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diff --git a/lib/isc/heap.c b/lib/isc/heap.c
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+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+/*! \file
+ * Heap implementation of priority queues adapted from the following:
+ *
+ *	\li "Introduction to Algorithms," Cormen, Leiserson, and Rivest,
+ *	MIT Press / McGraw Hill, 1990, ISBN 0-262-03141-8, chapter 7.
+ *
+ *	\li "Algorithms," Second Edition, Sedgewick, Addison-Wesley, 1988,
+ *	ISBN 0-201-06673-4, chapter 11.
+ */
+
+#include <stdbool.h>
+
+#include <isc/heap.h>
+#include <isc/magic.h>
+#include <isc/mem.h>
+#include <isc/string.h> /* Required for memmove. */
+#include <isc/util.h>
+
+/*@{*/
+/*%
+ * Note: to make heap_parent and heap_left easy to compute, the first
+ * element of the heap array is not used; i.e. heap subscripts are 1-based,
+ * not 0-based.  The parent is index/2, and the left-child is index*2.
+ * The right child is index*2+1.
+ */
+#define heap_parent(i) ((i) >> 1)
+#define heap_left(i)   ((i) << 1)
+/*@}*/
+
+#define SIZE_INCREMENT 1024
+
+#define HEAP_MAGIC    ISC_MAGIC('H', 'E', 'A', 'P')
+#define VALID_HEAP(h) ISC_MAGIC_VALID(h, 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 priority
+ * higher than or equal to that of i.
+ */
+#define HEAPCONDITION(i) \
+	((i) == 1 ||     \
+	 !heap->compare(heap->array[(i)], heap->array[heap_parent(i)]))
+
+/*% ISC heap structure. */
+struct isc_heap {
+	unsigned int magic;
+	isc_mem_t *mctx;
+	unsigned int size;
+	unsigned int size_increment;
+	unsigned int last;
+	void **array;
+	isc_heapcompare_t compare;
+	isc_heapindex_t index;
+};
+
+#ifdef ISC_HEAP_CHECK
+static void
+heap_check(isc_heap_t *heap) {
+	unsigned int i;
+	for (i = 1; i <= heap->last; i++) {
+		INSIST(HEAPCONDITION(i));
+	}
+}
+#else /* ifdef ISC_HEAP_CHECK */
+#define heap_check(x) (void)0
+#endif /* ifdef ISC_HEAP_CHECK */
+
+void
+isc_heap_create(isc_mem_t *mctx, isc_heapcompare_t compare, isc_heapindex_t idx,
+		unsigned int size_increment, isc_heap_t **heapp) {
+	isc_heap_t *heap;
+
+	REQUIRE(heapp != NULL && *heapp == NULL);
+	REQUIRE(compare != NULL);
+
+	heap = isc_mem_get(mctx, sizeof(*heap));
+	heap->magic = HEAP_MAGIC;
+	heap->size = 0;
+	heap->mctx = NULL;
+	isc_mem_attach(mctx, &heap->mctx);
+	if (size_increment == 0) {
+		heap->size_increment = SIZE_INCREMENT;
+	} else {
+		heap->size_increment = size_increment;
+	}
+	heap->last = 0;
+	heap->array = NULL;
+	heap->compare = compare;
+	heap->index = idx;
+
+	*heapp = heap;
+}
+
+void
+isc_heap_destroy(isc_heap_t **heapp) {
+	isc_heap_t *heap;
+
+	REQUIRE(heapp != NULL);
+	heap = *heapp;
+	*heapp = NULL;
+	REQUIRE(VALID_HEAP(heap));
+
+	if (heap->array != NULL) {
+		isc_mem_put(heap->mctx, heap->array,
+			    heap->size * sizeof(void *));
+	}
+	heap->magic = 0;
+	isc_mem_putanddetach(&heap->mctx, heap, sizeof(*heap));
+}
+
+static void
+resize(isc_heap_t *heap) {
+	void **new_array;
+	unsigned int new_size;
+
+	REQUIRE(VALID_HEAP(heap));
+
+	new_size = heap->size + heap->size_increment;
+	new_array = isc_mem_get(heap->mctx, new_size * sizeof(void *));
+	if (heap->array != NULL) {
+		memmove(new_array, heap->array, heap->size * sizeof(void *));
+		isc_mem_put(heap->mctx, heap->array,
+			    heap->size * sizeof(void *));
+	}
+	heap->size = new_size;
+	heap->array = new_array;
+}
+
+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))
+	{
+		heap->array[i] = heap->array[p];
+		if (heap->index != NULL) {
+			(heap->index)(heap->array[i], i);
+		}
+	}
+	heap->array[i] = elt;
+	if (heap->index != NULL) {
+		(heap->index)(heap->array[i], i);
+	}
+
+	INSIST(HEAPCONDITION(i));
+	heap_check(heap);
+}
+
+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 the smallest of the (at most) two children. */
+		j = heap_left(i);
+		if (j < size &&
+		    heap->compare(heap->array[j + 1], heap->array[j]))
+		{
+			j++;
+		}
+		if (heap->compare(elt, heap->array[j])) {
+			break;
+		}
+		heap->array[i] = heap->array[j];
+		if (heap->index != NULL) {
+			(heap->index)(heap->array[i], i);
+		}
+		i = j;
+	}
+	heap->array[i] = elt;
+	if (heap->index != NULL) {
+		(heap->index)(heap->array[i], i);
+	}
+
+	INSIST(HEAPCONDITION(i));
+	heap_check(heap);
+}
+
+void
+isc_heap_insert(isc_heap_t *heap, void *elt) {
+	unsigned int new_last;
+
+	REQUIRE(VALID_HEAP(heap));
+
+	heap_check(heap);
+	new_last = heap->last + 1;
+	RUNTIME_CHECK(new_last > 0); /* overflow check */
+	if (new_last >= heap->size) {
+		resize(heap);
+	}
+	heap->last = new_last;
+
+	float_up(heap, new_last, elt);
+}
+
+void
+isc_heap_delete(isc_heap_t *heap, unsigned int idx) {
+	void *elt;
+	bool less;
+
+	REQUIRE(VALID_HEAP(heap));
+	REQUIRE(idx >= 1 && idx <= heap->last);
+
+	heap_check(heap);
+	if (heap->index != NULL) {
+		(heap->index)(heap->array[idx], 0);
+	}
+	if (idx == heap->last) {
+		heap->array[heap->last] = NULL;
+		heap->last--;
+		heap_check(heap);
+	} else {
+		elt = heap->array[heap->last];
+		heap->array[heap->last] = NULL;
+		heap->last--;
+
+		less = heap->compare(elt, heap->array[idx]);
+		heap->array[idx] = elt;
+		if (less) {
+			float_up(heap, idx, heap->array[idx]);
+		} else {
+			sink_down(heap, idx, heap->array[idx]);
+		}
+	}
+}
+
+void
+isc_heap_increased(isc_heap_t *heap, unsigned int idx) {
+	REQUIRE(VALID_HEAP(heap));
+	REQUIRE(idx >= 1 && idx <= heap->last);
+
+	float_up(heap, idx, heap->array[idx]);
+}
+
+void
+isc_heap_decreased(isc_heap_t *heap, unsigned int idx) {
+	REQUIRE(VALID_HEAP(heap));
+	REQUIRE(idx >= 1 && idx <= heap->last);
+
+	sink_down(heap, idx, heap->array[idx]);
+}
+
+void *
+isc_heap_element(isc_heap_t *heap, unsigned int idx) {
+	REQUIRE(VALID_HEAP(heap));
+	REQUIRE(idx >= 1);
+
+	heap_check(heap);
+	if (idx <= heap->last) {
+		return (heap->array[idx]);
+	}
+	return (NULL);
+}
+
+void
+isc_heap_foreach(isc_heap_t *heap, isc_heapaction_t action, void *uap) {
+	unsigned int i;
+
+	REQUIRE(VALID_HEAP(heap));
+	REQUIRE(action != NULL);
+
+	for (i = 1; i <= heap->last; i++) {
+		(action)(heap->array[i], uap);
+	}
+}