308 lines
6.8 KiB
C
308 lines
6.8 KiB
C
/*-------------------------------------------------------------------------
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*
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* binaryheap.c
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* A simple binary heap implementaion
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*
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* Portions Copyright (c) 2012-2013, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* src/backend/lib/binaryheap.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include <math.h>
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#include "lib/binaryheap.h"
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static void sift_down(binaryheap *heap, int node_off);
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static void sift_up(binaryheap *heap, int node_off);
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static inline void swap_nodes(binaryheap *heap, int a, int b);
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/*
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* binaryheap_allocate
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*
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* Returns a pointer to a newly-allocated heap that has the capacity to
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* store the given number of nodes, with the heap property defined by
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* the given comparator function, which will be invoked with the additional
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* argument specified by 'arg'.
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*/
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binaryheap *
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binaryheap_allocate(int capacity, binaryheap_comparator compare, void *arg)
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{
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int sz;
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binaryheap *heap;
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sz = offsetof(binaryheap, bh_nodes) +sizeof(Datum) * capacity;
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heap = (binaryheap *) palloc(sz);
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heap->bh_space = capacity;
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heap->bh_compare = compare;
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heap->bh_arg = arg;
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heap->bh_size = 0;
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heap->bh_has_heap_property = true;
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return heap;
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}
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/*
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* binaryheap_reset
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*
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* Resets the heap to an empty state, losing its data content but not the
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* parameters passed at allocation.
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*/
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void
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binaryheap_reset(binaryheap *heap)
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{
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heap->bh_size = 0;
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heap->bh_has_heap_property = true;
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}
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/*
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* binaryheap_free
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*
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* Releases memory used by the given binaryheap.
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*/
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void
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binaryheap_free(binaryheap *heap)
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{
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pfree(heap);
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}
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/*
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* These utility functions return the offset of the left child, right
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* child, and parent of the node at the given index, respectively.
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*
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* The heap is represented as an array of nodes, with the root node
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* stored at index 0. The left child of node i is at index 2*i+1, and
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* the right child at 2*i+2. The parent of node i is at index (i-1)/2.
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*/
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static inline int
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left_offset(int i)
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{
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return 2 * i + 1;
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}
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static inline int
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right_offset(int i)
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{
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return 2 * i + 2;
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}
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static inline int
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parent_offset(int i)
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{
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return (i - 1) / 2;
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}
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/*
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* binaryheap_add_unordered
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*
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* Adds the given datum to the end of the heap's list of nodes in O(1) without
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* preserving the heap property. This is a convenience to add elements quickly
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* to a new heap. To obtain a valid heap, one must call binaryheap_build()
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* afterwards.
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*/
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void
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binaryheap_add_unordered(binaryheap *heap, Datum d)
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{
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if (heap->bh_size >= heap->bh_space)
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elog(ERROR, "out of binary heap slots");
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heap->bh_has_heap_property = false;
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heap->bh_nodes[heap->bh_size] = d;
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heap->bh_size++;
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}
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/*
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* binaryheap_build
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*
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* Assembles a valid heap in O(n) from the nodes added by
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* binaryheap_add_unordered(). Not needed otherwise.
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*/
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void
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binaryheap_build(binaryheap *heap)
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{
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int i;
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for (i = parent_offset(heap->bh_size - 1); i >= 0; i--)
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sift_down(heap, i);
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heap->bh_has_heap_property = true;
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}
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/*
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* binaryheap_add
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*
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* Adds the given datum to the heap in O(log n) time, while preserving
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* the heap property.
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*/
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void
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binaryheap_add(binaryheap *heap, Datum d)
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{
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if (heap->bh_size >= heap->bh_space)
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elog(ERROR, "out of binary heap slots");
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heap->bh_nodes[heap->bh_size] = d;
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heap->bh_size++;
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sift_up(heap, heap->bh_size - 1);
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}
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/*
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* binaryheap_first
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*
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* Returns a pointer to the first (root, topmost) node in the heap
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* without modifying the heap. The caller must ensure that this
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* routine is not used on an empty heap. Always O(1).
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*/
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Datum
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binaryheap_first(binaryheap *heap)
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{
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Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
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return heap->bh_nodes[0];
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}
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/*
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* binaryheap_remove_first
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*
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* Removes the first (root, topmost) node in the heap and returns a
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* pointer to it after rebalancing the heap. The caller must ensure
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* that this routine is not used on an empty heap. O(log n) worst
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* case.
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*/
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Datum
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binaryheap_remove_first(binaryheap *heap)
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{
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Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
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if (heap->bh_size == 1)
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{
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heap->bh_size--;
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return heap->bh_nodes[0];
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}
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/*
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* Swap the root and last nodes, decrease the size of the heap (i.e.
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* remove the former root node) and sift the new root node down to its
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* correct position.
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*/
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swap_nodes(heap, 0, heap->bh_size - 1);
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heap->bh_size--;
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sift_down(heap, 0);
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return heap->bh_nodes[heap->bh_size];
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}
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/*
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* binaryheap_replace_first
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*
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* Replace the topmost element of a non-empty heap, preserving the heap
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* property. O(1) in the best case, or O(log n) if it must fall back to
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* sifting the new node down.
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*/
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void
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binaryheap_replace_first(binaryheap *heap, Datum d)
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{
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Assert(!binaryheap_empty(heap) && heap->bh_has_heap_property);
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heap->bh_nodes[0] = d;
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if (heap->bh_size > 1)
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sift_down(heap, 0);
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}
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/*
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* Swap the contents of two nodes.
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*/
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static inline void
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swap_nodes(binaryheap *heap, int a, int b)
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{
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Datum swap;
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swap = heap->bh_nodes[a];
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heap->bh_nodes[a] = heap->bh_nodes[b];
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heap->bh_nodes[b] = swap;
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}
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/*
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* Sift a node up to the highest position it can hold according to the
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* comparator.
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*/
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static void
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sift_up(binaryheap *heap, int node_off)
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{
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while (node_off != 0)
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{
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int cmp;
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int parent_off;
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/*
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* If this node is smaller than its parent, the heap condition is
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* satisfied, and we're done.
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*/
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parent_off = parent_offset(node_off);
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cmp = heap->bh_compare(heap->bh_nodes[node_off],
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heap->bh_nodes[parent_off],
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heap->bh_arg);
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if (cmp <= 0)
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break;
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/*
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* Otherwise, swap the node and its parent and go on to check the
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* node's new parent.
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*/
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swap_nodes(heap, node_off, parent_off);
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node_off = parent_off;
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}
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}
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/*
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* Sift a node down from its current position to satisfy the heap
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* property.
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*/
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static void
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sift_down(binaryheap *heap, int node_off)
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{
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while (true)
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{
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int left_off = left_offset(node_off);
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int right_off = right_offset(node_off);
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int swap_off = 0;
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/* Is the left child larger than the parent? */
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if (left_off < heap->bh_size &&
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heap->bh_compare(heap->bh_nodes[node_off],
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heap->bh_nodes[left_off],
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heap->bh_arg) < 0)
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swap_off = left_off;
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/* Is the right child larger than the parent? */
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if (right_off < heap->bh_size &&
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heap->bh_compare(heap->bh_nodes[node_off],
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heap->bh_nodes[right_off],
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heap->bh_arg) < 0)
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{
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/* swap with the larger child */
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if (!swap_off ||
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heap->bh_compare(heap->bh_nodes[left_off],
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heap->bh_nodes[right_off],
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heap->bh_arg) < 0)
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swap_off = right_off;
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}
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/*
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* If we didn't find anything to swap, the heap condition is
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* satisfied, and we're done.
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*/
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if (!swap_off)
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break;
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/*
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* Otherwise, swap the node with the child that violates the heap
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* property; then go on to check its children.
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*/
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swap_nodes(heap, swap_off, node_off);
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node_off = swap_off;
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}
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}
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