m2-syn 0.2.0

//! Parsing boundaries over the shared raw token representation.
//!
//! [`Parser`] consumes a whole [`TokenStream`] and is the backend interface.
//! [`Parse`] consumes one value from a [`ParseStream`] and is the generated
//! dual of [`crate::Lex`]. Both native parsing and generated token parsing use the
//! same cursor facts instead of maintaining parallel token arrays.

use std::error::Error;
use std::fmt::{Display, Formatter};
use std::iter::FusedIterator;
use std::rc::Rc;

use crate::lexer::split_cells;
use crate::{
    ExternalCstNode, IdentToken, LexError, LiteralToken, Punct, Reconstruct, ReconstructError,
    SourceFile, Span, Spanned, Token, TokenGroup, TokenStream, TokenTree, Trivia, lex_str,
};

/// A parser backend that produces one typed syntax target.
///
/// Backends may construct `T` directly or adapt their concrete syntax tree
/// through [`ExternalCstNode`] and [`reconstruct`].
pub trait Parser<T = SourceFile> {
    /// The backend-specific parse failure.
    type Error: Error;

    /// Parses `tokens` into one typed syntax target.
    ///
    /// # Errors
    ///
    /// Returns the backend's [`Self::Error`] when it cannot produce `T`.
    fn parse_stream(&mut self, tokens: TokenStream) -> Result<T, Self::Error>;
}

/// Parses an existing token stream with an explicitly selected backend.
///
/// # Errors
///
/// Returns the selected backend's error when parsing fails.
pub fn parse_with<T, P>(parser: &mut P, tokens: TokenStream) -> Result<T, P::Error>
where
    P: Parser<T>,
{
    parser.parse_stream(tokens)
}

/// A typed syntax value that can consume itself from an M2 token cursor.
pub trait Parse: Sized {
    /// Consumes this value from `input`.
    ///
    /// # Errors
    ///
    /// Returns [`TokenParseError`] when the next tokens do not encode `Self`.
    fn parse(input: &mut ParseStream) -> Result<Self, TokenParseError>;
}

impl<T: Parse> Parse for Box<T> {
    fn parse(input: &mut ParseStream) -> Result<Self, TokenParseError> {
        T::parse(input).map(Self::new)
    }
}

impl Parse for TokenTree {
    fn parse(input: &mut ParseStream) -> Result<Self, TokenParseError> {
        input
            .next_raw_token()
            .ok_or_else(|| TokenParseError::unexpected_end("a token tree", input.eof_span()))
    }
}

macro_rules! parse_raw_variant {
    ($ty:ty, $variant:ident, $expected:literal) => {
        impl Parse for $ty {
            fn parse(input: &mut ParseStream) -> Result<Self, TokenParseError> {
                let token = TokenTree::parse(input)?;
                match token {
                    TokenTree::$variant(value) => Ok(value),
                    token => Err(TokenParseError::unexpected_token($expected, &token)),
                }
            }
        }
    };
}

parse_raw_variant!(TokenGroup, Group, "a delimited group");
parse_raw_variant!(LiteralToken, Literal, "a literal");
parse_raw_variant!(Punct, Punct, "punctuation");
parse_raw_variant!(IdentToken, Ident, "an identifier");
parse_raw_variant!(Trivia, Trivia, "trivia");

impl Parse for TokenStream {
    fn parse(input: &mut ParseStream) -> Result<Self, TokenParseError> {
        Ok(input.by_ref().collect())
    }
}

/// Parses one complete token stream as `T`.
///
/// # Errors
///
/// Returns [`TokenParseError`] when `T` cannot be parsed or leaves trailing
/// tokens.
pub fn parse2<T: Parse>(tokens: TokenStream) -> Result<T, TokenParseError> {
    let mut input = ParseStream::new(tokens);
    let value = T::parse(&mut input)?;
    input.finish()?;
    Ok(value)
}

/// Lexes one source cell and parses it as a typed syntax fragment.
///
/// This applies the grammar expressed by `T`'s [`Parse`] implementation. It
/// does not validate the fragment against the complete Macaulay2 grammar; use
/// [`crate::parse_native`] or [`crate::parse_file`] when parsing a source file.
/// A delimited fragment containing semicolons is still one cell because cell
/// boundaries are recognized only at the top level.
///
/// # Errors
///
/// Returns [`FragmentParseError`] when lexing fails, the source contains
/// multiple top-level cells, or the cell does not parse as `T`.
pub fn parse_fragment_str<T: Parse>(source: &str) -> Result<T, FragmentParseError> {
    let mut cells = split_cells(lex_str(source)?);
    if cells.len() > 1 {
        return Err(FragmentParseError::MultipleCells { count: cells.len() });
    }
    let tokens = cells
        .pop()
        .map_or_else(TokenStream::new, |cell| cell.stream);
    parse2(tokens).map_err(Into::into)
}

/// An error produced while lexing or parsing one typed syntax fragment.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum FragmentParseError {
    /// The source could not be converted into token trees.
    Lex(LexError),
    /// The token trees did not match the requested fragment type.
    Parse(TokenParseError),
    /// The source contained top-level cell boundaries and therefore was not a
    /// single fragment.
    MultipleCells {
        /// The number of cells produced by the lexer.
        count: usize,
    },
}

impl Display for FragmentParseError {
    fn fmt(&self, formatter: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Lex(error) => error.fmt(formatter),
            Self::Parse(error) => error.fmt(formatter),
            Self::MultipleCells { count } => {
                write!(formatter, "expected one source cell, found {count}")
            }
        }
    }
}

impl Error for FragmentParseError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            Self::Lex(error) => Some(error),
            Self::Parse(error) => Some(error),
            Self::MultipleCells { .. } => None,
        }
    }
}

impl From<LexError> for FragmentParseError {
    fn from(error: LexError) -> Self {
        Self::Lex(error)
    }
}

impl From<TokenParseError> for FragmentParseError {
    fn from(error: TokenParseError) -> Self {
        Self::Parse(error)
    }
}

/// One non-trivia token together with facts about the trivia immediately
/// before it. Parser backends share this view instead of maintaining their own
/// token vectors, cursor positions, and newline scanners.
#[derive(Debug, Clone)]
pub(crate) struct SignificantToken {
    pub(crate) token: TokenTree,
    pub(crate) crossed_newline: bool,
}

#[derive(Debug, Clone)]
pub(crate) enum Lookahead {
    Token(SignificantToken),
    End(Span),
}

impl Spanned for Lookahead {
    fn span(&self) -> Span {
        match self {
            Self::Token(token) => token.token.span(),
            Self::End(span) => span.clone(),
        }
    }
}

/// An owned, cheaply forkable cursor over one immutable token stream.
///
/// Forks share the token storage through [`Rc`] and copy only their current
/// position. Advancing a fork never changes the cursor from which it came.
#[derive(Clone)]
pub struct ParseStream {
    tokens: Rc<TokenStream>,
    position: usize,
    /// The zero-width position just past the last token.
    ///
    /// End of input is the absence of a token, which the cursor already
    /// reports. Only the position needs carrying, for diagnostics that point
    /// at where input ran out.
    eof_span: Span,
}

impl ParseStream {
    /// Creates a cursor at the beginning of `tokens`.
    pub fn new(tokens: TokenStream) -> Self {
        let eof_span = tokens
            .last()
            .and_then(|token| {
                let end = token.span().end()?;
                Some(Span::from_offset(end))
            })
            .unwrap_or_else(Span::detached);
        Self {
            tokens: Rc::new(tokens),
            position: 0,
            eof_span,
        }
    }

    /// Creates an independent cursor over the same immutable token storage.
    pub fn fork(&self) -> Self {
        self.clone()
    }

    /// Commits this cursor to a position reached by one of its forks.
    ///
    /// # Panics
    ///
    /// Panics if `fork` belongs to another token stream.
    pub fn advance_to(&mut self, fork: &Self) {
        assert!(
            Rc::ptr_eq(&self.tokens, &fork.tokens),
            "cannot advance to a cursor from another token stream"
        );
        self.position = fork.position;
    }

    /// Returns the next non-trivia token without consuming it.
    pub fn peek(&self) -> Option<&TokenTree> {
        self.tokens().get(self.significant_position())
    }

    pub(crate) fn lookahead(&self) -> Lookahead {
        let mut position = self.position;
        let mut crossed_newline = false;
        while let Some(TokenTree::Trivia(trivia)) = self.tokens().get(position) {
            crossed_newline |= trivia.contains_line_break();
            position += 1;
        }
        let Some(token) = self.tokens().get(position).cloned() else {
            return Lookahead::End(self.eof_span.clone());
        };
        Lookahead::Token(SignificantToken {
            token,
            crossed_newline,
        })
    }

    pub(crate) fn consume_lookahead(&mut self) -> Lookahead {
        let lookahead = self.lookahead();
        if matches!(lookahead, Lookahead::Token(_)) {
            self.set_position(self.significant_position() + 1);
        }
        lookahead
    }

    pub(crate) fn skip_trivia(&mut self) {
        self.set_position(self.significant_position());
    }

    pub(crate) fn eof_span(&self) -> Span {
        self.eof_span.clone()
    }

    /// Consumes and returns the next non-trivia token.
    pub fn next_token(&mut self) -> Option<TokenTree> {
        self.set_position(self.significant_position());
        self.next_raw_token()
    }

    /// Consumes the next token, including trivia.
    pub fn next_raw_token(&mut self) -> Option<TokenTree> {
        self.next()
    }

    /// Parses one generated typed token from its matching raw atom.
    ///
    /// # Errors
    ///
    /// Returns [`TokenParseError`] when input ends or the next significant token does
    /// not match `T`.
    pub fn parse_token<T: Token>(&mut self) -> Result<T, TokenParseError> {
        let token = self.next_token();
        let Some(token) = token else {
            return Err(TokenParseError::unexpected_end(
                T::SPELLING,
                self.eof_span(),
            ));
        };
        let error = TokenParseError::unexpected_token(T::SPELLING, &token);
        T::from_token_tree(token).ok_or(error)
    }

    /// Returns whether no significant token remains.
    pub fn is_eof(&self) -> bool {
        self.peek().is_none()
    }

    fn significant_position(&self) -> usize {
        let mut position = self.position;
        while matches!(self.tokens().get(position), Some(TokenTree::Trivia(_))) {
            position += 1;
        }
        position
    }

    fn tokens(&self) -> &TokenStream {
        &self.tokens
    }

    fn set_position(&mut self, position: usize) {
        self.position = position;
    }

    fn finish(&mut self) -> Result<(), TokenParseError> {
        for token in self.by_ref() {
            if matches!(token, TokenTree::Trivia(_) | TokenTree::Eoc(_)) {
                continue;
            }
            return Err(TokenParseError::TrailingToken {
                found: token.description(),
                span: token.span(),
            });
        }
        Ok(())
    }
}

impl Iterator for ParseStream {
    type Item = TokenTree;

    fn next(&mut self) -> Option<Self::Item> {
        let token = self.tokens().get(self.position)?.clone();
        self.position += 1;
        Some(token)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.tokens().len().saturating_sub(self.position);
        (remaining, Some(remaining))
    }
}

impl ExactSizeIterator for ParseStream {}
impl FusedIterator for ParseStream {}

#[derive(Debug, Clone, PartialEq, Eq, Spanned)]
/// A failure while consuming typed syntax from a [`ParseStream`].
pub enum TokenParseError {
    /// Input ended before the required syntax appeared.
    UnexpectedEnd {
        /// The expected syntax description or spelling.
        expected: &'static str,
        /// The end-of-input position.
        span: Span,
    },
    /// The next token did not match the required syntax.
    UnexpectedToken {
        /// The expected syntax description or spelling.
        expected: &'static str,
        /// A readable description of the received token.
        found: String,
        /// The received token span.
        span: Span,
    },
    /// A complete parse left a significant token unconsumed.
    TrailingToken {
        /// A readable description of the remaining token.
        found: String,
        /// The remaining token span.
        span: Span,
    },
    /// A parser backend reported a higher-level syntax failure.
    Syntax {
        /// The backend's diagnostic message.
        message: String,
        /// The syntax associated with the failure.
        span: Span,
    },
}

impl TokenParseError {
    pub(crate) fn unexpected_end(expected: &'static str, span: Span) -> Self {
        Self::UnexpectedEnd { expected, span }
    }

    pub(crate) fn unexpected(expected: &'static str, found: impl Into<String>, span: Span) -> Self {
        Self::UnexpectedToken {
            expected,
            found: found.into(),
            span,
        }
    }

    pub(crate) fn unexpected_token(expected: &'static str, token: &TokenTree) -> Self {
        Self::unexpected(expected, token.description(), token.span())
    }
}

impl Display for TokenParseError {
    fn fmt(&self, formatter: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::UnexpectedEnd { expected, .. } => {
                write!(formatter, "expected `{expected}`, found the end of input")
            }
            Self::UnexpectedToken {
                expected, found, ..
            } => write!(formatter, "expected `{expected}`, found `{found}`"),
            Self::TrailingToken { found, .. } => {
                write!(formatter, "expected the end of input, found `{found}`")
            }
            Self::Syntax { message, .. } => formatter.write_str(message),
        }
    }
}

impl Error for TokenParseError {}

/// Reconstructs a typed syntax target from a parser-specific CST root.
///
/// # Errors
///
/// Returns [`ReconstructError`] when the root cannot reconstruct `T`.
pub fn reconstruct<T, N>(root: N) -> Result<T, ReconstructError>
where
    N: ExternalCstNode,
    T: Reconstruct<N>,
{
    T::reconstruct(root)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{LiteralKind, quote_m2};

    #[test]
    fn parse_stream_iteration_excludes_its_eof_token() {
        let mut input = ParseStream::new(quote_m2!(name + 1));
        assert_eq!(input.len(), 3);
        let emitted = input.by_ref().collect::<TokenStream>();

        assert_eq!(emitted.to_string(), "name+1");
        assert!(input.is_eof());
        assert_eq!(input.next(), None);
        assert_eq!(input.next(), None);
    }

    #[test]
    fn exact_size_counts_every_remaining_token() {
        let mut input = ParseStream::new(quote_m2!(name));

        assert_eq!(input.len(), 1);
        assert!(input.next().is_some());
        assert_eq!(input.len(), 0);
    }

    #[test]
    fn raw_token_categories_implement_parse() {
        let identifier = parse2::<IdentToken>(quote_m2!(name)).unwrap();
        let literal = parse2::<LiteralToken>(quote_m2!(1)).unwrap();
        let punctuation = parse2::<Punct>(quote_m2!(+)).unwrap();
        let group = parse2::<TokenGroup>(quote_m2!((name))).unwrap();

        assert_eq!(identifier.text(), "name");
        assert_eq!(literal.kind, LiteralKind::Integer);
        assert_eq!(punctuation.text(), "+");
        assert_eq!(group.to_string(), "(name)");
    }

    #[test]
    fn lookahead_reports_the_stored_eof_span_through_spanned() {
        let tokens = lex_str("name").unwrap();
        let mut input = ParseStream::new(tokens);
        input.next_token();

        let span = input.lookahead().span();
        assert_eq!(span.start().unwrap(), "name".len());
        assert_eq!(span, input.eof_span());
    }
}