Change queues and stacks

- Make chain element private
- Remove JSON Marshaller from stack and queue
- Remove access to top chain element

containers/queues.go

@@ -1,13 +1,15 @@
 package containers
 
 import (
-	"encoding/json"
 	"errors"
 )
 
+var ErrInvalidQueue = errors.New("invalid queue")
+var ErrEmptyQueue = errors.New("empty queue")
+
 type Queue[T any] struct {
-	head *ChainElem[T]
-	tail *ChainElem[T]
+	head *chainElem[T]
+	tail *chainElem[T]
 }
 
 // isValid checks if the queue is still valid.
@@ -25,7 +27,7 @@ func (q *Queue[T]) IsEmpty() bool {
 // Push adds a new element to the end of the queue.
 func (q *Queue[T]) Push(elem *T) error {
 	if !q.isValid() {
-		return errors.New("invalid queue")
+		return ErrInvalidQueue
 	}
 
 	e := emptyElem[T]()
@@ -40,10 +42,10 @@ func (q *Queue[T]) Push(elem *T) error {
 // 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, errors.New("invalid queue")
+		return nil, ErrInvalidQueue
 	}
 	if q.IsEmpty() {
-		return nil, errors.New("empty queue")
+		return nil, ErrEmptyQueue
 	}
 	Elem := q.head.Elem
 	q.head = q.head.Next
@@ -54,38 +56,14 @@ func (q *Queue[T]) Pop() (*T, error) {
 // 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, errors.New("queue invalid")
+		return nil, ErrInvalidQueue
 	}
 	if q.IsEmpty() {
-		return nil, errors.New("queue empty")
+		return nil, ErrEmptyQueue
 	}
 	return q.head.Elem, nil
 }
 
-// HeadElem returns the first ChainElem of the queue without removing it.
-// It errors out if there is no element or the queue is invalid.
-func (q *Queue[T]) HeadElem() (*ChainElem[T], error) {
-	if !q.isValid() {
-		return nil, errors.New("queue invalid")
-	}
-	if q.IsEmpty() {
-		return nil, errors.New("queue empty")
-	}
-	return q.head, nil
-}
-
-// MarshalJSON is used for generating json data when using json.Marshal.
-func (q *Queue[T]) MarshalJSON() ([]byte, error) {
-	if !q.isValid() {
-		return nil, errors.New("queue invalid")
-	}
-	if q.IsEmpty() {
-		return nil, errors.New("queue empty")
-	}
-
-	return json.Marshal(q.head)
-}
-
 func BuildQueue[T any]() *Queue[T] {
 	empty := emptyElem[T]()
 	return &Queue[T]{