tempfork/heap: add copy of Go's container/heap but using generics

From Go commit 0a48e5cbfabd679e, then with some generics sprinkled
about.

Updates tailscale/corp#7354

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tempfork/heap/heap.go

@@ -0,0 +1,121 @@
+// 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.
+//
+// This package is a copy of the Go standard library's
+// container/heap, but using generics.
+package heap
+
+import "sort"
+
+// The Interface type describes the requirements
+// for a type using the routines in this package.
+// Any type that implements it 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[V any] interface {
+	sort.Interface
+	Push(x V) // add x as element Len()
+	Pop() V   // remove and return element Len() - 1.
+}
+
+// Init establishes the heap invariants required by the other routines in this package.
+// Init is idempotent with respect to the heap invariants
+// and may be called whenever the heap invariants may have been invalidated.
+// The complexity is O(n) where n = h.Len().
+func Init[V any](h Interface[V]) {
+	// 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[V any](h Interface[V], x V) {
+	h.Push(x)
+	up(h, h.Len()-1)
+}
+
+// Pop removes and returns the minimum element (according to Less) from the heap.
+// The complexity is O(log n) where n = h.Len().
+// Pop is equivalent to Remove(h, 0).
+func Pop[V any](h Interface[V]) V {
+	n := h.Len() - 1
+	h.Swap(0, n)
+	down(h, 0, n)
+	return h.Pop()
+}
+
+// Remove removes and returns the element at index i from the heap.
+// The complexity is O(log n) where n = h.Len().
+func Remove[V any](h Interface[V], i int) V {
+	n := h.Len() - 1
+	if n != i {
+		h.Swap(i, n)
+		if !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[V any](h Interface[V], i int) {
+	if !down(h, i, h.Len()) {
+		up(h, i)
+	}
+}
+
+func up[V any](h Interface[V], j int) {
+	for {
+		i := (j - 1) / 2 // parent
+		if i == j || !h.Less(j, i) {
+			break
+		}
+		h.Swap(i, j)
+		j = i
+	}
+}
+
+func down[V any](h Interface[V], i0, n int) bool {
+	i := i0
+	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(j2, j1) {
+			j = j2 // = 2*i + 2  // right child
+		}
+		if !h.Less(j, i) {
+			break
+		}
+		h.Swap(i, j)
+		i = j
+	}
+	return i > i0
+}

tempfork/heap/heap_test.go

@@ -0,0 +1,216 @@
+// 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
+
+import (
+	"math/rand"
+	"testing"
+
+	"golang.org/x/exp/constraints"
+)
+
+type myHeap[T constraints.Ordered] []T
+
+func (h *myHeap[T]) Less(i, j int) bool {
+	return (*h)[i] < (*h)[j]
+}
+
+func (h *myHeap[T]) Swap(i, j int) {
+	(*h)[i], (*h)[j] = (*h)[j], (*h)[i]
+}
+
+func (h *myHeap[T]) Len() int {
+	return len(*h)
+}
+
+func (h *myHeap[T]) Pop() (v T) {
+	*h, v = (*h)[:h.Len()-1], (*h)[h.Len()-1]
+	return
+}
+
+func (h *myHeap[T]) Push(v T) {
+	*h = append(*h, v)
+}
+
+func (h myHeap[T]) verify(t *testing.T, i int) {
+	t.Helper()
+	n := h.Len()
+	j1 := 2*i + 1
+	j2 := 2*i + 2
+	if j1 < n {
+		if h.Less(j1, i) {
+			t.Errorf("heap invariant invalidated [%d] = %v > [%d] = %v", i, h[i], j1, h[j1])
+			return
+		}
+		h.verify(t, j1)
+	}
+	if j2 < n {
+		if h.Less(j2, i) {
+			t.Errorf("heap invariant invalidated [%d] = %v > [%d] = %v", i, h[i], j1, h[j2])
+			return
+		}
+		h.verify(t, j2)
+	}
+}
+
+func TestInit0(t *testing.T) {
+	h := new(myHeap[int])
+	for i := 20; i > 0; i-- {
+		h.Push(0) // all elements are the same
+	}
+	Init[int](h)
+	h.verify(t, 0)
+
+	for i := 1; h.Len() > 0; i++ {
+		x := Pop[int](h)
+		h.verify(t, 0)
+		if x != 0 {
+			t.Errorf("%d.th pop got %d; want %d", i, x, 0)
+		}
+	}
+}
+
+func TestInit1(t *testing.T) {
+	h := new(myHeap[int])
+	for i := 20; i > 0; i-- {
+		h.Push(i) // all elements are different
+	}
+	Init[int](h)
+	h.verify(t, 0)
+
+	for i := 1; h.Len() > 0; i++ {
+		x := Pop[int](h)
+		h.verify(t, 0)
+		if x != i {
+			t.Errorf("%d.th pop got %d; want %d", i, x, i)
+		}
+	}
+}
+
+func Test(t *testing.T) {
+	h := new(myHeap[int])
+	h.verify(t, 0)
+
+	for i := 20; i > 10; i-- {
+		h.Push(i)
+	}
+	Init[int](h)
+	h.verify(t, 0)
+
+	for i := 10; i > 0; i-- {
+		Push[int](h, i)
+		h.verify(t, 0)
+	}
+
+	for i := 1; h.Len() > 0; i++ {
+		x := Pop[int](h)
+		if i < 20 {
+			Push[int](h, 20+i)
+		}
+		h.verify(t, 0)
+		if x != i {
+			t.Errorf("%d.th pop got %d; want %d", i, x, i)
+		}
+	}
+}
+
+func TestRemove0(t *testing.T) {
+	h := new(myHeap[int])
+	for i := 0; i < 10; i++ {
+		h.Push(i)
+	}
+	h.verify(t, 0)
+
+	for h.Len() > 0 {
+		i := h.Len() - 1
+		x := Remove[int](h, i)
+		if x != i {
+			t.Errorf("Remove(%d) got %d; want %d", i, x, i)
+		}
+		h.verify(t, 0)
+	}
+}
+
+func TestRemove1(t *testing.T) {
+	h := new(myHeap[int])
+	for i := 0; i < 10; i++ {
+		h.Push(i)
+	}
+	h.verify(t, 0)
+
+	for i := 0; h.Len() > 0; i++ {
+		x := Remove[int](h, 0)
+		if x != i {
+			t.Errorf("Remove(0) got %d; want %d", x, i)
+		}
+		h.verify(t, 0)
+	}
+}
+
+func TestRemove2(t *testing.T) {
+	N := 10
+
+	h := new(myHeap[int])
+	for i := 0; i < N; i++ {
+		h.Push(i)
+	}
+	h.verify(t, 0)
+
+	m := make(map[int]bool)
+	for h.Len() > 0 {
+		m[Remove[int](h, (h.Len()-1)/2)] = true
+		h.verify(t, 0)
+	}
+
+	if len(m) != N {
+		t.Errorf("len(m) = %d; want %d", len(m), N)
+	}
+	for i := 0; i < len(m); i++ {
+		if !m[i] {
+			t.Errorf("m[%d] doesn't exist", i)
+		}
+	}
+}
+
+func BenchmarkDup(b *testing.B) {
+	const n = 10000
+	h := make(myHeap[int], 0, n)
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			Push[int](&h, 0) // all elements are the same
+		}
+		for h.Len() > 0 {
+			Pop[int](&h)
+		}
+	}
+}
+
+func TestFix(t *testing.T) {
+	h := new(myHeap[int])
+	h.verify(t, 0)
+
+	for i := 200; i > 0; i -= 10 {
+		Push[int](h, i)
+	}
+	h.verify(t, 0)
+
+	if (*h)[0] != 10 {
+		t.Fatalf("Expected head to be 10, was %d", (*h)[0])
+	}
+	(*h)[0] = 210
+	Fix[int](h, 0)
+	h.verify(t, 0)
+
+	for i := 100; i > 0; i-- {
+		elem := rand.Intn(h.Len())
+		if i&1 == 0 {
+			(*h)[elem] *= 2
+		} else {
+			(*h)[elem] /= 2
+		}
+		Fix[int](h, elem)
+		h.verify(t, 0)
+	}
+}