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Why Are Golang Heaps So Sophisticated

Why Are Golang Heaps So Sophisticated

2023-12-01 17:26:57

Heaps are generally used to partially kind a set. Each
insertion/deletion from the set is adopted by a “fixup” to revive
both min-heap or max-heap integrity. For instance, a max-heap could be
represented as a binary tree the place each mother or father is “larger” than its
youngsters. It often takes a small variety of swaps to “fixup” a tree to
restore the max-heap property after an insertion or deletion. Even
although all the set parts are usually not globally ordered, the “largest”
worth is all the time on the high of a max-heap. Consequently, heaps have a
variety of practical


Heaps could be carried out as binary bushes with nodes and pointers, however
most programming languages present default heap implementations for

I personally discovered the usual library heap complicated after I began
utilizing Golang.

That is what I used to be used to coming from Python:

h = []
>>> heappush(h, (5, 'write code'))
>>> heappush(h, (7, 'launch product'))
>>> heappush(h, (1, 'write spec'))
>>> heappush(h, (3, 'create checks'))
>>> heappop(h)
(1, 'write spec')

For comparability, right here is the equal code in Go tailored from the
container/heap docs: (run

import (

kind Tuple struct {
	i int
	s string

kind TupleHeap []Tuple

func (h TupleHeap) Len() int { return len(h) }
func (h TupleHeap) Much less(i, j int) bool {
	if h[i].i != h[j].i {
		return h[i].i < h[j].i
	return h[i].s < h[j].s
func (h TupleHeap) Swap(i, j int) { h[i], h[j] = h[j], h[i] }
func (h *TupleHeap) Push(x any) {
	*h = append(*h, x.(Tuple))

func (h *TupleHeap) Pop() any {
	outdated := *h
	n := len(outdated)
	x := outdated[n-1]
	*h = outdated[0 : n-1]
	return x

func most important() {
	h := &TupleHeap{}
	heap.Push(h, Tuple{i: 5, s: "write code"})
	heap.Push(h, Tuple{i: 7, s: "launch product"})
	heap.Push(h, Tuple{i: 1, s: "write spec"})
	heap.Push(h, Tuple{i: 3, s: "create checks"})
	fmt.Printf("%d ", heap.Pop(h))


I used heaps for a number of engine optimizations not too long ago, and three years
later I nonetheless discover Go heaps complicated. So I took someday to analysis

One factor that stands out about collections/heap is that it’s broadly
generic however requires plenty of boilerplate. heap.Interface helps
binary heaps, array heaps, disk-backed heaps, with any consumer supported

A lot of Golang’s customary library goals to be maximally easy, so this
design characteristic is unsurprising. kind, for instance, is an identical
customary library package deal whose interfaces are equally generic. One key
distinction is that kind.Ints and kind.Strings present defaults that
take away boilerplate for the most typical circumstances.

Maximally versatile however good defaults sounds ideally suited. Why does not heap
assist easy defaults?

We now have documented prior to now how slices have sharp
. Half
of why slices are complicated is that they’re implicit tips to
underlying storage arrays. Considered one of Nick’s most important criticisms in that weblog is
how array mutations can have unpredictable results on reminiscence. An analogous
downside plagues the area of heap implementations.

As an example, listed below are two totally different append capabilities. One makes use of a
default slice pointer, and one makes use of a pointer to a slice pointer (run

func most important() {
	arr := []int{1, 2}
	append1(arr, 3)
	fmt.Printf("#%vn", arr) 
	append2(&arr, 3)
	fmt.Printf("#%vn", arr) 

func append1(arr []int, v int) {
	arr = append(arr, v)

func append2(arr *[]int, v int) {
	*arr = append(*arr, v)

With out going too deep into the small print, slice pointers act like several
different worth kind when handed to a operate . Mutating the underlying
array creates a brand new slice pointer. If that slice pointer was handed by
worth, all the adjustments are restricted to the inside closure (by
distinction, in-place modifying an array by way of a slice pointer maintains
the integrity of the unique reference). If the identical slice pointer is
handed by reference, the outer slice handle is up to date to the brand new

That is essential for heap implementations as a result of it limits the
design area. We both want:

  1. An indirection layer that maintains the most recent pointer after we modify reminiscence

  2. Return new references after we modify reminiscence, or

  3. Function explicitly on tips to slices so the unique references stay legitimate

The primary possibility, chosen by the Go customary library, outlines a
minimal attainable interface that abstracts pointless sorting particulars
however forces a consumer to acknowledge and keep the slice indirection:

kind IntHeap []int

kind Interface interface {
	Push(x any) 
	Pop() any   

As a substitute of passing a pointer of a pointer, we add a concrete kind to
make the extra reference specific. This makes it straightforward for
heap.Interface to focus on the slice’s edges whereas supporting
implementation flexibility.

The second possibility, returning and monitoring up to date slice pointers for
every heap operation, is one technique to implement a heap that handles []int

var h []int
h = heap.Push(h, x)
h, y := heap.Pop(h)

Immutable updates drive us to repeatedly monitor the most recent heap
ref. We additionally lose the flexibility to do the type of context administration,
locking, and concurrency management that we often use in manufacturing.
Provided that Go usually designs round in-place updating central objects, I
am not shocked the usual library prevented this.

See Also

The third sample passes slice tips to heap capabilities, just like
how the usual library
encoding/json passes string
pointers for JSON deserialization. I forked the usual library and
carried out this final design to persuade myself that it really works :

	h := []int{5, 7, 1, 3}
	Push(&h, 3)
	min := Pop(&h)

This package deal helps each heap.Interface and built-in Comparable
slices ([]int and []string for starters). The draw back is that
unsupported sorts panic (alternatively, heap capabilities may
deviate from the usual library interface and return an error).

Arguably the most important draw back of the final method is combining the
dynamic and static interfaces into one package deal. If one thing goes improper
within the wrapper I might be much more confused than earlier than doing all of

After I initially posted this text, one of many r/golang mods
jerf identified a pleasant
package that makes it
straightforward to heap kind built-in sorts:

    h := binheap.EmptyMaxHeap[int]()
    min := h.Pop() 

One other consumer named twek discovered an
outdated proposal from 2021 with
an identical interface design. He was good sufficient to create an example
implementation sandbox
for others to
strive if you’re curious about working it your self!

This diverges a bit from the present customary library by combining the helper
strategies and the heap.Interface into one. A heap implementation can
standardize Push(), Pop(), and Swap() for all slices. And the
generic object statically captures the comparable kind’s Much less().
The result’s zero effort heaps for all in-built sorts!

The identical technique additionally makes it straightforward to heap kind
arbitrary slices with an initialization callback just like
kind.Slice (run

import (


kind Struct struct {
	Idx         int
	Description string

func (s Struct) LessThan(r Struct) bool {
	if s.Idx < r.Idx {
		return true
	if s.Idx > r.Idx {
		return false
	return s.Description < r.Description

func most important() {
	h := binheap.EmptyHeap[Struct](func(a, b Struct) bool { return a.LessThan(b) })
	h.Push(Struct{5, "write code"})
	h.Push(Struct{7, "launch product"})
	h.Push(Struct{1, "write spec"})
	h.Push(Struct{3, "create checks"})

On the finish of the day, the way in which slices work makes Golang’s heaps a bit
extra complicated than different languages. Peeling again the curtain a bit
helped me perceive the design area given Golang’s language
limitations. The energetic neighborhood on r/golang helped present me with
extra context and higher implementations than I may discover alone. The generics
resolution makes use of a barely totally different design than the usual library,
however is an easy abstraction and removes a lot of the boilerplate in contrast
to container/heap as we speak. I linked the open proposal under if you’re
curious about following the Go core staff’s ideas[^4].

I’m positive I missed some issues within the technique of doing this writeup,
whether or not earlier design discussions or different blogs relating to this matter.
If in case you have feedback our questions be at liberty to succeed in out to us on
Discord, and

[^4] Proposal so as to add generics implementation to go customary library

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