use std::{fmt, ops::Deref};

use line_span::{LineSpan, LineSpans};

fn process<'raw>(raw: &'raw str) -> Document<'raw> {
	let tokens: Vec<TokenLine<'raw>> = raw
		.line_spans()
		.map(|span| TokenLine {
			tok: Token::detect(span.as_str()),
			span,
		})
		.collect();

	let mut blocks = vec![];
	let mut curr = &tokens[..];
	loop {
		if curr.is_empty() {
			break;
		}

		let tok = curr[0];
		match tok.tok {
			Token::Text => blocks.push(Block::Paragraph(tok.span.as_str())),
			Token::Blank => (),
			Token::Header { level, text } => blocks.push(Block::header(level, text)),
			Token::Escaped { text } => blocks.push(Block::Paragraph(text)),
			Token::HorizontalLine => blocks.push(Block::HorizontalLine),
			Token::ListItem { text } => {
				let mut mur = map_until(&curr[1..], extract_listitem);
				mur.items.insert(0, text);

				blocks.push(Block::list(mur.items));
				curr = mur.remaining;
				continue;
			}
			Token::CodeFence { lang } => {
				let mur = map_until(&curr[1..], extract_codeblock_body);
				let body = StrOrMany::from_spans(raw, &mur.items);
				blocks.push(Block::code(lang, body));

				// Advance remaining by one to skip the close
				curr = &mur.remaining[1..];
				continue;
			}
		}

		curr = &curr[1..];
	}

	Document { blocks }
}

fn maybe_header<'raw>(line: &'raw str) -> Option<Token<'raw>> {
	if line.starts_with('#') {
		if let Some((level_str, text)) = line.split_once(|c| c == ' ') {
			let level = level_str
				.chars()
				.fold(0, |acc, c| if c == '#' { acc + 1 } else { acc });

			// If the level (which is the count of '#' in the level_str) is not
			// equal to level_str.len(), not every character was a hash and this
			// header is invalid
			if level == level_str.len() {
				return Some(Token::Header {
					level: level as u8,
					text,
				});
			}
		}
	}

	None
}

fn extract_listitem<'raw>(tok: &TokenLine<'raw>) -> Option<&'raw str> {
	if let Token::ListItem { text } = tok.tok {
		Some(text)
	} else {
		None
	}
}

fn extract_codeblock_body<'raw>(tok: &TokenLine<'raw>) -> Option<SpanOrStr<'raw>> {
	match tok.tok {
		Token::CodeFence { .. } => None,
		_ => Some(tok.as_code_body()),
	}
}

fn map_until<'raw, 'tok, F, T>(toks: &'tok [TokenLine<'raw>], f: F) -> MapUntilReturn<'raw, 'tok, T>
where
	F: Fn(&TokenLine<'raw>) -> Option<T>,
{
	let mut items = vec![];
	for (idx, tok) in toks.iter().enumerate() {
		match f(tok) {
			Some(item) => items.push(item),
			None => {
				return MapUntilReturn {
					items,
					remaining: &toks[idx..],
				};
			}
		}
	}

	MapUntilReturn {
		items,
		remaining: &[],
	}
}

struct MapUntilReturn<'raw, 'tok, T> {
	items: Vec<T>,
	remaining: &'tok [TokenLine<'raw>],
}

#[derive(Copy, Clone, Debug)]
struct TokenLine<'raw> {
	span: LineSpan<'raw>,
	tok: Token<'raw>,
}

impl<'raw> Deref for TokenLine<'raw> {
	type Target = Token<'raw>;

	fn deref(&self) -> &Self::Target {
		&self.tok
	}
}

impl<'raw> TokenLine<'raw> {
	pub fn as_code_body(&self) -> SpanOrStr<'raw> {
		if let Token::Escaped { text } = self.tok {
			return text.into();
		}

		self.span.into()
	}
}

#[derive(Copy, Clone, Debug)]
enum Token<'raw> {
	Text,
	Blank,
	Escaped { text: &'raw str },
	Header { level: u8, text: &'raw str },
	CodeFence { lang: Option<&'raw str> },
	HorizontalLine,
	ListItem { text: &'raw str },
}

impl<'raw> Token<'raw> {
	pub fn detect(line: &'raw str) -> Self {
		if line.is_empty() {
			return Token::Blank;
		}

		let mut escaped = None;
		let working = if let Some(stripped) = line.strip_prefix('\\') {
			escaped = Some(stripped);
			stripped
		} else {
			line
		};

		macro_rules! maybe_escaped {
			() => {
				if let Some(text) = escaped {
					return Token::Escaped { text };
				}
			};
		}

		if let Some(header_tok) = maybe_header(working) {
			maybe_escaped!();
			return header_tok;
		}

		if let Some(stripped) = working.strip_prefix("```") {
			maybe_escaped!();
			return Token::CodeFence {
				lang: Self::str_empty_option(stripped),
			};
		}

		if working.trim_end().eq("---") {
			maybe_escaped!();
			return Token::HorizontalLine;
		}

		if let Some(stripped) = working.strip_prefix('-') {
			maybe_escaped!();
			return Token::ListItem { text: stripped };
		}

		Token::Text
	}

	pub fn str_empty_option(s: &str) -> Option<&str> {
		match s.is_empty() {
			true => None,
			false => Some(s),
		}
	}
}

enum SpanOrStr<'raw> {
	Span(LineSpan<'raw>),
	Str(&'raw str),
}

impl<'raw> SpanOrStr<'raw> {
	pub fn slice_has_str(slice: &[SpanOrStr<'raw>]) -> bool {
		for sos in slice {
			if let SpanOrStr::Str(_) = sos {
				return true;
			}
		}

		false
	}

	pub fn as_str(&self) -> &'raw str {
		match self {
			Self::Span(span) => span.as_str(),
			Self::Str(s) => s,
		}
	}
}

impl<'raw> From<LineSpan<'raw>> for SpanOrStr<'raw> {
	fn from(span: LineSpan<'raw>) -> Self {
		Self::Span(span)
	}
}

impl<'raw> From<&'raw str> for SpanOrStr<'raw> {
	fn from(s: &'raw str) -> Self {
		Self::Str(s)
	}
}

#[derive(Clone, Debug, PartialEq)]
pub struct Document<'raw> {
	blocks: Vec<Block<'raw>>,
}

impl<'raw> Document<'raw> {
	pub fn parse(raw: &'raw str) -> Self {
		process(raw)
	}

	pub fn blocks(&self) -> &[Block<'raw>] {
		&self.blocks
	}
}

#[derive(Clone, Debug, PartialEq)]
pub enum StrOrMany<'raw> {
	One(&'raw str),
	Many(Vec<&'raw str>),
}

impl<'raw> From<&'raw str> for StrOrMany<'raw> {
	fn from(s: &'raw str) -> Self {
		StrOrMany::One(s)
	}
}

impl<'raw> StrOrMany<'raw> {
	fn from_spans(raw: &'raw str, slice: &[SpanOrStr<'raw>]) -> Self {
		if SpanOrStr::slice_has_str(slice) {
			StrOrMany::Many(slice.iter().map(|sp| sp.as_str()).collect())
		} else {
			match [slice.first(), slice.last()] {
				// Slice is empty
				[None, _] => return StrOrMany::One(""),

				// These can't happen because we just checked if the slice has a Str variant
				[Some(SpanOrStr::Str(_)), _] => unreachable!(),
				[_, Some(SpanOrStr::Str(_))] => unreachable!(),

				// This can't happen because the last would be the first
				[Some(SpanOrStr::Span(_)), None] => unreachable!(),

				// Return one large slice encompassing all the Span in the slice
				[Some(SpanOrStr::Span(first)), Some(SpanOrStr::Span(last))] => {
					StrOrMany::One(&raw[first.start()..last.end()])
				}
			}
		}
	}
}

impl<'raw> fmt::Display for StrOrMany<'raw> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		match self {
			Self::One(s) => write!(f, "{s}"),
			Self::Many(strs) => match strs.len() {
				0 => write!(f, ""),
				1 => write!(f, "{}", strs[1]),
				len => {
					let last = strs.last().unwrap();
					for s in &strs[..len - 1] {
						writeln!(f, "{s}")?;
					}
					write!(f, "{last}")
				}
			},
		}
	}
}

#[derive(Clone, Debug, PartialEq)]
pub enum Block<'raw> {
	Header {
		level: u8,
		text: &'raw str,
	},
	Paragraph(&'raw str),
	HorizontalLine,
	List {
		items: Vec<&'raw str>,
	},
	Code {
		language: Option<&'raw str>,
		lines: StrOrMany<'raw>,
	},
}

impl<'raw> Block<'raw> {
	pub fn header(level: u8, text: &'raw str) -> Self {
		Self::Header { level, text }
	}

	pub fn para(text: &'raw str) -> Self {
		Self::Paragraph(text)
	}

	pub fn list(items: Vec<&'raw str>) -> Self {
		Self::List { items }
	}

	pub fn code<L: Into<StrOrMany<'raw>>>(language: Option<&'raw str>, lines: L) -> Self {
		Self::Code {
			language,
			lines: lines.into(),
		}
	}
}

#[cfg(test)]
mod test {
	use super::{Block, Document, process};

	macro_rules! doc {
		(1p $text:literal) => {
			Document {
				blocks: vec![Block::Paragraph($text)],
			}
		};

		(1 $someblock:expr) => {
			Document {
				blocks: vec![$someblock],
			}
		};
	}

	#[test]
	pub fn header() {
		let raw = "# Level one";
		let actual = process(raw);
		let expected = doc!(1 Block::header(1, "Level one"));

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn many_headers() {
		let raw = "# One\n## Two\n### Three";

		let actual = process(raw);
		let expected = Document {
			blocks: vec![
				Block::header(1, "One"),
				Block::header(2, "Two"),
				Block::header(3, "Three"),
			],
		};

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn paragraph() {
		let raw = "paragraph";
		let actual = process(raw);
		let expected = doc!(1p "paragraph");

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn horizontal_line() {
		let raw = "---";
		let actual = process(raw);
		let expected = doc!(1 Block::HorizontalLine);

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn list_item() {
		let raw = "- list item";
		let actual = process(raw);
		let expected = doc!(1 Block::list(vec![" list item"]));

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn empty_code_block_no_lang() {
		let raw = "```\n```";
		let actual = process(raw);
		let expected = doc!(1 Block::code(None, ""));

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn empty_code_block_with_lang() {
		let raw = "```lang\n```";
		let actual = process(raw);
		let expected = doc!(1 Block::code(Some("lang"), ""));

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn code_block_no_lang() {
		let raw = "```\nsome code\n```";
		let actual = process(raw);
		let expected = doc!(1 Block::code(None, "some code"));

		assert_eq!(actual, expected)
	}

	#[test]
	pub fn code_block_with_lang() {
		let raw = "```html\n<p>paragraph</p>\n```";
		let actual = process(raw);
		let expected = doc!(1 Block::code(Some("html"), "<p>paragraph</p>"));

		assert_eq!(actual, expected)
	}

	const ALL: &'static str = "
# Header h1
Paragraph!
- Item 1
- Item 2

## Header h2
```html
<p>some html</p>
```
";

	#[test]
	pub fn all_together_now() {
		let actual = process(ALL);
		let expected_blocks = vec![
			Block::header(1, "Header h1"),
			Block::para("Paragraph!"),
			Block::list(vec![" Item 1", " Item 2"]),
			Block::header(2, "Header h2"),
			Block::code(Some("html"), "<p>some html</p>"),
		];

		assert_eq!(
			actual,
			Document {
				blocks: expected_blocks
			}
		)
	}
}