cuttlefish-core 0.9.0

//! Turning spec text into tokens.
//!
//! # Why a lexer instead of splitting on punctuation
//!
//! The previous parser split statements on `;` and lists on `,`. That works
//! until a value *contains* one — and a description is prose, so it contains
//! semicolons routinely:
//!
//! ```text
//! description = "Use when summarizing; especially long files.";
//! ```
//!
//! Splitting on `;` cuts that in half and reports a confusing error about the
//! description not being a quoted string. A path containing a comma broke the
//! capability list the same way. Both were real bugs, not hypotheticals, and
//! neither is fixable by being cleverer about splitting: a separator inside a
//! string is only distinguishable from a separator between values by tracking
//! whether you are inside a string, which is what a lexer is.
//!
//! It also buys positions. "malformed spec" with no location is a poor error for
//! a file someone is editing by hand; every token here carries a line and column
//! so the parser can point at the problem.

/// A token's position in the source, for error messages.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Span {
    /// 1-based line.
    pub line: u32,
    /// 1-based column.
    pub column: u32,
}

impl std::fmt::Display for Span {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "line {}, column {}", self.line, self.column)
    }
}

/// What a token is.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Tok {
    /// A bare word: `spec`, `description`, `Ollama`, `Local_only`.
    Ident(String),
    /// A quoted string, with escapes already resolved.
    Str(String),
    /// `=`
    Equals,
    /// `{`
    OpenBrace,
    /// `}`
    CloseBrace,
    /// `[`
    OpenBracket,
    /// `]`
    CloseBracket,
    /// `,`
    Comma,
    /// `;`
    Semicolon,
    /// `->`
    Arrow,
}

impl Tok {
    /// How to name this in an error message.
    pub fn describe(&self) -> String {
        match self {
            Tok::Ident(name) => format!("`{name}`"),
            Tok::Str(_) => "a quoted string".into(),
            Tok::Equals => "`=`".into(),
            Tok::OpenBrace => "`{`".into(),
            Tok::CloseBrace => "`}`".into(),
            Tok::OpenBracket => "`[`".into(),
            Tok::CloseBracket => "`]`".into(),
            Tok::Comma => "`,`".into(),
            Tok::Semicolon => "`;`".into(),
            Tok::Arrow => "`->`".into(),
        }
    }
}

/// A token and where it came from.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Token {
    /// The token.
    pub tok: Tok,
    /// Where it started.
    pub span: Span,
}

/// Why lexing stopped.
#[derive(Debug, thiserror::Error, PartialEq, Eq)]
pub enum LexError {
    /// A string had no closing quote.
    #[error("unterminated string starting at {span}")]
    UnterminatedString {
        /// Where the string began.
        span: Span,
    },
    /// A character that cannot begin any token.
    #[error("unexpected character `{ch}` at {span}")]
    UnexpectedChar {
        /// The offending character.
        ch: char,
        /// Where it is.
        span: Span,
    },
    /// A backslash escape this format does not define.
    #[error("unknown escape `\\{ch}` at {span}")]
    UnknownEscape {
        /// The character after the backslash.
        ch: char,
        /// Where the escape is.
        span: Span,
    },
}

/// Tokenize spec source.
///
/// Comments run from `#` or `//` to end of line, and whitespace is
/// insignificant.
pub fn lex(src: &str) -> Result<Vec<Token>, LexError> {
    let mut tokens = Vec::new();
    let mut chars = src.chars().peekable();
    let (mut line, mut column) = (1u32, 1u32);

    // Consuming through a closure keeps line and column correct in one place;
    // tracking them at each call site is how they drift.
    macro_rules! bump {
        () => {{
            let c = chars.next();
            match c {
                Some('\n') => {
                    line += 1;
                    column = 1;
                }
                Some(_) => column += 1,
                None => {}
            }
            c
        }};
    }

    while let Some(&c) = chars.peek() {
        let span = Span { line, column };

        macro_rules! skip_to_end_of_line {
            () => {{
                while let Some(&c) = chars.peek() {
                    if c == '\n' {
                        break;
                    }
                    bump!();
                }
            }};
        }

        match c {
            c if c.is_whitespace() => {
                bump!();
            }
            // `#` and `//` both run to end of line. `//` is not redundant:
            // this grammar reads C-ish enough that people reach for it
            // first, and when it was unsupported the failure was maximally
            // confusing — the lexer skipped the slashes, then reported an
            // "unexpected character" pointing at some punctuation *inside
            // the comment's prose*, which names a character that is not the
            // problem and a position that is not where the mistake is.
            '#' => {
                skip_to_end_of_line!();
            }
            '/' if chars.clone().nth(1) == Some('/') => {
                skip_to_end_of_line!();
            }
            '"' => {
                bump!();
                let mut value = String::new();
                loop {
                    match bump!() {
                        None => return Err(LexError::UnterminatedString { span }),
                        Some('"') => break,
                        Some('\\') => {
                            let escape_span = Span { line, column };
                            match bump!() {
                                Some('"') => value.push('"'),
                                Some('\\') => value.push('\\'),
                                Some('n') => value.push('\n'),
                                Some('t') => value.push('\t'),
                                Some(other) => {
                                    return Err(LexError::UnknownEscape {
                                        ch: other,
                                        span: escape_span,
                                    })
                                }
                                None => return Err(LexError::UnterminatedString { span }),
                            }
                        }
                        // A newline inside a string is allowed: descriptions
                        // wrap, and requiring an escape for that would make the
                        // common case awkward.
                        Some(other) => value.push(other),
                    }
                }
                tokens.push(Token {
                    tok: Tok::Str(value),
                    span,
                });
            }
            '-' => {
                let mut la = chars.clone();
                la.next();
                if la.peek() == Some(&'>') {
                    bump!();
                    bump!();
                    tokens.push(Token {
                        tok: Tok::Arrow,
                        span,
                    });
                } else {
                    return Err(LexError::UnexpectedChar { ch: '-', span });
                }
            }
            c if c.is_alphanumeric() || c == '_' || c == '.' || c == '/' => {
                let mut word = String::new();
                while let Some(&c) = chars.peek() {
                    if c.is_alphanumeric() || matches!(c, '_' | '.' | '/' | ':') {
                        word.push(c);
                        bump!();
                    } else {
                        break;
                    }
                }
                tokens.push(Token {
                    tok: Tok::Ident(word),
                    span,
                });
            }
            _ => {
                let tok = match c {
                    '=' => Tok::Equals,
                    '{' => Tok::OpenBrace,
                    '}' => Tok::CloseBrace,
                    '[' => Tok::OpenBracket,
                    ']' => Tok::CloseBracket,
                    ',' => Tok::Comma,
                    ';' => Tok::Semicolon,
                    other => return Err(LexError::UnexpectedChar { ch: other, span }),
                };
                bump!();
                tokens.push(Token { tok, span });
            }
        }
    }

    Ok(tokens)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn an_arrow_lexes_as_one_token() {
        let tokens = lex(r#""pdf" -> handle_pdf"#).unwrap();
        assert_eq!(
            tokens.iter().map(|t| t.tok.clone()).collect::<Vec<_>>(),
            vec![
                Tok::Str("pdf".into()),
                Tok::Arrow,
                Tok::Ident("handle_pdf".into()),
            ]
        );
    }

    #[test]
    fn a_lone_hyphen_is_still_an_error() {
        // Confirms `-` alone (not followed by `>`) keeps today's behavior —
        // this plan only special-cases the two-character `->` sequence.
        assert!(lex("- foo").is_err());
    }
}

#[cfg(test)]
mod comment_tests {
    use super::*;

    /// Both comment syntaxes reach end of line and nothing else.
    ///
    /// `//` was added after a real spec failed to parse because its author
    /// reached for it. The lexer skipped the slashes and then reported an
    /// "unexpected character" pointing at a `(` inside the comment's own
    /// prose — a character that was not the problem, at a position that was
    /// not the mistake. That is the most confusing possible way to say
    /// "comments start with `#`".
    #[test]
    fn both_comment_syntaxes_run_to_end_of_line() {
        let hash = lex("# note\nspec").expect("`#` comments must lex");
        let slash = lex("// note\nspec").expect("`//` comments must lex");
        assert_eq!(
            hash.len(),
            slash.len(),
            "the two syntaxes must produce identical token streams"
        );

        // Punctuation inside a comment is prose, not syntax — this is the
        // exact input that used to fail.
        lex("// call infer(prompt, 32) here\nspec").expect("prose in a comment must be skipped");
        lex("# call infer(prompt, 32) here\nspec").unwrap();
    }

    #[test]
    fn a_comment_ends_at_the_newline_and_not_before_or_after() {
        // The token after a comment must still be seen: a comment that ate
        // the rest of the file would turn a typo into silent truncation.
        let tokens = lex("# gone\nkept").unwrap();
        assert_eq!(tokens.len(), 1, "{tokens:?}");
        // And a trailing comment with no newline must simply end.
        assert!(lex("kept # gone").is_ok());
    }

    #[test]
    fn a_single_slash_is_still_an_ordinary_identifier_character() {
        // Only a *doubled* slash opens a comment. `/` is a legal identifier
        // character here — namespaced catalog names like `team/cat-a@1` rely
        // on it — so making `/` special would have broken them. Pinned
        // because the comment rule is one character away from doing exactly
        // that.
        let tokens = lex("team/cat").expect("a slash inside an identifier must still lex");
        assert_eq!(tokens.len(), 1, "{tokens:?}");
        // And a slash that opens nothing still ends the identifier cleanly
        // rather than swallowing the rest of the line.
        assert_eq!(lex("a/b c").unwrap().len(), 2);
    }

    /// The exact input from the field log, which used to fail with
    /// ``unexpected character `(` at line 4, column 25``.
    ///
    /// Column 25 was a `(` inside the *comment's prose*. The lexer had taken
    /// `//` as an identifier (a slash is a legal identifier character), then
    /// `call`, then `infer`, and only tripped on the parenthesis — naming a
    /// character that was not the problem, at a position that was not the
    /// mistake, several tokens past the real cause.
    #[test]
    fn the_spec_that_failed_in_the_field_now_parses() {
        let src = r#"spec demo = {
  description = "Use when testing.";
  model = Stub "unused";
  // the model is called via infer(prompt, 32) inside the block
  data_policy = Local_only;
  capabilities = [ ];
  nodes = { check = { block = "./check.rhai"; }; };
}
"#;
        crate::spec::parse_spec(src).expect("a spec with `//` comments must parse");
    }
}