Imported Upstream version 1.4upstream/1.4

1 files changed, 117 insertions, 0 deletions

diff --git a/src/container/heap/heap.go b/src/container/heap/heap.go
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+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package heap provides heap operations for any type that implements
+// heap.Interface. A heap is a tree with the property that each node is the
+// minimum-valued node in its subtree.
+//
+// The minimum element in the tree is the root, at index 0.
+//
+// A heap is a common way to implement a priority queue. To build a priority
+// queue, implement the Heap interface with the (negative) priority as the
+// ordering for the Less method, so Push adds items while Pop removes the
+// highest-priority item from the queue. The Examples include such an
+// implementation; the file example_pq_test.go has the complete source.
+//
+package heap
+
+import "sort"
+
+// Any type that implements heap.Interface may be used as a
+// min-heap with the following invariants (established after
+// Init has been called or if the data is empty or sorted):
+//
+//	!h.Less(j, i) for 0 <= i < h.Len() and 2*i+1 <= j <= 2*i+2 and j < h.Len()
+//
+// Note that Push and Pop in this interface are for package heap's
+// implementation to call.  To add and remove things from the heap,
+// use heap.Push and heap.Pop.
+type Interface interface {
+	sort.Interface
+	Push(x interface{}) // add x as element Len()
+	Pop() interface{}   // remove and return element Len() - 1.
+}
+
+// A heap must be initialized before any of the heap operations
+// can be used. Init is idempotent with respect to the heap invariants
+// and may be called whenever the heap invariants may have been invalidated.
+// Its complexity is O(n) where n = h.Len().
+//
+func Init(h Interface) {
+	// heapify
+	n := h.Len()
+	for i := n/2 - 1; i >= 0; i-- {
+		down(h, i, n)
+	}
+}
+
+// Push pushes the element x onto the heap. The complexity is
+// O(log(n)) where n = h.Len().
+//
+func Push(h Interface, x interface{}) {
+	h.Push(x)
+	up(h, h.Len()-1)
+}
+
+// Pop removes the minimum element (according to Less) from the heap
+// and returns it. The complexity is O(log(n)) where n = h.Len().
+// It is equivalent to Remove(h, 0).
+//
+func Pop(h Interface) interface{} {
+	n := h.Len() - 1
+	h.Swap(0, n)
+	down(h, 0, n)
+	return h.Pop()
+}
+
+// Remove removes the element at index i from the heap.
+// The complexity is O(log(n)) where n = h.Len().
+//
+func Remove(h Interface, i int) interface{} {
+	n := h.Len() - 1
+	if n != i {
+		h.Swap(i, n)
+		down(h, i, n)
+		up(h, i)
+	}
+	return h.Pop()
+}
+
+// Fix re-establishes the heap ordering after the element at index i has changed its value.
+// Changing the value of the element at index i and then calling Fix is equivalent to,
+// but less expensive than, calling Remove(h, i) followed by a Push of the new value.
+// The complexity is O(log(n)) where n = h.Len().
+func Fix(h Interface, i int) {
+	down(h, i, h.Len())
+	up(h, i)
+}
+
+func up(h Interface, j int) {
+	for {
+		i := (j - 1) / 2 // parent
+		if i == j || !h.Less(j, i) {
+			break
+		}
+		h.Swap(i, j)
+		j = i
+	}
+}
+
+func down(h Interface, i, n int) {
+	for {
+		j1 := 2*i + 1
+		if j1 >= n || j1 < 0 { // j1 < 0 after int overflow
+			break
+		}
+		j := j1 // left child
+		if j2 := j1 + 1; j2 < n && !h.Less(j1, j2) {
+			j = j2 // = 2*i + 2  // right child
+		}
+		if !h.Less(j, i) {
+			break
+		}
+		h.Swap(i, j)
+		i = j
+	}
+}