package DataStructs

import (
	"encoding/json"
	"errors"
)

type Stack[T any] struct {
	top    *ChainElem[T]
	bottom *ChainElem[T]
}

// isValid checks if the stack is valid.
func (s *Stack[T]) isValid() bool {
	return s.top != nil &&
		s.bottom != nil &&
		reachable(s.top, s.bottom)
}

// IsEmpty checks if the stack is currently empty.
func (s *Stack[T]) IsEmpty() bool {
	return s.top == s.bottom
}

// Push adds a new element to the top of the stack.
// It errors out if the stack is invalid.
func (s *Stack[T]) Push(e *T) error {
	if !s.isValid() {
		return errors.New("stack invalid")
	}
	n := emptyElem[T]()
	n.Elem = e
	n.Next = s.top
	s.top = n
	return nil
}

// Pop removes the first element at the top of the stack and returns it.
// It errors out if the stack is invalid or empty.
func (s *Stack[T]) Pop() (*T, error) {
	if !s.isValid() {
		return nil, errors.New("stack invalid")
	}
	if s.IsEmpty() {
		return nil, errors.New("stack empty")
	}
	e := s.top.Elem
	s.top = s.top.Next
	return e, nil
}

// Top returns the first element at the top of the stack without removing it.
// It errors out if the stack is empty or invalid.
func (s *Stack[T]) Top() (*T, error) {
	if !s.isValid() {
		return nil, errors.New("stack invalid")
	}
	if s.IsEmpty() {
		return nil, errors.New("stack empty")
	}
	return s.top.Elem, nil
}

// MarshalJSON is used by json.Marshal to create a json representation.
func (s *Stack[T]) MarshalJSON() ([]byte, error) {
	if !s.isValid() {
		return nil, errors.New("queue invalid")
	}
	if s.IsEmpty() {
		return nil, errors.New("queue empty")
	}

	return json.Marshal(s.top)
}
func BuildStack[T any]() *Stack[T] {
	empty := emptyElem[T]()
	return &Stack[T]{
		top:    empty,
		bottom: empty,
	}
}