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,
}

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

    Program,
    Pipeline,
    Command,
    Word,

    /// 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.
#[derive(Debug)]
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! tailcall {
            ($rule:expr) => {{
                use NodeKind::*;
                self.stack.pop();
                self.stack.push($rule);
                Some(ParseEvent::StartNode($rule))
            }};
        }

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

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

        use TokenKind::*;

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

        match self.stack.last() {
            None => None,
            Some(nt) => match nt {
                NodeKind::Program => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Newlines => leaf!(Newlines),
                    Semicolon => leaf!(Semicolon),
                    Comment => leaf!(Comment),
                    PlainWord | EscapedChar => call!(Pipeline),
                    Pipe => error!(UnexpectedPipe),
                    Eof => chain!(None, ret!()), // return silently
                },
                NodeKind::Pipeline => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Comment => leaf!(Comment),
                    Pipe => chain!(leaf!(Pipe), call!(PipelineCont)),
                    PlainWord | EscapedChar => call!(Command),
                    Newlines | Semicolon | Eof => ret!(),
                },
                NodeKind::PipelineCont => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Newlines => leaf!(Newlines),
                    Comment => leaf!(Comment),
                    PlainWord | EscapedChar => tailcall!(Command),
                    Semicolon => chain_buf!(chain!(error!(UnexpectedSemicolon), ret!()), ret!()),
                    Pipe => chain!(error!(UnexpectedPipe), ret!()),
                    Eof => chain!(error!(UnexpectedEof), ret!()),
                },
                NodeKind::Command => match self.lookahead.kind {
                    Whitespace => leaf!(Whitespace),
                    Comment => leaf!(Comment),
                    PlainWord | EscapedChar => call!(Word),
                    Newlines | Semicolon | Eof => ret!(),
                    Pipe => ret!(),
                },
                NodeKind::Word => match self.lookahead.kind {
                    PlainWord => leaf!(PlainWord),
                    EscapedChar => leaf!(EscapedChar),
                    Comment | Whitespace | Newlines | Semicolon | Pipe | Eof => ret!(),
                },
                _ => unreachable!(),
            },
        }
    }
}