From c103d073819a2189d849c0a93d51c726be524c48 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 18 Sep 2023 14:06:08 -0700
Subject: [PATCH] Add function for removing arbitrary nodes in binaryheap.

This commit introduces binaryheap_remove_node(), which can be used
to remove any node from a binary heap.  The implementation is
straightforward.  The target node is replaced with the last node in
the heap, and then we sift as needed to preserve the heap property.
This new function is intended for use in a follow-up commit that
will improve the performance of pg_restore.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/3612876.1689443232%40sss.pgh.pa.us
---
 src/common/binaryheap.c      | 29 +++++++++++++++++++++++++++++
 src/include/lib/binaryheap.h |  3 +++
 2 files changed, 32 insertions(+)

diff --git a/src/common/binaryheap.c b/src/common/binaryheap.c
index 39a8243a6d5..19e095f1fb1 100644
--- a/src/common/binaryheap.c
+++ b/src/common/binaryheap.c
@@ -215,6 +215,35 @@ binaryheap_remove_first(binaryheap *heap)
 	return result;
 }
 
+/*
+ * binaryheap_remove_node
+ *
+ * Removes the nth (zero based) node from the heap.  The caller must ensure
+ * that there are at least (n + 1) nodes in the heap.  O(log n) worst case.
+ */
+void
+binaryheap_remove_node(binaryheap *heap, int n)
+{
+	int			cmp;
+
+	Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
+	Assert(n >= 0 && n < heap->bh_size);
+
+	/* compare last node to the one that is being removed */
+	cmp = heap->bh_compare(heap->bh_nodes[--heap->bh_size],
+						   heap->bh_nodes[n],
+						   heap->bh_arg);
+
+	/* remove the last node, placing it in the vacated entry */
+	heap->bh_nodes[n] = heap->bh_nodes[heap->bh_size];
+
+	/* sift as needed to preserve the heap property */
+	if (cmp > 0)
+		sift_up(heap, n);
+	else if (cmp < 0)
+		sift_down(heap, n);
+}
+
 /*
  * binaryheap_replace_first
  *
diff --git a/src/include/lib/binaryheap.h b/src/include/lib/binaryheap.h
index 3647aeae657..9525dcaec44 100644
--- a/src/include/lib/binaryheap.h
+++ b/src/include/lib/binaryheap.h
@@ -59,8 +59,11 @@ extern void binaryheap_build(binaryheap *heap);
 extern void binaryheap_add(binaryheap *heap, bh_node_type d);
 extern bh_node_type binaryheap_first(binaryheap *heap);
 extern bh_node_type binaryheap_remove_first(binaryheap *heap);
+extern void binaryheap_remove_node(binaryheap *heap, int n);
 extern void binaryheap_replace_first(binaryheap *heap, bh_node_type d);
 
 #define binaryheap_empty(h)			((h)->bh_size == 0)
+#define binaryheap_size(h)			((h)->bh_size)
+#define binaryheap_get_node(h, n)	((h)->bh_nodes[n])
 
 #endif							/* BINARYHEAP_H */