use crate::wald::{NodeRef, NodeStorage, Text};

#[derive(Copy, Clone, Eq, PartialEq)]
#[repr(u16)]
pub enum NodeKind {
	// Tokens
	// These are the "leaf" nodes
	LPar,
	RPar,
	Whitespace,
	Word,

	// Composite
	/// A generic node that wraps multiple tokens. This is useful for e.g.
	/// attaching whitespace to a Word.
	Multi,
	List,
	Root,

	#[doc(hidden)]
	_ErrFirst,
	ErrUnexpectedEOF,
	#[doc(hidden)]
	_ErrLast,
	#[doc(hidden)]
	_Last,
}
impl From<u16> for NodeKind {
	fn from(value: u16) -> Self {
		assert!(value < NodeKind::_Last as u16);

		unsafe { core::mem::transmute::<u16, NodeKind>(value) }
	}
}

impl From<NodeKind> for u16 {
	fn from(val: NodeKind) -> Self {
		val as u16
	}
}

pub fn breaks_word(b: u8) -> bool {
	b.is_ascii_whitespace() || b == b')' || b == b'('
}

pub struct Parser<'a> {
	input: &'a str,
	at: usize,
	nodes: &'a mut NodeStorage,
}

impl<'a> Parser<'a> {
	pub fn new(nodes: &'a mut NodeStorage, input: &'a str) -> Self {
		Self {
			nodes,
			input,
			at: 0,
		}
	}

	pub fn head(&self) -> Option<u8> {
		self.input[self.at..].bytes().next()
	}

	pub fn skip(&mut self, amt: usize) {
		self.at += amt;
	}

	pub fn skip_while(&mut self, mut f: impl FnMut(u8) -> bool) {
		while let Some(head) = self.head() {
			if !f(head) {
				break;
			}

			self.skip(1);
		}
	}

	pub fn parse_whitespace(&mut self) -> Option<NodeRef> {
		let start = self.at;
		self.skip_while(|b| b.is_ascii_whitespace());
		let end = self.at;
		let span = start..end;
		if span.is_empty() {
			return None;
		}

		let node = self.nodes.new_node(NodeKind::Whitespace.into());
		self.nodes.set_text(node, Text::Span(span));

		Some(node)
	}

	pub fn parse_word(&mut self, whitespace: Option<NodeRef>) -> NodeRef {
		let start = self.at;
		self.skip_while(|b| !breaks_word(b));
		let end = self.at;

		let node_word = self.nodes.new_node(NodeKind::Word.into());
		self.nodes.set_text(node_word, Text::Span(start..end));

		let node_multi = self.nodes.new_node(NodeKind::Multi.into());
		if let Some(node_whitespace) = whitespace {
			self.nodes.append_child(node_multi, node_whitespace);
		}
		self.nodes.append_child(node_multi, node_word);

		node_multi
	}

	pub fn parse_list(&mut self, whitespace: Option<NodeRef>) -> NodeRef {
		assert!(self.head().unwrap() == b'(');

		let node_list = self.nodes.new_node(NodeKind::List.into());
		if let Some(node_whitespace) = whitespace {
			self.nodes.append_child(node_list, node_whitespace);
		}

		// LPar
		let start = self.at;
		self.skip(1);
		let end = self.at;

		let node_lpar = self.nodes.new_node(NodeKind::LPar.into());
		self.nodes.set_text(node_lpar, Text::Span(start..end));
		self.nodes.append_child(node_list, node_lpar);

		loop {
			let head = match self.head() {
				None => {
					let node_err =
						self.nodes.new_node(NodeKind::ErrUnexpectedEOF.into());
					self.nodes.append_child(node_list, node_err);

					break;
				},
				Some(h) => h,
			};

			if head == b')' {
				let start = self.at;
				self.skip(1);
				let end = self.at;

				let node_rpar = self.nodes.new_node(NodeKind::RPar.into());
				self.nodes.set_text(node_rpar, Text::Span(start..end));
				self.nodes.append_child(node_list, node_rpar);

				break;
			}

			let node_child = self.parse_one().unwrap();
			self.nodes.append_child(node_list, node_child);
		}

		node_list
	}

	pub fn parse_one(&mut self) -> Option<NodeRef> {
		let whitespace = self.parse_whitespace();

		let head = self.head()?;
		let node = match head {
			b'(' => self.parse_list(whitespace),
			_ => self.parse_word(whitespace),
		};

		Some(node)
	}

	pub fn parse(&mut self) {
		assert!(self.nodes.nodes().next().is_none());

		let node_root = self.nodes.new_node(NodeKind::Root.into());

		while let Some(node) = self.parse_one() {
			self.nodes.append_child(node_root, node);
		}
	}
}

pub fn parse(storage: &mut NodeStorage, input: &str) {
	let mut parser = Parser::new(storage, input);
	parser.parse();
}

#[cfg(test)]
mod test {
	use crate::{
		syn::{parse, NodeKind},
		wald::NodeStorage,
	};

	#[test]
	fn simple_print_input_exactly() {
		let input = r#"(+ 3 4)"#;
		let mut storage = NodeStorage::new();
		parse(&mut storage, input);

		assert!(!storage
			.nodes()
			.any(|n| (NodeKind::_ErrFirst.into()..NodeKind::_ErrLast.into())
				.contains(&storage.tag(n))));

		let root = storage
			.nodes()
			.find(|&n| storage.tag(n) == NodeKind::Root.into())
			.unwrap();
		let display = storage.display_syntax(input, root);

		let output = format!("{display}");
		assert_eq!(input, output);
	}

	#[test]
	fn traversal() {
		let input = r#"(+ 3 4)"#;
		let mut nodes = NodeStorage::new();
		parse(&mut nodes, input);

		let three = nodes
			.nodes()
			.find(|&n| nodes.text(n).as_str(input) == "3")
			.unwrap();

		let three_multi = nodes.parent(three).unwrap();
		let four_multi = nodes.sibling_next(three_multi).unwrap();
		let four = nodes
			.children(four_multi)
			.find(|&n| nodes.tag(n) == NodeKind::Word.into())
			.unwrap();

		assert_eq!(nodes.text(four).as_str(input), "4");

		let list = nodes.parent(three_multi).unwrap();
		assert_eq!(list, nodes.parent(four_multi).unwrap());
	}
}