use std::fmt::{self, Display, Formatter};

use proc_macro2::TokenStream;
use proc_macro2_diagnostics::{Diagnostic, SpanDiagnosticExt};
use quote::ToTokens;
use syn::{
    braced, bracketed,
    ext::IdentExt,
    parenthesized,
    parse::{Lookahead1, Parse, ParseStream},
    punctuated::{Pair, Punctuated},
    spanned::Spanned,
    token::{
        At, Brace, Bracket, Colon, Comma, Dot, Else, Eq, FatArrow, For, If, In, Let, Match, Minus,
        Paren, Pound, Question, Semi, Slash, While,
    },
    Error, Expr, Ident, Lit, LitBool, LitInt, LitStr, Local, Pat, Stmt,
};

#[derive(Debug, Clone)]
pub struct Markups<E> {
    pub markups: Vec<Markup<E>>,
}

impl<E: MaybeElement> DiagnosticParse for Markups<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let mut markups = Vec::new();
        while !input.is_empty() {
            markups.push(Markup::diagnostic_parse_in_block(input, diagnostics)?)
        }
        Ok(Self { markups })
    }
}

impl<E: ToTokens> ToTokens for Markups<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        for markup in &self.markups {
            markup.to_tokens(tokens);
        }
    }
}

#[derive(Debug, Clone)]
#[allow(clippy::large_enum_variant)]
pub enum Markup<E> {
    Block(Block<E>),
    Lit(HtmlLit),
    Splice { paren_token: Paren, expr: Expr },
    Element(E),
    ControlFlow(ControlFlow<E>),
    Semi(Semi),
}

impl<E: MaybeElement> Markup<E> {
    pub fn diagnostic_parse_in_block(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        if input.peek(Let)
            || input.peek(If)
            || input.peek(Else)
            || input.peek(For)
            || input.peek(While)
            || input.peek(Match)
        {
            let kw = input.call(Ident::parse_any)?;
            diagnostics.push(
                kw.span()
                    .error(format!("found keyword `{kw}`"))
                    .help(format!("should this be `@{kw}`?")),
            );
        }

        let lookahead = input.lookahead1();

        if lookahead.peek(Brace) {
            input.diagnostic_parse(diagnostics).map(Self::Block)
        } else if lookahead.peek(Lit) {
            input.diagnostic_parse(diagnostics).map(Self::Lit)
        } else if lookahead.peek(Paren) {
            let content;
            Ok(Self::Splice {
                paren_token: parenthesized!(content in input),
                expr: content.parse()?,
            })
        } else if let Some(parse_element) = E::should_parse(&lookahead) {
            parse_element(input, diagnostics).map(Self::Element)
        } else if lookahead.peek(At) {
            input.diagnostic_parse(diagnostics).map(Self::ControlFlow)
        } else if lookahead.peek(Semi) {
            input.parse().map(Self::Semi)
        } else {
            Err(lookahead.error())
        }
    }
}

impl<E: MaybeElement> DiagnosticParse for Markup<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let markup = Self::diagnostic_parse_in_block(input, diagnostics)?;

        if let Self::ControlFlow(ControlFlow {
            kind: ControlFlowKind::Let(_),
            ..
        }) = &markup
        {
            diagnostics.push(
                markup
                    .span()
                    .error("`@let` bindings are only allowed inside blocks"),
            )
        }

        Ok(markup)
    }
}

impl<E: ToTokens> ToTokens for Markup<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Block(block) => block.to_tokens(tokens),
            Self::Lit(lit) => lit.to_tokens(tokens),
            Self::Splice { paren_token, expr } => {
                paren_token.surround(tokens, |tokens| {
                    expr.to_tokens(tokens);
                });
            }
            Self::Element(element) => element.to_tokens(tokens),
            Self::ControlFlow(control_flow) => control_flow.to_tokens(tokens),
            Self::Semi(semi) => semi.to_tokens(tokens),
        }
    }
}

/// Represents a context that may or may not allow elements.
///
/// An attribute accepts almost the same syntax as an element body, except child elements aren't
/// allowed. To enable code reuse, introduce a trait that abstracts over whether an element is
/// allowed or not.
pub trait MaybeElement: Sized + ToTokens {
    /// If an element can be parsed here, returns `Some` with a parser for the rest of the element.
    fn should_parse(lookahead: &Lookahead1<'_>) -> Option<DiagnosticParseFn<Self>>;
}

/// An implementation of `DiagnosticParse::diagnostic_parse`.
pub type DiagnosticParseFn<T> = fn(ParseStream, &mut Vec<Diagnostic>) -> syn::Result<T>;

/// Represents an attribute context, where elements are disallowed.
#[derive(Debug, Clone)]
pub enum NoElement {}

impl MaybeElement for NoElement {
    fn should_parse(
        _lookahead: &Lookahead1<'_>,
    ) -> Option<fn(ParseStream, &mut Vec<Diagnostic>) -> syn::Result<Self>> {
        None
    }
}

impl ToTokens for NoElement {
    fn to_tokens(&self, _tokens: &mut TokenStream) {
        match *self {}
    }
}

#[derive(Debug, Clone)]
pub struct Element {
    pub name: Option<HtmlName>,
    pub attrs: Vec<Attribute>,
    pub body: ElementBody,
}

impl From<NoElement> for Element {
    fn from(value: NoElement) -> Self {
        match value {}
    }
}

impl MaybeElement for Element {
    fn should_parse(
        lookahead: &Lookahead1<'_>,
    ) -> Option<fn(ParseStream, &mut Vec<Diagnostic>) -> syn::Result<Self>> {
        if lookahead.peek(Ident::peek_any) || lookahead.peek(Dot) || lookahead.peek(Pound) {
            Some(Element::diagnostic_parse)
        } else {
            None
        }
    }
}

impl DiagnosticParse for Element {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            name: if input.peek(Ident::peek_any) {
                Some(input.diagnostic_parse(diagnostics)?)
            } else {
                None
            },
            attrs: {
                let mut id_pushed = false;
                let mut attrs = Vec::new();

                while input.peek(Ident::peek_any)
                    || input.peek(Lit)
                    || input.peek(Dot)
                    || input.peek(Pound)
                {
                    let attr = input.diagnostic_parse(diagnostics)?;

                    if let Attribute::Id { .. } = attr {
                        if id_pushed {
                            return Err(Error::new_spanned(
                                attr,
                                "duplicate id (`#`) attribute specified",
                            ));
                        }
                        id_pushed = true;
                    }

                    attrs.push(attr);
                }

                if !(input.peek(Brace) || input.peek(Semi) || input.peek(Slash)) {
                    let lookahead = input.lookahead1();

                    lookahead.peek(Ident::peek_any);
                    lookahead.peek(Lit);
                    lookahead.peek(Dot);
                    lookahead.peek(Pound);

                    lookahead.peek(Brace);
                    lookahead.peek(Semi);

                    return Err(lookahead.error());
                }

                attrs
            },
            body: input.diagnostic_parse(diagnostics)?,
        })
    }
}

impl ToTokens for Element {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        if let Some(name) = &self.name {
            name.to_tokens(tokens);
        }
        for attr in &self.attrs {
            attr.to_tokens(tokens);
        }
        self.body.to_tokens(tokens);
    }
}

#[derive(Debug, Clone)]
pub enum ElementBody {
    Void(Semi),
    Block(Block<Element>),
}

impl DiagnosticParse for ElementBody {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Semi) {
            input.parse().map(Self::Void)
        } else if lookahead.peek(Brace) {
            input.diagnostic_parse(diagnostics).map(Self::Block)
        } else if lookahead.peek(Slash) {
            diagnostics.push(
                input
                    .parse::<Slash>()?
                    .span()
                    .error("void elements must use `;`, not `/`")
                    .help("change this to `;`")
                    .help("see https://github.com/lambda-fairy/maud/pull/315 for details"),
            );

            Ok(Self::Void(<Semi>::default()))
        } else {
            Err(lookahead.error())
        }
    }
}

impl ToTokens for ElementBody {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Void(semi) => semi.to_tokens(tokens),
            Self::Block(block) => block.to_tokens(tokens),
        }
    }
}

#[derive(Debug, Clone)]
pub struct Block<E> {
    pub brace_token: Brace,
    pub markups: Markups<E>,
}

impl<E: MaybeElement> DiagnosticParse for Block<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let content;
        Ok(Self {
            brace_token: braced!(content in input),
            markups: content.diagnostic_parse(diagnostics)?,
        })
    }
}

impl<E: ToTokens> ToTokens for Block<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.brace_token.surround(tokens, |tokens| {
            self.markups.to_tokens(tokens);
        });
    }
}

#[derive(Debug, Clone)]
pub enum Attribute {
    Class {
        dot_token: Dot,
        name: HtmlNameOrMarkup,
        toggler: Option<Toggler>,
    },
    Id {
        pound_token: Pound,
        name: HtmlNameOrMarkup,
    },
    Named {
        name: HtmlName,
        attr_type: AttributeType,
    },
}

impl DiagnosticParse for Attribute {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Dot) {
            Ok(Self::Class {
                dot_token: input.parse()?,
                name: input.diagnostic_parse(diagnostics)?,
                toggler: {
                    let lookahead = input.lookahead1();

                    if lookahead.peek(Bracket) {
                        Some(input.diagnostic_parse(diagnostics)?)
                    } else {
                        None
                    }
                },
            })
        } else if lookahead.peek(Pound) {
            Ok(Self::Id {
                pound_token: input.parse()?,
                name: input.diagnostic_parse(diagnostics)?,
            })
        } else {
            let name = input.diagnostic_parse::<HtmlName>(diagnostics)?;

            if input.peek(Question) {
                input.parse::<Question>()?;
            }

            let fork = input.fork();

            let attr = Self::Named {
                name: name.clone(),
                attr_type: input.diagnostic_parse(diagnostics)?,
            };

            if fork.peek(Eq) && fork.peek2(LitBool) {
                diagnostics.push(
                    attr.span()
                        .error("attribute value must be a string")
                        .help(format!("to declare an empty attribute, omit the equals sign: `{name}`"))
                        .help(format!("to toggle the attribute, use square brackets: `{name}[some_boolean_flag]`"))
                );
            }

            Ok(attr)
        }
    }
}

impl ToTokens for Attribute {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Class {
                dot_token,
                name,
                toggler,
            } => {
                dot_token.to_tokens(tokens);
                name.to_tokens(tokens);
                if let Some(toggler) = toggler {
                    toggler.to_tokens(tokens);
                }
            }
            Self::Id { pound_token, name } => {
                pound_token.to_tokens(tokens);
                name.to_tokens(tokens);
            }
            Self::Named { name, attr_type } => {
                name.to_tokens(tokens);
                attr_type.to_tokens(tokens);
            }
        }
    }
}

#[derive(Debug, Clone)]
#[allow(clippy::large_enum_variant)]
pub enum HtmlNameOrMarkup {
    HtmlName(HtmlName),
    Markup(Markup<NoElement>),
}

impl DiagnosticParse for HtmlNameOrMarkup {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        if input.peek(Ident::peek_any) || input.peek(Lit) {
            input.diagnostic_parse(diagnostics).map(Self::HtmlName)
        } else {
            input.diagnostic_parse(diagnostics).map(Self::Markup)
        }
    }
}

impl Parse for HtmlNameOrMarkup {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Self::diagnostic_parse(input, &mut Vec::new())
    }
}

impl ToTokens for HtmlNameOrMarkup {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::HtmlName(name) => name.to_tokens(tokens),
            Self::Markup(markup) => markup.to_tokens(tokens),
        }
    }
}

impl Display for HtmlNameOrMarkup {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::HtmlName(name) => name.fmt(f),
            Self::Markup(markup) => markup.to_token_stream().fmt(f),
        }
    }
}

#[derive(Debug, Clone)]
#[allow(clippy::large_enum_variant)]
pub enum AttributeType {
    Normal {
        eq_token: Eq,
        value: Markup<NoElement>,
    },
    Optional {
        eq_token: Eq,
        toggler: Toggler,
    },
    Empty(Option<Toggler>),
}

impl DiagnosticParse for AttributeType {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Eq) {
            let eq_token = input.parse()?;

            if input.peek(Bracket) {
                Ok(Self::Optional {
                    eq_token,
                    toggler: input.diagnostic_parse(diagnostics)?,
                })
            } else {
                Ok(Self::Normal {
                    eq_token,
                    value: input.diagnostic_parse(diagnostics)?,
                })
            }
        } else if lookahead.peek(Bracket) {
            Ok(Self::Empty(Some(input.diagnostic_parse(diagnostics)?)))
        } else {
            Ok(Self::Empty(None))
        }
    }
}

impl ToTokens for AttributeType {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Normal { eq_token, value } => {
                eq_token.to_tokens(tokens);
                value.to_tokens(tokens);
            }
            Self::Optional { eq_token, toggler } => {
                eq_token.to_tokens(tokens);
                toggler.to_tokens(tokens);
            }
            Self::Empty(toggler) => {
                if let Some(toggler) = toggler {
                    toggler.to_tokens(tokens);
                }
            }
        }
    }
}

#[derive(Debug, Clone)]
pub struct HtmlName {
    pub name: Punctuated<HtmlNameFragment, HtmlNamePunct>,
}

impl DiagnosticParse for HtmlName {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            name: {
                let mut punctuated = Punctuated::new();

                loop {
                    punctuated.push_value(input.diagnostic_parse(diagnostics)?);

                    if !(input.peek(Minus) || input.peek(Colon)) {
                        break;
                    }

                    let punct = input.diagnostic_parse(diagnostics)?;
                    punctuated.push_punct(punct);
                }

                punctuated
            },
        })
    }
}

impl Parse for HtmlName {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Self::diagnostic_parse(input, &mut Vec::new())
    }
}

impl ToTokens for HtmlName {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.name.to_tokens(tokens);
    }
}

impl Display for HtmlName {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        for pair in self.name.pairs() {
            match pair {
                Pair::Punctuated(fragment, punct) => {
                    fragment.fmt(f)?;
                    punct.fmt(f)?;
                }
                Pair::End(fragment) => {
                    fragment.fmt(f)?;
                }
            }
        }

        Ok(())
    }
}

#[derive(Debug, Clone)]
pub enum HtmlNameFragment {
    Ident(Ident),
    LitInt(LitInt),
    LitStr(LitStr),
    Empty,
}

impl DiagnosticParse for HtmlNameFragment {
    fn diagnostic_parse(
        input: ParseStream,
        _diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Ident::peek_any) {
            input.call(Ident::parse_any).map(Self::Ident)
        } else if lookahead.peek(LitInt) {
            input.parse().map(Self::LitInt)
        } else if lookahead.peek(LitStr) {
            input.parse().map(Self::LitStr)
        } else if lookahead.peek(Minus) || lookahead.peek(Colon) {
            Ok(Self::Empty)
        } else {
            Err(lookahead.error())
        }
    }
}

impl ToTokens for HtmlNameFragment {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Ident(ident) => ident.to_tokens(tokens),
            Self::LitInt(lit) => lit.to_tokens(tokens),
            Self::LitStr(lit) => lit.to_tokens(tokens),
            Self::Empty => {}
        }
    }
}

impl Display for HtmlNameFragment {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::Ident(ident) => ident.fmt(f),
            Self::LitInt(lit) => lit.fmt(f),
            Self::LitStr(lit) => lit.value().fmt(f),
            Self::Empty => Ok(()),
        }
    }
}

#[derive(Debug, Clone)]
pub struct HtmlLit {
    pub lit: LitStr,
}

impl DiagnosticParse for HtmlLit {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Lit) {
            let lit = input.parse()?;
            match lit {
                Lit::Str(lit) => Ok(Self { lit }),
                Lit::Int(lit) => {
                    diagnostics.push(
                        lit.span()
                            .error(format!(r#"literal must be double-quoted: `"{lit}"`"#)),
                    );
                    Ok(Self {
                        lit: LitStr::new("", lit.span()),
                    })
                }
                Lit::Float(lit) => {
                    diagnostics.push(
                        lit.span()
                            .error(format!(r#"literal must be double-quoted: `"{lit}"`"#)),
                    );
                    Ok(Self {
                        lit: LitStr::new("", lit.span()),
                    })
                }
                Lit::Char(lit) => {
                    diagnostics.push(lit.span().error(format!(
                        r#"literal must be double-quoted: `"{}"`"#,
                        lit.value()
                    )));
                    Ok(Self {
                        lit: LitStr::new("", lit.span()),
                    })
                }
                Lit::Bool(_) => {
                    // diagnostic handled earlier with more information
                    Ok(Self {
                        lit: LitStr::new("", lit.span()),
                    })
                }
                _ => {
                    diagnostics.push(lit.span().error("expected string"));
                    Ok(Self {
                        lit: LitStr::new("", lit.span()),
                    })
                }
            }
        } else {
            Err(lookahead.error())
        }
    }
}

impl ToTokens for HtmlLit {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.lit.to_tokens(tokens);
    }
}

impl Display for HtmlLit {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        self.lit.value().fmt(f)
    }
}

#[derive(Debug, Clone)]
pub enum HtmlNamePunct {
    Colon(Colon),
    Hyphen(Minus),
}

impl DiagnosticParse for HtmlNamePunct {
    fn diagnostic_parse(input: ParseStream, _: &mut Vec<Diagnostic>) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(Colon) {
            input.parse().map(Self::Colon)
        } else if lookahead.peek(Minus) {
            input.parse().map(Self::Hyphen)
        } else {
            Err(lookahead.error())
        }
    }
}

impl ToTokens for HtmlNamePunct {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::Colon(token) => token.to_tokens(tokens),
            Self::Hyphen(token) => token.to_tokens(tokens),
        }
    }
}

impl Display for HtmlNamePunct {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::Colon(_) => f.write_str(":"),
            Self::Hyphen(_) => f.write_str("-"),
        }
    }
}

#[derive(Debug, Clone)]
pub struct Toggler {
    pub bracket_token: Bracket,
    pub cond: Expr,
}

impl DiagnosticParse for Toggler {
    fn diagnostic_parse(input: ParseStream, _: &mut Vec<Diagnostic>) -> syn::Result<Self> {
        let content;
        Ok(Self {
            bracket_token: bracketed!(content in input),
            cond: content.parse()?,
        })
    }
}

impl ToTokens for Toggler {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.bracket_token.surround(tokens, |tokens| {
            self.cond.to_tokens(tokens);
        });
    }
}

#[derive(Debug, Clone)]
pub struct ControlFlow<E> {
    pub at_token: At,
    pub kind: ControlFlowKind<E>,
}

impl<E: MaybeElement> DiagnosticParse for ControlFlow<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            at_token: input.parse()?,
            kind: {
                let lookahead = input.lookahead1();

                if lookahead.peek(If) {
                    ControlFlowKind::If(input.diagnostic_parse(diagnostics)?)
                } else if lookahead.peek(For) {
                    ControlFlowKind::For(input.diagnostic_parse(diagnostics)?)
                } else if lookahead.peek(While) {
                    ControlFlowKind::While(input.diagnostic_parse(diagnostics)?)
                } else if lookahead.peek(Match) {
                    ControlFlowKind::Match(input.diagnostic_parse(diagnostics)?)
                } else if lookahead.peek(Let) {
                    let Stmt::Local(local) = input.parse()? else {
                        unreachable!()
                    };

                    ControlFlowKind::Let(local)
                } else {
                    return Err(lookahead.error());
                }
            },
        })
    }
}

impl<E: ToTokens> ToTokens for ControlFlow<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.at_token.to_tokens(tokens);
        match &self.kind {
            ControlFlowKind::Let(local) => local.to_tokens(tokens),
            ControlFlowKind::If(if_) => if_.to_tokens(tokens),
            ControlFlowKind::For(for_) => for_.to_tokens(tokens),
            ControlFlowKind::While(while_) => while_.to_tokens(tokens),
            ControlFlowKind::Match(match_) => match_.to_tokens(tokens),
        }
    }
}

#[derive(Debug, Clone)]
pub enum ControlFlowKind<E> {
    Let(Local),
    If(IfExpr<E>),
    For(ForExpr<E>),
    While(WhileExpr<E>),
    Match(MatchExpr<E>),
}

#[derive(Debug, Clone)]
pub struct IfExpr<E> {
    pub if_token: If,
    pub cond: Expr,
    pub then_branch: Block<E>,
    pub else_branch: Option<(At, Else, Box<IfOrBlock<E>>)>,
}

impl<E: MaybeElement> DiagnosticParse for IfExpr<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            if_token: input.parse()?,
            cond: input.call(Expr::parse_without_eager_brace)?,
            then_branch: input.diagnostic_parse(diagnostics)?,
            else_branch: {
                if input.peek(At) && input.peek2(Else) {
                    Some((
                        input.parse()?,
                        input.parse()?,
                        input.diagnostic_parse(diagnostics)?,
                    ))
                } else {
                    None
                }
            },
        })
    }
}

impl<E: ToTokens> ToTokens for IfExpr<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.if_token.to_tokens(tokens);
        self.cond.to_tokens(tokens);
        self.then_branch.to_tokens(tokens);
        if let Some((at_token, else_token, else_branch)) = &self.else_branch {
            at_token.to_tokens(tokens);
            else_token.to_tokens(tokens);
            else_branch.to_tokens(tokens);
        }
    }
}

#[derive(Debug, Clone)]
pub enum IfOrBlock<E> {
    If(IfExpr<E>),
    Block(Block<E>),
}

impl<E: MaybeElement> DiagnosticParse for IfOrBlock<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let lookahead = input.lookahead1();

        if lookahead.peek(If) {
            input.diagnostic_parse(diagnostics).map(Self::If)
        } else if lookahead.peek(Brace) {
            input.diagnostic_parse(diagnostics).map(Self::Block)
        } else {
            Err(lookahead.error())
        }
    }
}

impl<E: ToTokens> ToTokens for IfOrBlock<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Self::If(if_) => if_.to_tokens(tokens),
            Self::Block(block) => block.to_tokens(tokens),
        }
    }
}

#[derive(Debug, Clone)]
pub struct ForExpr<E> {
    pub for_token: For,
    pub pat: Pat,
    pub in_token: In,
    pub expr: Expr,
    pub body: Block<E>,
}

impl<E: MaybeElement> DiagnosticParse for ForExpr<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            for_token: input.parse()?,
            pat: input.call(Pat::parse_multi_with_leading_vert)?,
            in_token: input.parse()?,
            expr: input.call(Expr::parse_without_eager_brace)?,
            body: input.diagnostic_parse(diagnostics)?,
        })
    }
}

impl<E: ToTokens> ToTokens for ForExpr<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.for_token.to_tokens(tokens);
        self.pat.to_tokens(tokens);
        self.in_token.to_tokens(tokens);
        self.expr.to_tokens(tokens);
        self.body.to_tokens(tokens);
    }
}

#[derive(Debug, Clone)]
pub struct WhileExpr<E> {
    pub while_token: While,
    pub cond: Expr,
    pub body: Block<E>,
}

impl<E: MaybeElement> DiagnosticParse for WhileExpr<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            while_token: input.parse()?,
            cond: input.call(Expr::parse_without_eager_brace)?,
            body: input.diagnostic_parse(diagnostics)?,
        })
    }
}

impl<E: ToTokens> ToTokens for WhileExpr<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.while_token.to_tokens(tokens);
        self.cond.to_tokens(tokens);
        self.body.to_tokens(tokens);
    }
}

#[derive(Debug, Clone)]
pub struct MatchExpr<E> {
    pub match_token: Match,
    pub expr: Expr,
    pub brace_token: Brace,
    pub arms: Vec<MatchArm<E>>,
}

impl<E: MaybeElement> DiagnosticParse for MatchExpr<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        let match_token = input.parse()?;
        let expr = input.call(Expr::parse_without_eager_brace)?;

        let content;
        let brace_token = braced!(content in input);

        let mut arms = Vec::new();
        while !content.is_empty() {
            arms.push(content.diagnostic_parse(diagnostics)?);
        }

        Ok(Self {
            match_token,
            expr,
            brace_token,
            arms,
        })
    }
}

impl<E: ToTokens> ToTokens for MatchExpr<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.match_token.to_tokens(tokens);
        self.expr.to_tokens(tokens);
        self.brace_token.surround(tokens, |tokens| {
            for arm in &self.arms {
                arm.to_tokens(tokens);
            }
        });
    }
}

#[derive(Debug, Clone)]
pub struct MatchArm<E> {
    pub pat: Pat,
    pub guard: Option<(If, Expr)>,
    pub fat_arrow_token: FatArrow,
    pub body: Markup<E>,
    pub comma_token: Option<Comma>,
}

impl<E: MaybeElement> DiagnosticParse for MatchArm<E> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Self {
            pat: Pat::parse_multi_with_leading_vert(input)?,
            guard: {
                if input.peek(If) {
                    Some((input.parse()?, input.parse()?))
                } else {
                    None
                }
            },
            fat_arrow_token: input.parse()?,
            body: Markup::diagnostic_parse_in_block(input, diagnostics)?,
            comma_token: if input.peek(Comma) {
                Some(input.parse()?)
            } else {
                None
            },
        })
    }
}

impl<E: ToTokens> ToTokens for MatchArm<E> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.pat.to_tokens(tokens);
        if let Some((if_token, guard)) = &self.guard {
            if_token.to_tokens(tokens);
            guard.to_tokens(tokens);
        }
        self.fat_arrow_token.to_tokens(tokens);
        self.body.to_tokens(tokens);
        if let Some(comma_token) = &self.comma_token {
            comma_token.to_tokens(tokens);
        }
    }
}

pub trait DiagnosticParse: Sized {
    fn diagnostic_parse(input: ParseStream, diagnostics: &mut Vec<Diagnostic>)
        -> syn::Result<Self>;
}

impl<T: DiagnosticParse> DiagnosticParse for Box<T> {
    fn diagnostic_parse(
        input: ParseStream,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<Self> {
        Ok(Box::new(input.diagnostic_parse(diagnostics)?))
    }
}

trait DiagonsticParseExt: Sized {
    fn diagnostic_parse<T: DiagnosticParse>(
        self,
        diagnostics: &mut Vec<Diagnostic>,
    ) -> syn::Result<T>;
}

impl DiagonsticParseExt for ParseStream<'_> {
    fn diagnostic_parse<T>(self, diagnostics: &mut Vec<Diagnostic>) -> syn::Result<T>
    where
        T: DiagnosticParse,
    {
        T::diagnostic_parse(self, diagnostics)
    }
}
