package containers

import (
	"errors"
)

var ErrInvalidQueue = errors.New("invalid queue")
var ErrEmptyQueue = errors.New("empty queue")

type Queue[T any] struct {
	head *chainElem[T]
	tail *chainElem[T]
}

// isValid checks if the queue is still valid.
func (q *Queue[T]) isValid() bool {
	return q.head != nil &&
		q.tail != nil &&
		reachable(q.head, q.tail)
}

// IsEmpty checks if the queue is empty.
func (q *Queue[T]) IsEmpty() bool {
	return q.head == q.tail
}

// Push adds a new element to the end of the queue.
func (q *Queue[T]) Push(elem *T) error {
	if !q.isValid() {
		return ErrInvalidQueue
	}

	e := emptyElem[T]()
	q.tail.Elem = elem
	q.tail.Next = e
	q.tail = e

	return nil
}

// Pop removes the first element of the queue.
// It errors out if there is no element or the queue is invalid.
func (q *Queue[T]) Pop() (*T, error) {
	if !q.isValid() {
		return nil, ErrInvalidQueue
	}
	if q.IsEmpty() {
		return nil, ErrEmptyQueue
	}
	Elem := q.head.Elem
	q.head = q.head.Next
	return Elem, nil
}

// Top returns the first element of the queue without removing it.
// It errors out if there is no element or the queue is invalid.
func (q *Queue[T]) Top() (*T, error) {
	if !q.isValid() {
		return nil, ErrInvalidQueue
	}
	if q.IsEmpty() {
		return nil, ErrEmptyQueue
	}
	return q.head.Elem, nil
}

func BuildQueue[T any]() *Queue[T] {
	empty := emptyElem[T]()
	return &Queue[T]{
		head: empty,
		tail: empty,
	}
}