use std::borrow::Cow;
use std::str;

use nom::branch::alt;
use nom::bytes::complete::{tag, take_until};
use nom::character::complete::char;
use nom::combinator::{
    complete, consumed, cut, eof, map, map_res, not, opt, peek, recognize, value,
};
use nom::error::{Error, ErrorKind};
use nom::error_position;
use nom::multi::{fold_many0, many0, many1, separated_list0, separated_list1};
use nom::sequence::{delimited, pair, preceded, terminated, tuple};

use crate::{ErrorContext, ParseResult};

use super::{
    bool_lit, char_lit, identifier, is_ws, keyword, num_lit, path_or_identifier, skip_till,
    str_lit, ws, Expr, PathOrIdentifier, State,
};

#[derive(Debug, PartialEq)]
pub enum Node<'a> {
    Lit(Lit<'a>),
    Comment(Comment<'a>),
    Expr(Ws, Expr<'a>),
    Call(Call<'a>),
    Let(Let<'a>),
    If(If<'a>),
    Match(Match<'a>),
    Loop(Box<Loop<'a>>),
    Extends(Extends<'a>),
    BlockDef(BlockDef<'a>),
    Include(Include<'a>),
    Import(Import<'a>),
    Macro(Macro<'a>),
    Raw(Raw<'a>),
    Break(Ws),
    Continue(Ws),
}

impl<'a> Node<'a> {
    pub(super) fn many(i: &'a str, s: &State<'_>) -> ParseResult<'a, Vec<Self>> {
        complete(many0(alt((
            map(|i| Lit::parse(i, s), Self::Lit),
            map(|i| Comment::parse(i, s), Self::Comment),
            |i| Self::expr(i, s),
            |i| Self::parse(i, s),
        ))))(i)
    }

    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = delimited(
            |i| s.tag_block_start(i),
            alt((
                map(|i| Call::parse(i, s), Self::Call),
                map(|i| Let::parse(i, s), Self::Let),
                map(|i| If::parse(i, s), Self::If),
                map(|i| Loop::parse(i, s), |l| Self::Loop(Box::new(l))),
                map(|i| Match::parse(i, s), Self::Match),
                map(Extends::parse, Self::Extends),
                map(Include::parse, Self::Include),
                map(Import::parse, Self::Import),
                map(|i| BlockDef::parse(i, s), Self::BlockDef),
                map(|i| Macro::parse(i, s), Self::Macro),
                map(|i| Raw::parse(i, s), Self::Raw),
                |i| Self::r#break(i, s),
                |i| Self::r#continue(i, s),
            )),
            cut(|i| s.tag_block_end(i)),
        );

        s.nest(i)?;
        let result = p(i);
        s.leave();

        result
    }

    fn r#break(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("break")),
            opt(Whitespace::parse),
        ));
        let (j, (pws, _, nws)) = p(i)?;
        if !s.is_in_loop() {
            return Err(nom::Err::Failure(error_position!(i, ErrorKind::Tag)));
        }
        Ok((j, Self::Break(Ws(pws, nws))))
    }

    fn r#continue(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("continue")),
            opt(Whitespace::parse),
        ));
        let (j, (pws, _, nws)) = p(i)?;
        if !s.is_in_loop() {
            return Err(nom::Err::Failure(error_position!(i, ErrorKind::Tag)));
        }
        Ok((j, Self::Continue(Ws(pws, nws))))
    }

    fn expr(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            |i| s.tag_expr_start(i),
            cut(tuple((
                opt(Whitespace::parse),
                ws(|i| Expr::parse(i, s.level.get())),
                opt(Whitespace::parse),
                |i| s.tag_expr_end(i),
            ))),
        ));
        let (i, (_, (pws, expr, nws, _))) = p(i)?;
        Ok((i, Self::Expr(Ws(pws, nws), expr)))
    }
}

#[derive(Clone, Debug, PartialEq)]
pub enum Target<'a> {
    Name(&'a str),
    Tuple(Vec<&'a str>, Vec<Target<'a>>),
    Struct(Vec<&'a str>, Vec<(&'a str, Target<'a>)>),
    NumLit(&'a str),
    StrLit(&'a str),
    CharLit(&'a str),
    BoolLit(&'a str),
    Path(Vec<&'a str>),
    OrChain(Vec<Target<'a>>),
}

impl<'a> Target<'a> {
    /// Parses multiple targets with `or` separating them
    pub(super) fn parse(i: &'a str) -> ParseResult<'a, Self> {
        map(
            separated_list1(ws(tag("or")), Self::parse_one),
            |mut opts| match opts.len() {
                1 => opts.pop().unwrap(),
                _ => Self::OrChain(opts),
            },
        )(i)
    }

    /// Parses a single target without an `or`, unless it is wrapped in parentheses.
    fn parse_one(i: &'a str) -> ParseResult<'a, Self> {
        let mut opt_opening_paren = map(opt(ws(char('('))), |o| o.is_some());
        let mut opt_closing_paren = map(opt(ws(char(')'))), |o| o.is_some());
        let mut opt_opening_brace = map(opt(ws(char('{'))), |o| o.is_some());

        let (i, lit) = opt(Self::lit)(i)?;
        if let Some(lit) = lit {
            return Ok((i, lit));
        }

        // match tuples and unused parentheses
        let (i, target_is_tuple) = opt_opening_paren(i)?;
        if target_is_tuple {
            let (i, is_empty_tuple) = opt_closing_paren(i)?;
            if is_empty_tuple {
                return Ok((i, Self::Tuple(Vec::new(), Vec::new())));
            }

            let (i, first_target) = Self::parse(i)?;
            let (i, is_unused_paren) = opt_closing_paren(i)?;
            if is_unused_paren {
                return Ok((i, first_target));
            }

            let mut targets = vec![first_target];
            let (i, _) = cut(tuple((
                fold_many0(
                    preceded(ws(char(',')), Self::parse),
                    || (),
                    |_, target| {
                        targets.push(target);
                    },
                ),
                opt(ws(char(','))),
                ws(cut(char(')'))),
            )))(i)?;
            return Ok((i, Self::Tuple(Vec::new(), targets)));
        }

        let path = |i| {
            map_res(path_or_identifier, |v| match v {
                PathOrIdentifier::Path(v) => Ok(v),
                PathOrIdentifier::Identifier(v) => Err(v),
            })(i)
        };

        // match structs
        let (i, path) = opt(path)(i)?;
        if let Some(path) = path {
            let i_before_matching_with = i;
            let (i, _) = opt(ws(keyword("with")))(i)?;

            let (i, is_unnamed_struct) = opt_opening_paren(i)?;
            if is_unnamed_struct {
                let (i, targets) = alt((
                    map(char(')'), |_| Vec::new()),
                    terminated(
                        cut(separated_list1(ws(char(',')), Self::parse)),
                        pair(opt(ws(char(','))), ws(cut(char(')')))),
                    ),
                ))(i)?;
                return Ok((i, Self::Tuple(path, targets)));
            }

            let (i, is_named_struct) = opt_opening_brace(i)?;
            if is_named_struct {
                let (i, targets) = alt((
                    map(char('}'), |_| Vec::new()),
                    terminated(
                        cut(separated_list1(ws(char(',')), Self::named)),
                        pair(opt(ws(char(','))), ws(cut(char('}')))),
                    ),
                ))(i)?;
                return Ok((i, Self::Struct(path, targets)));
            }

            return Ok((i_before_matching_with, Self::Path(path)));
        }

        // neither literal nor struct nor path
        let (new_i, name) = identifier(i)?;
        Ok((new_i, Self::verify_name(i, name)?))
    }

    fn lit(i: &'a str) -> ParseResult<'a, Self> {
        alt((
            map(str_lit, Self::StrLit),
            map(char_lit, Self::CharLit),
            map(num_lit, Self::NumLit),
            map(bool_lit, Self::BoolLit),
        ))(i)
    }

    fn named(init_i: &'a str) -> ParseResult<'a, (&str, Self)> {
        let (i, (src, target)) =
            pair(identifier, opt(preceded(ws(char(':')), Self::parse)))(init_i)?;
        let target = match target {
            Some(target) => target,
            None => Self::verify_name(init_i, src)?,
        };
        Ok((i, (src, target)))
    }

    fn verify_name(input: &'a str, name: &'a str) -> Result<Self, nom::Err<ErrorContext<'a>>> {
        match name {
            "self" | "writer" => Err(nom::Err::Failure(ErrorContext {
                input,
                message: Some(Cow::Owned(format!("Cannot use `{name}` as a name"))),
            })),
            _ => Ok(Self::Name(name)),
        }
    }
}

#[derive(Debug, PartialEq)]
pub struct When<'a> {
    pub ws: Ws,
    pub target: Target<'a>,
    pub nodes: Vec<Node<'a>>,
}

impl<'a> When<'a> {
    fn r#match(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            |i| s.tag_block_start(i),
            opt(Whitespace::parse),
            ws(keyword("else")),
            cut(tuple((
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                cut(|i| Node::many(i, s)),
            ))),
        ));
        let (i, (_, pws, _, (nws, _, nodes))) = p(i)?;
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                target: Target::Name("_"),
                nodes,
            },
        ))
    }

    #[allow(clippy::self_named_constructors)]
    fn when(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            |i| s.tag_block_start(i),
            opt(Whitespace::parse),
            ws(keyword("when")),
            cut(tuple((
                ws(Target::parse),
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                cut(|i| Node::many(i, s)),
            ))),
        ));
        let (i, (_, pws, _, (target, nws, _, nodes))) = p(i)?;
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                target,
                nodes,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Cond<'a> {
    pub ws: Ws,
    pub cond: Option<CondTest<'a>>,
    pub nodes: Vec<Node<'a>>,
}

impl<'a> Cond<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            |i| s.tag_block_start(i),
            opt(Whitespace::parse),
            ws(alt((keyword("else"), |i| {
                let _ = keyword("elif")(i)?;
                Err(nom::Err::Failure(ErrorContext {
                    input: i,
                    message: Some(Cow::Borrowed(
                        "unknown `elif` keyword; did you mean `else if`?",
                    )),
                }))
            }))),
            cut(tuple((
                opt(|i| CondTest::parse(i, s)),
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                cut(|i| Node::many(i, s)),
            ))),
        ));
        let (i, (_, pws, _, (cond, nws, _, nodes))) = p(i)?;
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                cond,
                nodes,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct CondTest<'a> {
    pub target: Option<Target<'a>>,
    pub expr: Expr<'a>,
}

impl<'a> CondTest<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = preceded(
            ws(keyword("if")),
            cut(tuple((
                opt(delimited(
                    ws(alt((keyword("let"), keyword("set")))),
                    ws(Target::parse),
                    ws(char('=')),
                )),
                ws(|i| Expr::parse(i, s.level.get())),
            ))),
        );
        let (i, (target, expr)) = p(i)?;
        Ok((i, Self { target, expr }))
    }
}

#[derive(Clone, Copy, Debug, PartialEq)]
pub enum Whitespace {
    Preserve,
    Suppress,
    Minimize,
}

impl Whitespace {
    fn parse(i: &str) -> ParseResult<'_, Self> {
        alt((
            value(Self::Preserve, char('+')),
            value(Self::Suppress, char('-')),
            value(Self::Minimize, char('~')),
        ))(i)
    }
}

#[derive(Debug, PartialEq)]
pub struct Loop<'a> {
    pub ws1: Ws,
    pub var: Target<'a>,
    pub iter: Expr<'a>,
    pub cond: Option<Expr<'a>>,
    pub body: Vec<Node<'a>>,
    pub ws2: Ws,
    pub else_nodes: Vec<Node<'a>>,
    pub ws3: Ws,
}

impl<'a> Loop<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        fn content<'a>(i: &'a str, s: &State<'_>) -> ParseResult<'a, Vec<Node<'a>>> {
            s.enter_loop();
            let result = Node::many(i, s);
            s.leave_loop();
            result
        }

        let if_cond = preceded(
            ws(keyword("if")),
            cut(ws(|i| Expr::parse(i, s.level.get()))),
        );
        let else_block = |i| {
            let mut p = preceded(
                ws(keyword("else")),
                cut(tuple((
                    opt(Whitespace::parse),
                    delimited(
                        |i| s.tag_block_end(i),
                        |i| Node::many(i, s),
                        |i| s.tag_block_start(i),
                    ),
                    opt(Whitespace::parse),
                ))),
            );
            let (i, (pws, nodes, nws)) = p(i)?;
            Ok((i, (pws, nodes, nws)))
        };
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("for")),
            cut(tuple((
                ws(Target::parse),
                ws(keyword("in")),
                cut(tuple((
                    ws(|i| Expr::parse(i, s.level.get())),
                    opt(if_cond),
                    opt(Whitespace::parse),
                    |i| s.tag_block_end(i),
                    cut(tuple((
                        |i| content(i, s),
                        cut(tuple((
                            |i| s.tag_block_start(i),
                            opt(Whitespace::parse),
                            opt(else_block),
                            ws(keyword("endfor")),
                            opt(Whitespace::parse),
                        ))),
                    ))),
                ))),
            ))),
        ));
        let (i, (pws1, _, (var, _, (iter, cond, nws1, _, (body, (_, pws2, else_block, _, nws2)))))) =
            p(i)?;
        let (nws3, else_block, pws3) = else_block.unwrap_or_default();
        Ok((
            i,
            Self {
                ws1: Ws(pws1, nws1),
                var,
                iter,
                cond,
                body,
                ws2: Ws(pws2, nws3),
                else_nodes: else_block,
                ws3: Ws(pws3, nws2),
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Macro<'a> {
    pub ws1: Ws,
    pub name: &'a str,
    pub args: Vec<&'a str>,
    pub nodes: Vec<Node<'a>>,
    pub ws2: Ws,
}

impl<'a> Macro<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        fn parameters(i: &str) -> ParseResult<'_, Vec<&str>> {
            delimited(
                ws(char('(')),
                separated_list0(char(','), ws(identifier)),
                ws(char(')')),
            )(i)
        }

        let mut start = tuple((
            opt(Whitespace::parse),
            ws(keyword("macro")),
            cut(tuple((
                ws(identifier),
                opt(ws(parameters)),
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
            ))),
        ));
        let (i, (pws1, _, (name, params, nws1, _))) = start(i)?;

        let mut end = cut(tuple((
            |i| Node::many(i, s),
            cut(tuple((
                |i| s.tag_block_start(i),
                opt(Whitespace::parse),
                ws(keyword("endmacro")),
                cut(preceded(
                    opt(|before| {
                        let (after, end_name) = ws(identifier)(before)?;
                        check_end_name(before, after, name, end_name, "macro")
                    }),
                    opt(Whitespace::parse),
                )),
            ))),
        )));
        let (i, (contents, (_, pws2, _, nws2))) = end(i)?;

        if name == "super" {
            // TODO: yield a a better error message here
            return Err(ErrorContext::from_err(nom::Err::Failure(Error::new(
                i,
                ErrorKind::Fail,
            ))));
        }

        Ok((
            i,
            Self {
                ws1: Ws(pws1, nws1),
                name,
                args: params.unwrap_or_default(),
                nodes: contents,
                ws2: Ws(pws2, nws2),
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Import<'a> {
    pub ws: Ws,
    pub path: &'a str,
    pub scope: &'a str,
}

impl<'a> Import<'a> {
    fn parse(i: &'a str) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("import")),
            cut(tuple((
                ws(str_lit),
                ws(keyword("as")),
                cut(pair(ws(identifier), opt(Whitespace::parse))),
            ))),
        ));
        let (i, (pws, _, (path, _, (scope, nws)))) = p(i)?;
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                path,
                scope,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Call<'a> {
    pub ws: Ws,
    pub scope: Option<&'a str>,
    pub name: &'a str,
    pub args: Vec<Expr<'a>>,
}

impl<'a> Call<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("call")),
            cut(tuple((
                opt(tuple((ws(identifier), ws(tag("::"))))),
                ws(identifier),
                opt(ws(|nested| Expr::arguments(nested, s.level.get(), true))),
                opt(Whitespace::parse),
            ))),
        ));
        let (i, (pws, _, (scope, name, args, nws))) = p(i)?;
        let scope = scope.map(|(scope, _)| scope);
        let args = args.unwrap_or_default();
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                scope,
                name,
                args,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Match<'a> {
    pub ws1: Ws,
    pub expr: Expr<'a>,
    pub arms: Vec<When<'a>>,
    pub ws2: Ws,
}

impl<'a> Match<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("match")),
            cut(tuple((
                ws(|i| Expr::parse(i, s.level.get())),
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                cut(tuple((
                    ws(many0(ws(value((), |i| Comment::parse(i, s))))),
                    many1(|i| When::when(i, s)),
                    cut(tuple((
                        opt(|i| When::r#match(i, s)),
                        cut(tuple((
                            ws(|i| s.tag_block_start(i)),
                            opt(Whitespace::parse),
                            ws(keyword("endmatch")),
                            opt(Whitespace::parse),
                        ))),
                    ))),
                ))),
            ))),
        ));
        let (i, (pws1, _, (expr, nws1, _, (_, arms, (else_arm, (_, pws2, _, nws2)))))) = p(i)?;

        let mut arms = arms;
        if let Some(arm) = else_arm {
            arms.push(arm);
        }

        Ok((
            i,
            Self {
                ws1: Ws(pws1, nws1),
                expr,
                arms,
                ws2: Ws(pws2, nws2),
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct BlockDef<'a> {
    pub ws1: Ws,
    pub name: &'a str,
    pub nodes: Vec<Node<'a>>,
    pub ws2: Ws,
}

impl<'a> BlockDef<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut start = tuple((
            opt(Whitespace::parse),
            ws(keyword("block")),
            cut(tuple((ws(identifier), opt(Whitespace::parse), |i| {
                s.tag_block_end(i)
            }))),
        ));
        let (i, (pws1, _, (name, nws1, _))) = start(i)?;

        let mut end = cut(tuple((
            |i| Node::many(i, s),
            cut(tuple((
                |i| s.tag_block_start(i),
                opt(Whitespace::parse),
                ws(keyword("endblock")),
                cut(tuple((
                    opt(|before| {
                        let (after, end_name) = ws(identifier)(before)?;
                        check_end_name(before, after, name, end_name, "block")
                    }),
                    opt(Whitespace::parse),
                ))),
            ))),
        )));
        let (i, (nodes, (_, pws2, _, (_, nws2)))) = end(i)?;

        Ok((
            i,
            BlockDef {
                ws1: Ws(pws1, nws1),
                name,
                nodes,
                ws2: Ws(pws2, nws2),
            },
        ))
    }
}

fn check_end_name<'a>(
    before: &'a str,
    after: &'a str,
    name: &'a str,
    end_name: &'a str,
    kind: &str,
) -> ParseResult<'a> {
    if name == end_name {
        return Ok((after, end_name));
    }
    let message = if name.is_empty() && !end_name.is_empty() {
        format!("unexpected name `{end_name}` in `end{kind}` tag for unnamed `{kind}`")
    } else {
        format!("expected name `{name}` in `end{kind}` tag, found `{end_name}`")
    };
    Err(nom::Err::Failure(ErrorContext {
        input: before,
        message: Some(Cow::Owned(message)),
    }))
}

#[derive(Debug, PartialEq)]
pub struct Lit<'a> {
    pub lws: &'a str,
    pub val: &'a str,
    pub rws: &'a str,
}

impl<'a> Lit<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let p_start = alt((
            tag(s.syntax.block_start),
            tag(s.syntax.comment_start),
            tag(s.syntax.expr_start),
        ));

        let (i, _) = not(eof)(i)?;
        let (i, content) = opt(recognize(skip_till(p_start)))(i)?;
        let (i, content) = match content {
            Some("") => {
                // {block,comment,expr}_start follows immediately.
                return Err(nom::Err::Error(error_position!(i, ErrorKind::TakeUntil)));
            }
            Some(content) => (i, content),
            None => ("", i), // there is no {block,comment,expr}_start: take everything
        };
        Ok((i, Self::split_ws_parts(content)))
    }

    pub(crate) fn split_ws_parts(s: &'a str) -> Self {
        let trimmed_start = s.trim_start_matches(is_ws);
        let len_start = s.len() - trimmed_start.len();
        let trimmed = trimmed_start.trim_end_matches(is_ws);
        Self {
            lws: &s[..len_start],
            val: trimmed,
            rws: &trimmed_start[trimmed.len()..],
        }
    }
}

#[derive(Debug, PartialEq)]
pub struct Raw<'a> {
    pub ws1: Ws,
    pub lit: Lit<'a>,
    pub ws2: Ws,
}

impl<'a> Raw<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let endraw = tuple((
            |i| s.tag_block_start(i),
            opt(Whitespace::parse),
            ws(keyword("endraw")),
            opt(Whitespace::parse),
            peek(|i| s.tag_block_end(i)),
        ));

        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("raw")),
            cut(tuple((
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                consumed(skip_till(endraw)),
            ))),
        ));

        let (_, (pws1, _, (nws1, _, (contents, (i, (_, pws2, _, nws2, _)))))) = p(i)?;
        let lit = Lit::split_ws_parts(contents);
        let ws1 = Ws(pws1, nws1);
        let ws2 = Ws(pws2, nws2);
        Ok((i, Self { ws1, lit, ws2 }))
    }
}

#[derive(Debug, PartialEq)]
pub struct Let<'a> {
    pub ws: Ws,
    pub var: Target<'a>,
    pub val: Option<Expr<'a>>,
}

impl<'a> Let<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(alt((keyword("let"), keyword("set")))),
            cut(tuple((
                ws(Target::parse),
                opt(preceded(
                    ws(char('=')),
                    ws(|i| Expr::parse(i, s.level.get())),
                )),
                opt(Whitespace::parse),
            ))),
        ));
        let (i, (pws, _, (var, val, nws))) = p(i)?;

        Ok((
            i,
            Let {
                ws: Ws(pws, nws),
                var,
                val,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct If<'a> {
    pub ws: Ws,
    pub branches: Vec<Cond<'a>>,
}

impl<'a> If<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            |i| CondTest::parse(i, s),
            cut(tuple((
                opt(Whitespace::parse),
                |i| s.tag_block_end(i),
                cut(tuple((
                    |i| Node::many(i, s),
                    many0(|i| Cond::parse(i, s)),
                    cut(tuple((
                        |i| s.tag_block_start(i),
                        opt(Whitespace::parse),
                        ws(keyword("endif")),
                        opt(Whitespace::parse),
                    ))),
                ))),
            ))),
        ));

        let (i, (pws1, cond, (nws1, _, (nodes, elifs, (_, pws2, _, nws2))))) = p(i)?;
        let mut branches = vec![Cond {
            ws: Ws(pws1, nws1),
            cond: Some(cond),
            nodes,
        }];
        branches.extend(elifs);

        Ok((
            i,
            Self {
                ws: Ws(pws2, nws2),
                branches,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Include<'a> {
    pub ws: Ws,
    pub path: &'a str,
}

impl<'a> Include<'a> {
    fn parse(i: &'a str) -> ParseResult<'a, Self> {
        let mut p = tuple((
            opt(Whitespace::parse),
            ws(keyword("include")),
            cut(pair(ws(str_lit), opt(Whitespace::parse))),
        ));
        let (i, (pws, _, (path, nws))) = p(i)?;
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                path,
            },
        ))
    }
}

#[derive(Debug, PartialEq)]
pub struct Extends<'a> {
    pub path: &'a str,
}

impl<'a> Extends<'a> {
    fn parse(i: &'a str) -> ParseResult<'a, Self> {
        let (i, path) = preceded(ws(keyword("extends")), cut(ws(str_lit)))(i)?;
        Ok((i, Self { path }))
    }
}

#[derive(Debug, PartialEq)]
pub struct Comment<'a> {
    pub ws: Ws,
    pub content: &'a str,
}

impl<'a> Comment<'a> {
    fn parse(i: &'a str, s: &State<'_>) -> ParseResult<'a, Self> {
        fn body<'a>(mut i: &'a str, s: &State<'_>) -> ParseResult<'a> {
            let mut level = 0;
            loop {
                let (end, tail) = take_until(s.syntax.comment_end)(i)?;
                match take_until::<_, _, ErrorContext<'_>>(s.syntax.comment_start)(i) {
                    Ok((start, _)) if start.as_ptr() < end.as_ptr() => {
                        level += 1;
                        i = &start[2..];
                    }
                    _ if level > 0 => {
                        level -= 1;
                        i = &end[2..];
                    }
                    _ => return Ok((end, tail)),
                }
            }
        }

        let mut p = tuple((
            |i| s.tag_comment_start(i),
            cut(tuple((
                opt(Whitespace::parse),
                |i| body(i, s),
                |i| s.tag_comment_end(i),
            ))),
        ));
        let (i, (content, (pws, tail, _))) = p(i)?;
        let nws = if tail.ends_with('-') {
            Some(Whitespace::Suppress)
        } else if tail.ends_with('+') {
            Some(Whitespace::Preserve)
        } else if tail.ends_with('~') {
            Some(Whitespace::Minimize)
        } else {
            None
        };
        Ok((
            i,
            Self {
                ws: Ws(pws, nws),
                content,
            },
        ))
    }
}

/// First field is "minus/plus sign was used on the left part of the item".
///
/// Second field is "minus/plus sign was used on the right part of the item".
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Ws(pub Option<Whitespace>, pub Option<Whitespace>);