// package stack provides a generic first-in / first-out stack.
package stack

// stack interface

type Stack[V any] interface {
	Depth() int
	Push(V)
	Pop() V
	Peek(int) V
	Replace(int, V)
}

// simple slice implementation

type stack[V any] struct {
	tos  int
	data []V
}

func NewStack[V any]() *stack[V] {
	return &stack[V]{
		tos:  -1,
		data: make([]V, 0, 5),
	}
}

func (st *stack[V]) Depth() int {
	return st.tos + 1
}

func (st *stack[V]) Push(v V) {
	st.tos += 1
	if st.tos >= len(st.data) {
		st.data = append(st.data, v)
	} else {
		st.data[st.tos] = v
	}
}

func (st *stack[V]) Pop() V {
	v := st.data[st.tos]
	st.tos -= 1
	return v
}

func (st *stack[V]) Peek(i int) V {
	return st.data[st.tos-i]
}

func (st *stack[V]) Replace(i int, v V) {
	st.data[st.tos-i] = v
}