How to implement a queue in Go?

The current Go library doesn't provide the queue container.
To implement a simple queue, I use circle array as the underlying data structure.
It follows algorithms mentioned in TAOCP:

Insert Y into queue X: X[R]<-Y; R<-(R+1)%M; if R=F then OVERFLOW.
Delete Y from queue X: if F=R then UNDERFLOW; Y<-X[F]; F<-(F+1) % M.
F: Front, R: Rear, M: Array length.

Following is the code:

package main

import (
    "fmt"
)

type Queue struct {
    len        int 
    head, tail int 
    q          []int
}

func New(n int) *Queue {
    return &Queue{n, 0, 0, make([]int, n)} 
}

func (p *Queue) Enqueue(x int) bool {
    p.q[p.tail] = x 
    p.tail = (p.tail + 1) % p.len
    return p.head != p.tail
}

func (p *Queue) Dequeue() (int, bool) {
    if p.head == p.tail {
        return 0, false
    }   
    x := p.q[p.head]
    p.head = (p.head + 1) % p.len
    return x, true
}

func main() {
    q := New(10)
    for i := 1; i < 13; i++ {
        fmt.Println(i, q.Enqueue(i))
    }   
    fmt.Println()
    for i := 1; i < 13; i++ {
        fmt.Println(q.Dequeue())
    }   
}

But the output is obviously wrong:

1 true
2 true
3 true
4 true
5 true
6 true
7 true
8 true
9 true
10 false
11 true
12 true

11 true
12 true
0 false
0 false
0 false
0 false
0 false
0 false
0 false
0 false
0 false
0 false

I think I need one more field to make the code work properly.
What do you suggest?

The improved code has a small shortcoming: an array of size n can contain only n-1 elements.

package main

import (
    "fmt"
)

type Queue struct {
    len        int 
    head, tail int 
    q          []int
}

func New(n int) *Queue {
    return &Queue{n, 0, 0, make([]int, n)} 
}

func (p *Queue) Enqueue(x int) bool {
    p.q[p.tail] = x 
    ntail := (p.tail + 1) % p.len
    ok := false
    if ntail != p.head {
        p.tail = ntail
        ok = true
    }   
    return ok
}

func (p *Queue) Dequeue() (int, bool) {
    if p.head == p.tail {
        return 0, false
    }   
    x := p.q[p.head]
    p.head = (p.head + 1) % p.len
    return x, true
}

func main() {
    q := New(10)
    for i := 1; i < 13; i++ {
        fmt.Println(i, q.Enqueue(i))
    }   
    fmt.Println()
    for i := 1; i < 13; i++ {
        fmt.Println(q.Dequeue())
    }   
}

You do not need all this hustle in any reasonable go version (after 1.x). Everything can be achieved with slices.

queue := []int{}

Add to a queue:

queue = append(queue, 6)

Pop from a queue:

el := queue[0]
queue = queue[1:]

Here is implementation that shows that pop does not take a lot of time (in fact here it is shorter than push because in my opinion of reallocation of memory when the queue is growing).

package main

import (
	"fmt"
	"time"
)

func main() {
	n := 10000000
	queue := []int{1, 2, 3}

	start := time.Now()
	for i := 0; i < n; i++ {
		queue = append(queue, i)
	}
	elapsed := time.Since(start)
	fmt.Println(elapsed)

	start = time.Now()
	for i := 0; i < n; i++ {
		_ = queue[0]
		queue = queue[1:]
	}
	elapsed = time.Since(start)
	fmt.Println(elapsed)
	fmt.Println(queue)
}

On my machine the numbers are:

216.611664ms
13.441106ms

From @DaveC's comment:

> This is simple and works very well for everything except critical code
> where allocations (pressure on the garbage collector) is
> undesirable.Two things to note, first it does keep re-allocating the
> underlying array on push (although efficiently and not on every call)
> and pop doesn't free any space until that happens. This leads to the
> second thing, if (as is common) the queue contains a pointer to
> something, then it's good to do queue[0] = nil; queue = queue[1:] to
> have the queue stop referencing the pointer right away.

A buffered channel makes a fine queue, though it has a fixed maximum queue length, chosen at creation time. A channel has the useful property that dequeues are threadsafe (your code isn't).

When Enqueue fails, you're still incrementing p.tail, so next time it will appear not to fail -- that explains the single false in your first loop (and messes everything up for the second one). The original algorithm says OVERFLOW meaning "give everything up", not "just keep going as if nothing untowards happened";-).

All you need to do is decrement p.tail if you've checked that failure's occurring -- or put the incremented value in a local temporary and move it to p.tail only if failure is not occurring, that may be more elegant. This way, the failing Enqueue does not enqueue the new value, but the queue itself (without that overflowing value) is still semantically intact and correct for future operations.

It's true that there's no package called queue, but either vector or list would make fine queues. Also see this question.

> First you need to create a struct for Queue for holding queue properties. Then create a initQueue function to initialize default values, which will also take memory size from the user. Create a function to enqueue values, create function to dequeue values. Create a display function to display queues values.

type Queue struct {
	front  int
	rear   int
	size   int
	QArray []int
}

func (q *Queue) initQueue(size int) {
	q.size = size
	q.QArray = make([]int, q.size)
	q.front = -1
	q.rear = -1
}

func (q *Queue) enqueue(value int) {
	if q.rear == q.size-1 {
		fmt.Println("Queue is Full")
		return
	} else {
		q.rear++
		q.QArray[q.rear] = value
	}
}

func (q *Queue) dequeue() int {
	var x int = -1
	if q.front == q.rear {
		fmt.Println("Queue is Empty!")
	} else {
		q.front++
		x = q.QArray[q.front]
	}
	return x
}

I modify the original implementation to make a dynamic queue. I.e. when the queue fills up it will allocate a larger queue and move all the items over.

package main

import (
    "fmt"
)

type Queue struct {
    len        uint
    head, tail uint
    q          []int
}

func NextPowerOfTwo(v uint) uint {
    if v == 0 {
        return 1
    }
    v--
    v |= v >> 1
    v |= v >> 2
    v |= v >> 4
    v |= v >> 8
    v |= v >> 16
    v++
    return v
}

func NewQueue(n uint) *Queue {
    n = NextPowerOfTwo(n)
    if n < 4 {
        n = 4
    }
    println("create queue of", n)
    return &Queue{n, 0, 0, make([]int, n)}
}

func (p *Queue) Resize() {
    if p.head == (p.tail + 1) % p.len {
        new_len := p.len * 2;
        new_q := make([]int, new_len)
        // currently there are (len - 1) items in the queue
        var i uint
        for i = 0; i < p.len - 1; i++ {
            n, _ := p.Dequeue()
            new_q[i] = n
        }
        p.q = new_q
        p.head, p.tail = 0, p.len - 1
        p.len = new_len
        println("queue resized to ", p.len)
    }
}

func (p *Queue) Enqueue(x int) {
    p.Resize();
    p.q[p.tail] = x
    p.tail = (p.tail + 1) % p.len
}

func (p *Queue) Dequeue() (int, bool) {
    if p.head == p.tail {
        return -1, false
    }
    x := p.q[p.head]
    p.head = (p.head + 1) % p.len
    return x, true
}

func main() {
    q := NewQueue(1)
    for i := 1; i < 13; i++ {
        q.Enqueue(2 * i + 1)
        println("enqueued item ", i)
    }
    println("** queue content **")
    for i := 1; i < 13 + 1; i++ {
        fmt.Println(q.Dequeue())
    }
}