1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Package heap provides heap operations for any type that implements 6 // heap.Interface. A heap is a tree with the property that each node is the 7 // minimum-valued node in its subtree. 8 // 9 // The minimum element in the tree is the root, at index 0. 10 // 11 // A heap is a common way to implement a priority queue. To build a priority 12 // queue, implement the Heap interface with the (negative) priority as the 13 // ordering for the Less method, so Push adds items while Pop removes the 14 // highest-priority item from the queue. The Examples include such an 15 // implementation; the file example_pq_test.go has the complete source. 16 // 17 package heap 18 19 import "sort" 20 21 // The Interface type describes the requirements 22 // for a type using the routines in this package. 23 // Any type that implements it may be used as a 24 // min-heap with the following invariants (established after 25 // Init has been called or if the data is empty or sorted): 26 // 27 // !h.Less(j, i) for 0 <= i < h.Len() and 2*i+1 <= j <= 2*i+2 and j < h.Len() 28 // 29 // Note that Push and Pop in this interface are for package heap's 30 // implementation to call. To add and remove things from the heap, 31 // use heap.Push and heap.Pop. 32 type Interface interface { 33 sort.Interface 34 Push(x interface{}) // add x as element Len() 35 Pop() interface{} // remove and return element Len() - 1. 36 } 37 38 // Init establishes the heap invariants required by the other routines in this package. 39 // Init is idempotent with respect to the heap invariants 40 // and may be called whenever the heap invariants may have been invalidated. 41 // The complexity is O(n) where n = h.Len(). 42 func Init(h Interface) { 43 // heapify 44 n := h.Len() 45 for i := n/2 - 1; i >= 0; i-- { 46 down(h, i, n) 47 } 48 } 49 50 // Push pushes the element x onto the heap. 51 // The complexity is O(log n) where n = h.Len(). 52 func Push(h Interface, x interface{}) { 53 h.Push(x) 54 up(h, h.Len()-1) 55 } 56 57 // Pop removes and returns the minimum element (according to Less) from the heap. 58 // The complexity is O(log n) where n = h.Len(). 59 // Pop is equivalent to Remove(h, 0). 60 func Pop(h Interface) interface{} { 61 n := h.Len() - 1 62 h.Swap(0, n) 63 down(h, 0, n) 64 return h.Pop() 65 } 66 67 // Remove removes and returns the element at index i from the heap. 68 // The complexity is O(log n) where n = h.Len(). 69 func Remove(h Interface, i int) interface{} { 70 n := h.Len() - 1 71 if n != i { 72 h.Swap(i, n) 73 if !down(h, i, n) { 74 up(h, i) 75 } 76 } 77 return h.Pop() 78 } 79 80 // Fix re-establishes the heap ordering after the element at index i has changed its value. 81 // Changing the value of the element at index i and then calling Fix is equivalent to, 82 // but less expensive than, calling Remove(h, i) followed by a Push of the new value. 83 // The complexity is O(log n) where n = h.Len(). 84 func Fix(h Interface, i int) { 85 if !down(h, i, h.Len()) { 86 up(h, i) 87 } 88 } 89 90 func up(h Interface, j int) { 91 for { 92 i := (j - 1) / 2 // parent 93 if i == j || !h.Less(j, i) { 94 break 95 } 96 h.Swap(i, j) 97 j = i 98 } 99 } 100 101 func down(h Interface, i0, n int) bool { 102 i := i0 103 for { 104 j1 := 2*i + 1 105 if j1 >= n || j1 < 0 { // j1 < 0 after int overflow 106 break 107 } 108 j := j1 // left child 109 if j2 := j1 + 1; j2 < n && h.Less(j2, j1) { 110 j = j2 // = 2*i + 2 // right child 111 } 112 if !h.Less(j, i) { 113 break 114 } 115 h.Swap(i, j) 116 i = j 117 } 118 return i > i0 119 } 120