oysh/crates/oyster_parser/src/parser.rs

use thiserror::Error;

use crate::{lexer::Lexer, Token, TokenKind};

/// Errors that might occur during parsing.
#[derive(Debug, Error)]
pub enum ParseError {
    #[error("unexpected pipe")]
    UnexpectedPipe,
    #[error("unexpected end of file")]
    UnexpectedEof,
    #[error("unexpected semicolon in the middle of statement")]
    UnexpectedSemicolon,
    #[error("unmatched closing parenthesis")]
    UnexpectedClosingParenthesis,
}

/// Type of the node.
#[derive(Debug)]
pub enum NodeKind {
    Whitespace,
    Newlines,
    Semicolon,
    Pipe,
    DoubleQuote,
    OpeningParenthesis,
    ClosingParenthesis,
    PlainText,
    EscapedChar,
    Comment,

    Program,
    CommandSubstitution,
    Pipeline,
    Command,
    Word,
    DQuotedString,

    /// Read a pipe but didn't start word yet.
    PipelineCont,
}

/// Events required to build a syntax tree.
#[derive(Debug)]
pub enum ParseEvent {
    Error(ParseError, usize),
    StartNode(NodeKind),
    EndNode,
    NewLeaf(NodeKind, usize),
}

/// Parse a given source code string.
/// The parsing is decoupled from building the AST and parse tree,
/// instead of returning the tree directly, it returns a linear representation,
/// containing instructions on how to construct the trees.
/// StartNode(Program) and EndNode at the end is implied.
pub struct Parser<'a> {
    lex: Lexer<'a>,
    lookahead: Token,
    buffer: Option<ParseEvent>,
    stack: Vec<NodeKind>,
}

impl Parser<'_> {
    /// Create a new parser.
    pub fn new(source: &str) -> Parser {
        let mut lex = Lexer::new(source);
        let lookahead = lex.next_command_token();
        let mut stack = Vec::with_capacity(5);
        stack.push(NodeKind::Program);

        Parser {
            lex,
            lookahead,
            buffer: None,
            stack,
        }
    }
}

impl Iterator for Parser<'_> {
    type Item = ParseEvent;

    fn next(&mut self) -> Option<Self::Item> {
        // This is essentially a recursive descent parser.
        // The macros somewhat represent things you would do in a true RD implementation.

        macro_rules! chain {
            ($a:expr, $b:expr) => {{
                $b;
                $a
            }};
        }

        macro_rules! chain_buf {
            ($a:expr, $b:expr) => {{
                self.buffer = $b;
                $a
            }};
        }

        macro_rules! call {
            ($rule:expr) => {{
                use NodeKind::*;
                self.stack.push($rule);
                Some(ParseEvent::StartNode($rule))
            }};
        }

        macro_rules! ret {
            () => {{
                self.stack.pop();
                Some(ParseEvent::EndNode)
            }};
        }

        macro_rules! leaf {
            ($type:expr $(, $mode:ident)?) => {{
                use NodeKind::*;
                let len = self.lookahead.len;
                self.lookahead = next_token!($($mode)?);
                Some(ParseEvent::NewLeaf($type, len))
            }};
        }

        macro_rules! error {
            ($type:expr $(, $mode:ident)?) => {{
                use ParseError::*;
                let len = self.lookahead.len;
                self.lookahead = next_token!($($mode)?);
                Some(ParseEvent::Error($type, len))
            }};
        }

        macro_rules! next_token {
            () => {
                self.lex.next_command_token()
            };
            (Command) => {
                self.lex.next_command_token()
            };
            (String) => {
                self.lex.next_string_token()
            };
        }

        use TokenKind::*;

        if let Some(ev) = self.buffer.take() {
            return Some(ev);
        }

        match self.stack.last() {
            None => None,
            Some(nt) => match nt {
                // XXX: unify Program and CommandSubstitution to avoid duplication
                NodeKind::Program => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Newlines => leaf!(Newlines),
                    Semicolon => leaf!(Semicolon),
                    Comment => leaf!(Comment),
                    PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Pipeline),
                    Pipe => error!(UnexpectedPipe),
                    ClosingParenthesis => error!(UnexpectedClosingParenthesis),
                    Eof => chain!(None, ret!()), // return silently
                },
                NodeKind::CommandSubstitution => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Newlines => leaf!(Newlines),
                    Semicolon => leaf!(Semicolon),
                    Comment => leaf!(Comment),
                    PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Pipeline),
                    ClosingParenthesis => match self.stack.get(self.stack.len() - 2) {
                        Some(&NodeKind::DQuotedString) => {
                            chain_buf!(leaf!(ClosingParenthesis, String), ret!())
                        }
                        _ => chain_buf!(leaf!(ClosingParenthesis, Command), ret!()),
                    },
                    Pipe => error!(UnexpectedPipe),
                    Eof => chain_buf!(error!(UnexpectedEof), ret!()),
                },
                NodeKind::Pipeline => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Comment => leaf!(Comment),
                    Pipe => chain!(leaf!(Pipe), call!(PipelineCont)),
                    PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Command),
                    Newlines | Semicolon | ClosingParenthesis | Eof => ret!(),
                },
                NodeKind::PipelineCont => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Newlines => leaf!(Newlines),
                    Comment => leaf!(Comment),
                    PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => {
                        chain!(call!(Command), ret!())
                    }
                    Semicolon => chain_buf!(chain!(error!(UnexpectedSemicolon), ret!()), ret!()),
                    Pipe => chain!(error!(UnexpectedPipe), ret!()),
                    ClosingParenthesis => chain!(error!(UnexpectedClosingParenthesis), ret!()),
                    Eof => chain!(error!(UnexpectedEof), ret!()),
                },
                NodeKind::Command => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Comment => leaf!(Comment),
                    PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Word),
                    Newlines | Semicolon | Pipe | ClosingParenthesis | Eof => ret!(),
                },
                NodeKind::Word => match self.lookahead.kind {
                    PlainText => leaf!(PlainText),
                    EscapedChar => leaf!(EscapedChar),
                    DoubleQuote => chain_buf!(call!(DQuotedString), leaf!(DoubleQuote, String)),
                    OpeningParenthesis => {
                        chain_buf!(call!(CommandSubstitution), leaf!(OpeningParenthesis))
                    }
                    Comment | Whitespace | Newlines | Semicolon | Pipe | ClosingParenthesis
                    | Eof => ret!(),
                },
                NodeKind::DQuotedString => match self.lookahead.kind {
                    PlainText => leaf!(PlainText, String),
                    EscapedChar => leaf!(EscapedChar, String),
                    DoubleQuote => chain_buf!(leaf!(DoubleQuote, Command), ret!()),
                    OpeningParenthesis => chain_buf!(
                        call!(CommandSubstitution),
                        leaf!(OpeningParenthesis, Command)
                    ),
                    ClosingParenthesis => error!(UnexpectedClosingParenthesis, String),
                    Eof => chain_buf!(error!(UnexpectedEof, Command), ret!()),
                    _ => unreachable!(),
                },
                _ => unreachable!(),
            },
        }
    }
}