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199. [bug] isc_heap_delete() sometimes violated the heap

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invariant, causing timer events not to be posted
			when due.

Specifically, isc_heap_delete() moved the last element of the heap to
the vacated position and then attempted to restore the heap invariant
by calling sink_down().  This works when the last element has a lower
priority than the one being deleted, and this is often the case
because the last element tends to have a low priority.  However, it is
not guaranteed to be the lowest.  When deleting an item of a lower
priority than the last item, restoring the invariant requires a call
to float_up(), not sink_down().

isc_heap_delete_now calls either float_up() or sink_down() as needed,
and INSISTs have been added to verify that the heap invariant indeed
holds after the calls.
This commit is contained in:
Andreas Gustafsson
2000-05-22 02:53:52 +00:00
parent be8d4578da
commit 4c94069644
2 changed files with 34 additions and 9 deletions
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39
lib/isc/heap.c
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@@ -46,6 +46,15 @@
#define VALID_HEAP(h) ((h) != NULL && \
(h)->magic == HEAP_MAGIC)
/*
* When the heap is in a consistent state, the following invariant
* holds true: for every element i > 1, heap_parent(i) has a higher
* priority than i.
*/
#define HEAPCONDITION(i) ((i) == 1 || \
heap->compare(heap->array[heap_parent(i)], \
heap->array[(i)]))
struct isc_heap {
unsigned int magic;
isc_mem_t * mctx;
@@ -130,9 +139,9 @@ static void
float_up(isc_heap_t *heap, unsigned int i, void *elt) {
unsigned int p;
for ( p = heap_parent(i);
i > 1 && heap->compare(elt, heap->array[p]);
i = p, p = heap_parent(i) ) {
for (p = heap_parent(i);
i > 1 && heap->compare(elt, heap->array[p]);
i = p, p = heap_parent(i)) {
heap->array[i] = heap->array[p];
if (heap->index != NULL)
(heap->index)(heap->array[i], i);
@@ -140,19 +149,20 @@ float_up(isc_heap_t *heap, unsigned int i, void *elt) {
heap->array[i] = elt;
if (heap->index != NULL)
(heap->index)(heap->array[i], i);
INSIST(HEAPCONDITION(i));
}
static void
sink_down(isc_heap_t *heap, unsigned int i, void *elt) {
unsigned int j, size, half_size;
size = heap->last;
half_size = size / 2;
while (i <= half_size) {
/* find smallest of the (at most) two children */
/* Find the smallest of the (at most) two children. */
j = heap_left(i);
if (j < size && heap->compare(heap->array[j+1],
heap->array[j]))
heap->array[j]))
j++;
if (heap->compare(elt, heap->array[j]))
break;
@@ -164,6 +174,8 @@ sink_down(isc_heap_t *heap, unsigned int i, void *elt) {
heap->array[i] = elt;
if (heap->index != NULL)
(heap->index)(heap->array[i], i);
INSIST(HEAPCONDITION(i));
}
isc_result_t
@@ -184,13 +196,22 @@ isc_heap_insert(isc_heap_t *heap, void *elt) {
void
isc_heap_delete(isc_heap_t *heap, unsigned int i) {
void *elt;
isc_boolean_t less;
REQUIRE(VALID_HEAP(heap));
REQUIRE(i >= 1 && i <= heap->last);
elt = heap->array[heap->last];
if (--heap->last > 0)
sink_down(heap, i, elt);
if (i == heap->last) {
heap->last--;
} else {
elt = heap->array[heap->last--];
less = heap->compare(elt, heap->array[i]);
heap->array[i] = elt;
if (less)
float_up(heap, i, heap->array[i]);
else
sink_down(heap, i, heap->array[i]);
}
}
void