if (!*q)
  62                 return -ENOMEM;
  63
  64         return 0;
  65 }
  66
  67 static void swap(Prioq *q, unsigned j, unsigned k) {
  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         SWAP_TWO(q->items[j].data, q->items[k].data);
  76         SWAP_TWO(q->items[j].idx, q->items[k].idx);
  77
  78         if (q->items[j].idx)
  79                 *q->items[j].idx = j;
  80
  81         if (q->items[k].idx)
  82                 *q->items[k].idx = k;
  83 }
  84
  85 static unsigned shuffle_up(Prioq *q, unsigned idx) {
  86         assert(q);
  87         assert(idx < q->n_items);
  88
  89         while (idx > 0) {
  90                 unsigned k;
  91
  92                 k = (idx-1)/2;
  93
  94                 if (q->compare_func(q->items[k].data, q->items[idx].data) <= 0)
  95                         break;
  96
  97                 swap(q, idx, k);
  98                 idx = k;
  99         }
 100
 101         return idx;
 102 }
 103
 104 static unsigned shuffle_down(Prioq *q, unsigned idx) {
 105         assert(q);
 106
 107         for (;;) {
 108                 unsigned j, k, s;
 109
 110                 k = (idx+1)*2; /* right child */
 111                 j = k-1;       /* left child */
 112
 113                 if (j >= q->n_items)
 114                         break;
 115
 116                 if (q->compare_func(q->items[j].data, q->items[idx].data) < 0)
 117
 118                         /* So our left child is smaller than we are, let's
 119                          * remember this fact */
 120                         s = j;
 121                 else
 122                         s = idx;
 123
 124                 if (k < q->n_items &&
 125                     q->compare_func(q->items[k].data, q->items[s].data) < 0)
 126
 127                         /* So our right child is smaller than we are, let's
 128                          * remember this fact */
 129                         s = k;
 130
 131                 /* s now points to the smallest of the three items */
 132
 133                 if (s == idx)
 134                         /* No swap necessary, we're done */
 135                         break;
 136
 137                 swap(q, idx, s);
 138                 idx = s;
 139         }
 140
 141         return idx;
 142 }
 143
 144 int prioq_put(Prioq *q, void *data, unsigned *idx) {
 145         struct prioq_item *i;
 146         unsigned k;
 147
 148         assert(q);
 149
 150         if (q->n_items >= q->n_allocated) {
 151                 unsigned n;
 152                 struct prioq_item *j;
 153
 154                 n = MAX((q->n_items+1) * 2, 16u);
 155                 j = reallocarray(q->items, n, sizeof(struct prioq_item));
 156                 if (!j)
 157                         return -ENOMEM;
 158
 159                 q->items = j;
 160                 q->n_allocated = n;
 161         }
 162
 163         k = q->n_items++;
 164         i = q->items + k;
 165         i->data = data;
 166         i->idx = idx;
 167
 168         if (idx)
 169                 *idx = k;
 170
 171         shuffle_up(q, k);
 172
 173         return 0;
 174 }
 175
 176 static void remove_item(Prioq *q, struct prioq_item *i) {
 177         struct prioq_item *l;
 178
 179         assert(q);
 180         assert(i);
 181
 182         l = q->items + q->n_items - 1;
 183
 184         if (i == l)
 185                 /* Last entry, let's just remove it */
 186                 q->n_items--;
 187         else {
 188                 unsigned k;
 189
 190                 /* Not last entry, let's replace the last entry with
 191                  * this one, and reshuffle */
 192
 193                 k = i - q->items;
 194
 195                 i->data = l->data;
 196                 i->idx = l->idx;
 197                 if (i->idx)
 198                         *i->idx = k;
 199                 q->n_items--;
 200
 201                 k = shuffle_down(q, k);
 202                 shuffle_up(q, k);
 203         }
 204 }
 205
 206 _pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx) {
 207         struct prioq_item *i;
 208
 209         assert(q);
 210
 211         if (q->n_items <= 0)
 212                 return NULL;
 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_by_index(Prioq *q, unsigned idx) {
 263         if (!q)
 264                 return NULL;
 265
 266         if (idx >= q->n_items)
 267                 return NULL;
 268
 269         return q->items[idx].data;
 270 }
 271
 272 void *prioq_pop(Prioq *q) {
 273         void *data;
 274
 275         if (!q)
 276                 return NULL;
 277
 278         if (q->n_items <= 0)
 279                 return NULL;
 280
 281         data = q->items[0].data;
 282         remove_item(q, q->items);
 283         return data;
 284 }
 285
 286 unsigned prioq_size(Prioq *q) {
 287
 288         if (!q)
 289                 return 0;
 290
 291         return q->n_items;
 292 }
 293
 294 bool prioq_isempty(Prioq *q) {
 295
 296         if (!q)
 297                 return true;
 298
 299         return q->n_items <= 0;
 300 }