prioq.c revision b5efdb8af40ea759a1ea584c1bc44ecc81dd00ce
/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
/***
This file is part of systemd.
Copyright 2013 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
/*
* Priority Queue
* The prioq object implements a priority queue. That is, it orders objects by
* their priority and allows O(1) access to the object with the highest
* priority. Insertion and removal are Θ(log n). Optionally, the caller can
* provide a pointer to an index which will be kept up-to-date by the prioq.
*
* The underlying algorithm used in this implementation is a Heap.
*/
#include "alloc-util.h"
#include "prioq.h"
#include "util.h"
struct prioq_item {
void *data;
unsigned *idx;
};
struct Prioq {
compare_func_t compare_func;
unsigned n_items, n_allocated;
struct prioq_item *items;
};
Prioq *prioq_new(compare_func_t compare_func) {
Prioq *q;
q = new0(Prioq, 1);
if (!q)
return q;
q->compare_func = compare_func;
return q;
}
Prioq* prioq_free(Prioq *q) {
if (!q)
return NULL;
free(q->items);
free(q);
return NULL;
}
int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {
assert(q);
if (*q)
return 0;
*q = prioq_new(compare_func);
if (!*q)
return -ENOMEM;
return 0;
}
static void swap(Prioq *q, unsigned j, unsigned k) {
void *saved_data;
unsigned *saved_idx;
assert(q);
assert(j < q->n_items);
assert(k < q->n_items);
assert(!q->items[j].idx || *(q->items[j].idx) == j);
assert(!q->items[k].idx || *(q->items[k].idx) == k);
saved_data = q->items[j].data;
saved_idx = q->items[j].idx;
q->items[j].data = q->items[k].data;
q->items[j].idx = q->items[k].idx;
q->items[k].data = saved_data;
q->items[k].idx = saved_idx;
if (q->items[j].idx)
*q->items[j].idx = j;
if (q->items[k].idx)
*q->items[k].idx = k;
}
static unsigned shuffle_up(Prioq *q, unsigned idx) {
assert(q);
while (idx > 0) {
unsigned k;
k = (idx-1)/2;
if (q->compare_func(q->items[k].data, q->items[idx].data) <= 0)
break;
swap(q, idx, k);
idx = k;
}
return idx;
}
static unsigned shuffle_down(Prioq *q, unsigned idx) {
assert(q);
for (;;) {
unsigned j, k, s;
k = (idx+1)*2; /* right child */
j = k-1; /* left child */
if (j >= q->n_items)
break;
if (q->compare_func(q->items[j].data, q->items[idx].data) < 0)
/* So our left child is smaller than we are, let's
* remember this fact */
s = j;
else
s = idx;
if (k < q->n_items &&
q->compare_func(q->items[k].data, q->items[s].data) < 0)
/* So our right child is smaller than we are, let's
* remember this fact */
s = k;
/* s now points to the smallest of the three items */
if (s == idx)
/* No swap necessary, we're done */
break;
swap(q, idx, s);
idx = s;
}
return idx;
}
int prioq_put(Prioq *q, void *data, unsigned *idx) {
struct prioq_item *i;
unsigned k;
assert(q);
if (q->n_items >= q->n_allocated) {
unsigned n;
struct prioq_item *j;
n = MAX((q->n_items+1) * 2, 16u);
j = realloc(q->items, sizeof(struct prioq_item) * n);
if (!j)
return -ENOMEM;
q->items = j;
q->n_allocated = n;
}
k = q->n_items++;
i = q->items + k;
i->data = data;
i->idx = idx;
if (idx)
*idx = k;
shuffle_up(q, k);
return 0;
}
static void remove_item(Prioq *q, struct prioq_item *i) {
struct prioq_item *l;
assert(q);
assert(i);
l = q->items + q->n_items - 1;
if (i == l)
/* Last entry, let's just remove it */
q->n_items--;
else {
unsigned k;
/* Not last entry, let's replace the last entry with
* this one, and reshuffle */
k = i - q->items;
i->data = l->data;
i->idx = l->idx;
if (i->idx)
*i->idx = k;
q->n_items--;
k = shuffle_down(q, k);
shuffle_up(q, k);
}
}
_pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx) {
struct prioq_item *i;
assert(q);
if (idx) {
if (*idx == PRIOQ_IDX_NULL ||
*idx > q->n_items)
return NULL;
i = q->items + *idx;
if (i->data != data)
return NULL;
return i;
} else {
for (i = q->items; i < q->items + q->n_items; i++)
if (i->data == data)
return i;
return NULL;
}
}
int prioq_remove(Prioq *q, void *data, unsigned *idx) {
struct prioq_item *i;
if (!q)
return 0;
i = find_item(q, data, idx);
if (!i)
return 0;
remove_item(q, i);
return 1;
}
int prioq_reshuffle(Prioq *q, void *data, unsigned *idx) {
struct prioq_item *i;
unsigned k;
assert(q);
i = find_item(q, data, idx);
if (!i)
return 0;
k = i - q->items;
k = shuffle_down(q, k);
shuffle_up(q, k);
return 1;
}
void *prioq_peek(Prioq *q) {
if (!q)
return NULL;
if (q->n_items <= 0)
return NULL;
return q->items[0].data;
}
void *prioq_pop(Prioq *q) {
void *data;
if (!q)
return NULL;
if (q->n_items <= 0)
return NULL;
data = q->items[0].data;
remove_item(q, q->items);
return data;
}
unsigned prioq_size(Prioq *q) {
if (!q)
return 0;
return q->n_items;
}
bool prioq_isempty(Prioq *q) {
if (!q)
return true;
return q->n_items <= 0;
}