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py/pairheap: Properly unlink node on pop and delete.

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This fixes a bug in the pairing-heap implementation when nodes are deleted
with mp_pairheap_delete and then reinserted later on.
This commit is contained in:
Damien George 2020-03-13 14:35:03 +11:00
parent 98ab7643a7
commit c47a3ddf4a
2 changed files with 17 additions and 6 deletions
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14
py/pairheap.c
View File

@ -72,9 +72,11 @@ mp_pairheap_t *mp_pairheap_pairing(mp_pairheap_lt_t lt, mp_pairheap_t *child) {
while (!NEXT_IS_RIGHTMOST_PARENT(child)) { while (!NEXT_IS_RIGHTMOST_PARENT(child)) {
mp_pairheap_t *n1 = child; mp_pairheap_t *n1 = child;
child = child->next; child = child->next;
n1->next = NULL;
if (!NEXT_IS_RIGHTMOST_PARENT(child)) { if (!NEXT_IS_RIGHTMOST_PARENT(child)) {
mp_pairheap_t *n2 = child; mp_pairheap_t *n2 = child;
child = child->next; child = child->next;
n2->next = NULL;
n1 = mp_pairheap_meld(lt, n1, n2); n1 = mp_pairheap_meld(lt, n1, n2);
} }
heap = mp_pairheap_meld(lt, heap, n1); heap = mp_pairheap_meld(lt, heap, n1);
@ -87,7 +89,9 @@ mp_pairheap_t *mp_pairheap_pairing(mp_pairheap_lt_t lt, mp_pairheap_t *child) {
mp_pairheap_t *mp_pairheap_delete(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_pairheap_t *node) { mp_pairheap_t *mp_pairheap_delete(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_pairheap_t *node) {
// Simple case of the top being the node to delete // Simple case of the top being the node to delete
if (node == heap) { if (node == heap) {
return mp_pairheap_pairing(lt, heap->child); mp_pairheap_t *child = heap->child;
node->child = NULL;
return mp_pairheap_pairing(lt, child);
} }
// Case where node is not in the heap // Case where node is not in the heap
@ -113,18 +117,22 @@ mp_pairheap_t *mp_pairheap_delete(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_p
node->next = NULL; node->next = NULL;
return heap; return heap;
} else if (node == parent->child) { } else if (node == parent->child) {
mp_pairheap_t *child = node->child;
next = node->next; next = node->next;
node->child = NULL;
node->next = NULL; node->next = NULL;
node = mp_pairheap_pairing(lt, node->child); node = mp_pairheap_pairing(lt, child);
parent->child = node; parent->child = node;
} else { } else {
mp_pairheap_t *n = parent->child; mp_pairheap_t *n = parent->child;
while (node != n->next) { while (node != n->next) {
n = n->next; n = n->next;
} }
mp_pairheap_t *child = node->child;
next = node->next; next = node->next;
node->child = NULL;
node->next = NULL; node->next = NULL;
node = mp_pairheap_pairing(lt, node->child); node = mp_pairheap_pairing(lt, child);
if (node == NULL) { if (node == NULL) {
node = n; node = n;
} else { } else {

9
py/pairheap.h
View File

@ -35,6 +35,7 @@
// Algorithmica 1:111-129, 1986. // Algorithmica 1:111-129, 1986.
// https://www.cs.cmu.edu/~sleator/papers/pairing-heaps.pdf // https://www.cs.cmu.edu/~sleator/papers/pairing-heaps.pdf
#include <assert.h>
#include "py/obj.h" #include "py/obj.h"
// This struct forms the nodes of the heap and is intended to be extended, by // This struct forms the nodes of the heap and is intended to be extended, by
@ -77,14 +78,16 @@ static inline mp_pairheap_t *mp_pairheap_peek(mp_pairheap_lt_t lt, mp_pairheap_t
// Push new node onto existing heap. Returns the new heap. // Push new node onto existing heap. Returns the new heap.
static inline mp_pairheap_t *mp_pairheap_push(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_pairheap_t *node) { static inline mp_pairheap_t *mp_pairheap_push(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_pairheap_t *node) {
node->child = NULL; assert(node->child == NULL && node->next == NULL);
node->next = NULL;
return mp_pairheap_meld(lt, node, heap); // node is first to be stable return mp_pairheap_meld(lt, node, heap); // node is first to be stable
} }
// Pop the top off the heap, which must not be empty. Returns the new heap. // Pop the top off the heap, which must not be empty. Returns the new heap.
static inline mp_pairheap_t *mp_pairheap_pop(mp_pairheap_lt_t lt, mp_pairheap_t *heap) { static inline mp_pairheap_t *mp_pairheap_pop(mp_pairheap_lt_t lt, mp_pairheap_t *heap) {
return mp_pairheap_pairing(lt, heap->child); assert(heap->next == NULL);
mp_pairheap_t *child = heap->child;
heap->child = NULL;
return mp_pairheap_pairing(lt, child);
} }
#endif // MICROPY_INCLUDED_PY_PAIRHEAP_H #endif // MICROPY_INCLUDED_PY_PAIRHEAP_H