# An abstract node
abstract class MdNode

	# Node location in original markdown
	var location: MdLocation

	# Node parent
	var parent: nullable MdNode = null is writable

	# First child
	var first_child: nullable MdNode = null is writable

	# Last child
	var last_child: nullable MdNode = null is writable

	# Previous node
	var prev: nullable MdNode = null is writable

	# Next node
	var next: nullable MdNode = null is writable

	# Children nodes of `self`
	fun children: Array[MdNode] do
		var nodes = new Array[MdNode]

		var node = first_child
		while node != null do
			nodes.add node
			node = node.next
		end

		return nodes
	end

	# Append a child to `self`
	fun append_child(child: MdNode) do
		child.unlink
		child.parent = self
		if last_child != null then
			last_child.as(not null).next = child
			child.prev = last_child
			last_child = child
		else
			first_child = child
			last_child = child
		end
	end

	# Prepend a child to `self`
	fun prepend_child(child: MdNode) do
		child.unlink
		child.parent = self
		if first_child != null then
			first_child.as(not null).prev = child
			child.next = first_child
			first_child = child
		else
			first_child = child
			last_child = child
		end
	end

	# Unlink `self` from its `parent`
	fun unlink do
		if prev != null then
			prev.as(not null).next = next
		else if parent != null then
			parent.as(not null).first_child = next
		end
		if next != null then
			next.as(not null).prev = prev
		else if parent != null then
			parent.as(not null).last_child = prev
		end
		parent = null
		next = null
		prev = null
	end

	# Insert `sibling` after `self`.
	fun insert_after(sibling: MdNode) do
		sibling.unlink
		sibling.next = next
		if sibling.next != null then
			sibling.next.as(not null).prev = sibling
		end
		sibling.prev = self
		next = sibling
		sibling.parent = parent
		if sibling.next == null then
			sibling.parent.as(not null).last_child = sibling
		end
	end

	# Insert `sibling` before `self`.
	fun insert_before(sibling: MdNode) do
		sibling.unlink
		sibling.prev = prev
		if sibling.prev != null then
			sibling.prev.as(not null).next = sibling
		end
		sibling.next = self
		prev = sibling
		sibling.parent = parent
		if sibling.prev == null then
			sibling.parent.as(not null).first_child = sibling
		end
	end

	# Visit all children or `self`
	fun visit_all(v: MdVisitor) do
		var node = first_child
		while node != null do
			var next = node.next
			v.visit(node)
			node = next
		end
	end

	redef fun to_s do return "{super}\{{to_s_attrs}\}"

	# Returns `self` attributes as a String
	#
	# Mainly used for debug purposes.
	fun to_s_attrs: String do return "loc: {location}"

	# Print `self` AST
	fun debug do
		var v = new MdASTPrinter
		v.enter_visit(self)
	end
end