/*
 * A generalized stack.  This is as close as we get to a generic stack.
 */
package genstack

/*
 * Node for the linked structure.  This is private, since the client
 * never needs to refer to it.
 */
type node struct {
	data interface {}
	next *node
}

// This is the stack type.  The type Stack is exported, but the field
// head is not, so the client cannot look inside the stack.
type Stack struct {
	head *node
}

// Push method for the stack.  Notice that the receiver, stack, is
// effectively a pointer to a pointer to node.
func (stack *Stack) Push(d interface {}) {
	nn := new(node)
	nn.data = d
	nn.next = stack.head
	stack.head = nn
}

// Pop method for stack.  If the stack is not empty, return the top
// item and true.  If it's empty, return 0 and false, without changing
// the stack.
func (stack *Stack) Pop() (v interface {}, ok bool) {
	if stack.head == nil {
		return nil, false
	} else {
		v = stack.head.data
		stack.head = stack.head.next
		ok = true
		return
	}
}

// Empty method.
func (stack *Stack) Empty() bool {
	return stack.head == nil
}