Question

Based on Rob Pike’s load balancer demo, I implemented my own priority queue, but my Pop method is not right, can anyone tell me what’s wrong?

package main

import (
    "fmt"
    "container/heap"
)

type ClassRecord struct {
    name  string
    grade int
}

type RecordHeap []*ClassRecord

func (p RecordHeap) Len() int { return len(p) }

func (p RecordHeap) Less(i, j int) bool {
    return p[i].grade < p[j].grade
}

func (p *RecordHeap) Swap(i, j int) {
    a := *p
    a[i], a[j] = a[j], a[i]
}

func (p *RecordHeap) Push(x interface{}) {
    a := *p
    n := len(a)
    a = a[0 : n+1]
    r := x.(*ClassRecord)
    a[n] = r
    *p = a
}

func (p *RecordHeap) Pop() interface{} {
    a := *p
    *p = a[0 : len(a)-1]
    r := a[len(a)-1]
    return r
}

func main() {
    a := make([]ClassRecord, 6)
    a[0] = ClassRecord{"John", 80}
    a[1] = ClassRecord{"Dan", 85}
    a[2] = ClassRecord{"Aron", 90}
    a[3] = ClassRecord{"Mark", 65}
    a[4] = ClassRecord{"Rob", 99}
    a[5] = ClassRecord{"Brian", 78}
    h := make(RecordHeap, 0, 100)
    for _, c := range a {
        fmt.Println(c)
        heap.Push(&h, &c)
        fmt.Println("Push: heap has", h.Len(), "items")
    }
    for i, x := 0, heap.Pop(&h).(*ClassRecord); i < 10 && x != nil; i++ {
        fmt.Println("Pop: heap has", h.Len(), "items")
        fmt.Println(*x)
    }
}

EDIT: besides the way cthom06 pointed out, another way to fix this is to create a pointer array as follows,

a := make([]*ClassRecord, 6)
a[0] = &ClassRecord{"John", 80}
a[1] = &ClassRecord{"Dan", 85}
......

Answer 1

EDIT:

Oh I should’ve seen this right away.

heap.Push(&h, &c)

You push the address of c, which gets reused on each iteration of range. Every record in the heap is a pointer to the same area in memory, which ends up being Brian. I’m not sure if this is intended behavior or a compiler bug, but

t := c
heap.Push(&h, &t)

works around it.

Also: Your for loop is wrong.

for h.Len() > 0 {
    x := heap.Pop(&h...

should fix it.