use itertools::Itertools;
use orchid_base::error::{OrcErr, OrcErrv, OrcRes, mk_errv};
use orchid_base::interner::is;
use orchid_base::location::SrcRange;
use orchid_base::name::Sym;
use orchid_base::sym;
use orchid_base::tree::{Paren, wrap_tokv};
use orchid_extension::gen_expr::sym_ref;
use orchid_extension::lexer::{LexContext, Lexer, err_not_applicable};
use orchid_extension::parser::p_tree2gen;
use orchid_extension::tree::{GenTok, GenTokTree, ref_tok, x_tok};

use super::str_atom::IntStrAtom;

/// Reasons why [parse_string] might fail. See [StringError]
#[derive(Clone)]
enum StringErrorKind {
	/// A unicode escape sequence wasn't followed by 4 hex digits
	NotHex,
	/// A unicode escape sequence contained an unassigned code point
	BadCodePoint,
	/// An unrecognized escape sequence was found
	BadEscSeq,
}

/// Error produced by [parse_string]
#[derive(Clone)]
struct StringError {
	/// Character where the error occured
	pos: u32,
	/// Reason for the error
	kind: StringErrorKind,
}

impl StringError {
	/// Convert into project error for reporting
	pub async fn into_proj(self, path: &Sym, pos: u32) -> OrcErrv {
		let start = pos + self.pos;
		mk_errv(
			is("Failed to parse string").await,
			match self.kind {
				StringErrorKind::NotHex => "Expected a hex digit",
				StringErrorKind::BadCodePoint => "The specified number is not a Unicode code point",
				StringErrorKind::BadEscSeq => "Unrecognized escape sequence",
			},
			[SrcRange::new(start..start + 1, path).pos()],
		)
	}
}

/// Process escape sequences in a string literal
fn parse_string(str: &str) -> Result<String, StringError> {
	let mut target = String::new();
	let mut iter = str.char_indices().map(|(i, c)| (i as u32, c));
	while let Some((_, c)) = iter.next() {
		if c != '\\' {
			target.push(c);
			continue;
		}
		let (mut pos, code) = iter.next().expect("lexer would have continued");
		let next = match code {
			c @ ('\\' | '"' | '\'' | '$') => c,
			'b' => '\x08',
			'f' => '\x0f',
			'n' => '\n',
			'r' => '\r',
			't' => '\t',
			'\n' => 'skipws: loop {
				match iter.next() {
					None => return Ok(target),
					Some((_, c)) =>
						if !c.is_whitespace() {
							break 'skipws c;
						},
				}
			},
			'u' => {
				let acc = ((0..4).rev())
					.map(|radical| {
						let (j, c) = (iter.next()).ok_or(StringError { pos, kind: StringErrorKind::NotHex })?;
						pos = j;
						let b = u32::from_str_radix(&String::from(c), 16)
							.map_err(|_| StringError { pos, kind: StringErrorKind::NotHex })?;
						Ok(16u32.pow(radical) + b)
					})
					.fold_ok(0, u32::wrapping_add)?;
				char::from_u32(acc).ok_or(StringError { pos, kind: StringErrorKind::BadCodePoint })?
			},
			_ => return Err(StringError { pos, kind: StringErrorKind::BadEscSeq }),
		};
		target.push(next);
	}
	Ok(target)
}

#[derive(Debug, Default)]
pub struct StringLexer;
impl Lexer for StringLexer {
	const CHAR_FILTER: &'static [std::ops::RangeInclusive<char>] = &['"'..='"', '`'..='`'];
	async fn lex<'a>(all: &'a str, lctx: &'a LexContext<'a>) -> OrcRes<(&'a str, GenTokTree)> {
		let Some(mut tail) = all.strip_prefix('"') else {
			return Err(err_not_applicable().await);
		};
		let mut ret = None;
		let mut cur = String::new();
		let mut errors = vec![];
		async fn str_to_gen<'a>(
			str: &mut String,
			tail: &str,
			err: &mut Vec<OrcErr>,
			ctx: &'a LexContext<'a>,
		) -> GenTokTree {
			let str_val_res = parse_string(&str.split_off(0));
			if let Err(e) = &str_val_res {
				err.extend(e.clone().into_proj(ctx.src(), ctx.pos(tail) - str.len() as u32).await);
			}
			let str_val = str_val_res.unwrap_or_default();
			x_tok(IntStrAtom::from(is(&str_val).await)).await.at(ctx.pos_lt(str.len() as u32, tail))
				as GenTokTree
		}
		let add_frag = |prev: Option<GenTokTree>, new: GenTokTree| async {
			let Some(prev) = prev else { return new };
			let concat_fn =
				ref_tok(sym!(std::string::concat)).await.at(SrcRange::zw(prev.sr.path(), prev.sr.start()));
			wrap_tokv([concat_fn, prev, new])
		};
		loop {
			if let Some(rest) = tail.strip_prefix('"') {
				return Ok((
					rest,
					add_frag(ret, str_to_gen(&mut cur, tail, &mut errors, lctx).await).await,
				));
			} else if let Some(rest) = tail.strip_prefix('$') {
				ret = Some(add_frag(ret, str_to_gen(&mut cur, tail, &mut errors, lctx).await).await);
				let (new_tail, tree) = lctx.recurse(rest).await?;
				tail = new_tail;
				// wrap the received token in a call to to_str
				let to_str = sym_ref(sym!(std::string::to_str));
				let sr = tree.sr();
				let inj_to_str_tok = GenTok::NewExpr(to_str).at(sr.map_range(|_| sr.start()..sr.start()));
				let to_str_call = GenTok::S(Paren::Round, vec![inj_to_str_tok, p_tree2gen(tree)]).at(sr);
				ret = Some(add_frag(ret, to_str_call).await);
			} else if tail.starts_with('\\') {
				// parse_string will deal with it, we just have to skip the next char
				tail = &tail[2..];
			} else {
				let mut ch = tail.chars();
				if let Some(c) = ch.next() {
					cur.push(c);
					tail = ch.as_str();
				} else {
					let range = lctx.pos(all)..lctx.pos("");
					return Err(mk_errv(is("No string end").await, "String never terminated with \"", [
						SrcRange::new(range.clone(), lctx.src()),
					]));
				}
			}
		}
	}
}