Doughlang/src/parser/pat.rs

use super::{PResult, PResultExt, Parse, ParseError, Parser};
use crate::{
    ast::*,
    token::{TKind, Token},
};

/// Precedence levels of value and type pattern expressions.
///
/// Lower (toward [Prec::Min]) precedence levels can contain
/// all higher (toward [Prec::Max]) precedence levels.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum Prec {
    /// The lowest precedence
    Min,
    /// "Alternate" pattern: `Pat | Pat`
    Alt,
    /// Tuple pattern: `Pat,+`
    Tuple,
    /// Type annotation: `Pat : Pat`
    Typed,
    /// Function pattern: `Pat -> Pat`
    Fn,
    /// Range pattern: `Pat .. Pat`, `Pat ..= Pat`
    Range,
    /// The highest precedence
    Max,
}

macro_rules! intify {
    ($enum:ident($value:path) = $min:ident, $max: ident, $($variant:ident),*$(,)?) => {
        #[expect(non_upper_case_globals)] {
        const $min: u32 = $enum::$min as _;
        const $max: u32 = $enum::$max as _;
        $(const $variant: u32 = $enum::$variant as _;)*
        match $value {
            ..=$min => $enum::$min,
            $($variant => $enum::$variant,)*
            $max.. => $enum::$max,
        }
    }};
}

impl Prec {
    const fn from_int(value: u32) -> Self {
        intify! {Prec(value) = Min, Max, Alt, Tuple, Typed, Fn, Range}
    }

    /// Returns the level of precedence higher than this one
    const fn next(self) -> Self {
        Self::from_int(self as u32 + 1)
    }
}

/// Tries to map the incoming Token to a [pattern operator](PatOp)
/// and its [precedence level](Prec)
fn from_infix(token: &Token) -> Option<(PatOp, Prec)> {
    Some(match token.kind {
        TKind::Arrow => (PatOp::Fn, Prec::Fn),
        TKind::Bar => (PatOp::Alt, Prec::Alt),
        TKind::Colon => (PatOp::Typed, Prec::Typed),
        TKind::Comma => (PatOp::Tuple, Prec::Tuple),
        TKind::DotDot => (PatOp::RangeEx, Prec::Range),
        TKind::DotDotEq => (PatOp::RangeIn, Prec::Range),
        _ => None?,
    })
}

impl<'t> Parse<'t> for Pat {
    type Prec = Prec;

    fn parse(p: &mut Parser<'t>, level: Prec) -> PResult<Self> {
        let tok = p.peek()?;

        // Prefix
        let mut head = match tok.kind {
            TKind::Fn => return p.consume().parse(Prec::Fn),
            TKind::True | TKind::False | TKind::Character | TKind::Integer | TKind::String => {
                Pat::Lit(p.parse(())?)
            }
            TKind::Bar => p.consume().parse(level)?,
            TKind::Bang => p.consume().then(Pat::Never),
            TKind::Amp => Pat::Op(PatOp::Ref, vec![p.consume().parse(Prec::Max)?]),
            TKind::Star => Pat::Op(PatOp::Ptr, vec![p.consume().parse(Prec::Max)?]),
            TKind::Mut => Pat::Op(PatOp::Mut, vec![p.consume().parse(Prec::Max)?]),
            TKind::Pub => Pat::Op(PatOp::Pub, vec![p.consume().parse(Prec::Max)?]),
            TKind::AmpAmp => Pat::Op(
                PatOp::Ref,
                vec![Pat::Op(PatOp::Ref, vec![p.consume().parse(Prec::Max)?])],
            ),
            TKind::Identifier => match tok.lexeme.str() {
                Some("_") => p.consume().then(Pat::Ignore),
                _ => {
                    let mut path: Path = p.parse(())?;
                    // TODO: make these postfix.
                    match p.peek().map(Token::kind) {
                        Ok(TKind::LParen) => Pat::NamedTuple(path, p.parse(Prec::Typed)?),
                        Ok(TKind::LCurly) if level <= Prec::Tuple.next() => Pat::NamedStruct(
                            path,
                            p.consume()
                                .opt(Prec::Tuple, TKind::RCurly)?
                                .unwrap_or_else(|| Box::new(Pat::Op(PatOp::Tuple, vec![]))),
                        ),
                        Ok(_) | Err(ParseError::EOF(_)) => match path.parts.len() {
                            1 => Self::Name(path.parts.pop().expect("name has 1 part")),
                            _ => Self::Path(path),
                        },
                        Err(e) => Err(e)?,
                    }
                }
            },
            TKind::Grave => Pat::MetId(p.consume().next()?.lexeme.to_string()),
            TKind::DotDot => Pat::Op(
                PatOp::Rest,
                // Identifier in Rest position always becomes binder
                match p.consume().peek().allow_eof()?.map(Token::kind) {
                    Some(TKind::Identifier) => vec![Pat::Name(
                        p.take_lexeme()
                            .expect("should have lexeme")
                            .string()
                            .expect("should be string"),
                    )],
                    Some(TKind::Grave | TKind::Integer | TKind::Character) => vec![p.parse(level)?],
                    _ => vec![],
                },
            ),
            TKind::DotDotEq => Pat::Op(
                PatOp::RangeIn,
                match p.consume().peek().allow_eof()?.map(Token::kind) {
                    Some(TKind::Grave | TKind::Integer | TKind::Character) => vec![p.parse(level)?],
                    _ => vec![],
                },
            ),
            TKind::LParen => Pat::Op(
                PatOp::Tuple,
                p.consume()
                    .list(vec![], Prec::Typed, TKind::Comma, TKind::RParen)?,
            ),
            TKind::LBrack => parse_array_pat(p)?,
            _ => Err(ParseError::NotPattern(tok.kind, tok.span))?,
        };

        while let Ok(Some(tok)) = p.peek().allow_eof()
            && let Some((op, prec)) = from_infix(tok)
            && level <= prec
        {
            let kind = tok.kind;
            head = match op {
                PatOp::Typed => Pat::Op(PatOp::Typed, vec![head, p.consume().parse(prec.next())?]),
                PatOp::Fn => Pat::Op(PatOp::Fn, vec![head, p.consume().parse(Prec::Fn.next())?]),
                op @ (PatOp::RangeEx | PatOp::RangeIn) => Pat::Op(
                    op,
                    match p.consume().peek().map(Token::kind) {
                        Ok(TKind::Integer | TKind::Character | TKind::Identifier) => {
                            vec![head, p.parse(prec.next())?]
                        }
                        _ => vec![head],
                    },
                ),
                op => Pat::Op(op, p.consume().list_bare(vec![head], prec.next(), kind)?),
            }
        }
        Ok(head)
    }
}

fn parse_array_pat(p: &mut Parser<'_>) -> PResult<Pat> {
    if p.consume().peek()?.kind == TKind::RBrack {
        p.consume();
        return Ok(Pat::Op(PatOp::Slice, vec![]));
    }

    let item = p.parse(Prec::Tuple)?;
    let repeat = p.opt_if(Prec::Tuple, TKind::Semi)?;
    p.expect(TKind::RBrack)?;

    Ok(match (repeat, item) {
        (Some(repeat), item) => Pat::Op(PatOp::ArRep, vec![item, repeat]),
        (None, Pat::Op(PatOp::Tuple, items)) => Pat::Op(PatOp::Slice, items),
        (None, item) => Pat::Op(PatOp::Slice, vec![item]),
    })
}