From c47a3ddf4ac1bdaf74080454e3993b0cb2a97d66 Mon Sep 17 00:00:00 2001 From: Damien George Date: Fri, 13 Mar 2020 14:35:03 +1100 Subject: [PATCH] py/pairheap: Properly unlink node on pop and delete. This fixes a bug in the pairing-heap implementation when nodes are deleted with mp_pairheap_delete and then reinserted later on. --- py/pairheap.c | 14 +++++++++++--- py/pairheap.h | 9 ++++++--- 2 files changed, 17 insertions(+), 6 deletions(-) diff --git a/py/pairheap.c b/py/pairheap.c index a1eececa0f..d3a011c4ae 100644 --- a/py/pairheap.c +++ b/py/pairheap.c @@ -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)) { mp_pairheap_t *n1 = child; child = child->next; + n1->next = NULL; if (!NEXT_IS_RIGHTMOST_PARENT(child)) { mp_pairheap_t *n2 = child; child = child->next; + n2->next = NULL; n1 = mp_pairheap_meld(lt, n1, n2); } 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) { // Simple case of the top being the node to delete 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 @@ -113,18 +117,22 @@ mp_pairheap_t *mp_pairheap_delete(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_p node->next = NULL; return heap; } else if (node == parent->child) { + mp_pairheap_t *child = node->child; next = node->next; + node->child = NULL; node->next = NULL; - node = mp_pairheap_pairing(lt, node->child); + node = mp_pairheap_pairing(lt, child); parent->child = node; } else { mp_pairheap_t *n = parent->child; while (node != n->next) { n = n->next; } + mp_pairheap_t *child = node->child; next = node->next; + node->child = NULL; node->next = NULL; - node = mp_pairheap_pairing(lt, node->child); + node = mp_pairheap_pairing(lt, child); if (node == NULL) { node = n; } else { diff --git a/py/pairheap.h b/py/pairheap.h index 8a9138b178..16ae788097 100644 --- a/py/pairheap.h +++ b/py/pairheap.h @@ -35,6 +35,7 @@ // Algorithmica 1:111-129, 1986. // https://www.cs.cmu.edu/~sleator/papers/pairing-heaps.pdf +#include #include "py/obj.h" // 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. static inline mp_pairheap_t *mp_pairheap_push(mp_pairheap_lt_t lt, mp_pairheap_t *heap, mp_pairheap_t *node) { - node->child = NULL; - node->next = NULL; + assert(node->child == NULL && node->next == NULL); 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. 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