from .doubly_linked_base import _DoublyLinkedBase

class PositionalList(_DoublyLinkedBase):
	"""A sequential container of elements allowing positional access."""
	class Position:
		"""An abstraction representing the location of a single element.

		Note that two position instaces may represent the same inherent
		location in the list.  Therefore, users should always rely on
		syntax 'p == q' rather than 'p is q' when testing equivalence of
		positions.
		"""

		def __init__(self, container, node):
			"""Constructor should not be invoked by user."""
			self._container = container
			self._node = node
		
		def element(self):
			"""Return the element stored at this Position."""
			return self._node._element
			
		def __eq__(self, other):
			"""Return True if other is a Position representing the same location."""
			return type(other) is type(self) and other._node is self._node

		def __ne__(self, other):
			"""Return True if other does not represent the same location."""
			return not (self == other)

	def _validate(self, p):
		"""Return position's node, or raise appropriate error if invalid."""
		if not isinstance(p, self.Position):
			raise TypeError('p must be proper Position type')
		if p._container is not self:
			raise ValueError('p does not belong to this container')
		if p._node._next is None:
			raise ValueError('p is no longer valid')
		return p._node

	def _make_position(self, node):
		"""Return Position instance for given node (or None if sentinel)."""
		if node is self._header or node is self._trailer:
			return None
		else:
			return self.Position(self, node)

	def first(self):
		"""Return the first Position in the list (or None if list is empty)."""
		return self._make_position(self._header._next)

	def last(self):
		"""Return the last Position in the list (or None if list is empty)."""
		return self._make_position(self._trailer._prev)

	def before(self, p):
		"""Return the Position just before Position p (or None if p is first)."""
		node = self._validate(p)
		return self._make_position(node._prev)

	def after(self, p):
		"""Return the Position just after Position p (or None if p is last)."""
		node = self._validate(p)
		return self._make_position(node._next)

	def __iter__(self):
		"""Generate a forward iteration of the elements of the list."""
		cursor = self.first()
		while cursor is not None:
			yield cursor.element()
			cursor = self.after(cursor)

	def _insert_between(self, e, predecessor, successor):
		"""Add element between existing nodes and return new Position."""
		node = _DoublyLinkedBase._insert_between(self, e, predecessor, successor)
		return self._make_position(node)

	def add_first(self, e):
		"""Insert element e at the front of the list and return new Position."""
		return self._insert_between(e, self._header, self._header._next)

	def add_last(self, e):
		"""Insert element e at the back of the list and return new Position."""
		return self._insert_between(e, self._trailer._prev, self._trailer)

	def add_before(self, p, e):
		"""Insert element e into list before Position p and return new Position."""
		original = self._validate(p)
		return self._insert_between(e, original._prev, original)

	def add_after(self, p, e):
		"""Insert element e into list after Position p and return new Position."""
		original = self._validate(p)
		return self._insert_between(e, original, original._next)

	def delete(self, p):
		"""Remove and return the element at Position p."""
		original = self._validate(p)
		return self._delete_node(original)

	def replace(self, p, e):
		"""Replace the element at Position p with e.

		Return the element formerly at Position p.
		"""
		original = self._validate(p)
		old_value = original._element
		original._element = e
		return old_value
