lexer.rs

//! This module takes care of lexing Python source text.
//!
//! This means source code is scanned and translated into separate tokens. The rules
//! governing what is and is not a valid token are defined in the Python reference
//! guide section on [Lexical analysis].
//!
//! The primary function in this module is [`lex`], which takes a string slice
//! and returns an iterator over the tokens in the source code. The tokens are currently returned
//! as a `Result<Spanned, LexicalError>`, where [`Spanned`] is a tuple containing the
//! start and end [`TextSize`] and a [`Tok`] denoting the token.
//!
//! # Example
//!
//! ```
//! use rustpython_parser::{lexer::lex, Tok, Mode, StringKind};
//!
//! let source = "x = 'RustPython'";
//! let tokens = lex(source, Mode::Module)
//!     .map(|tok| tok.expect("Failed to lex"))
//!     .collect::<Vec<_>>();
//!
//! for (token, range) in tokens {
//!     println!(
//!         "{token:?}@{range:?}",
//!     );
//! }
//! ```
//!
//! [Lexical analysis]: https://docs.python.org/3/reference/lexical_analysis.html
use crate::{
    ast::bigint::BigInt,
    soft_keywords::SoftKeywordTransformer,
    string::FStringErrorType,
    text_size::{TextLen, TextRange, TextSize},
    token::{StringKind, Tok},
    Mode,
};
use log::trace;
use num_traits::{Num, Zero};
use std::{char, cmp::Ordering, ops::Index, slice::SliceIndex, str::FromStr};
use unic_emoji_char::is_emoji_presentation;
use unic_ucd_ident::{is_xid_continue, is_xid_start};

// Indentations are tracked by a stack of indentation levels. IndentationLevel keeps
// track of the number of tabs and spaces at the current level.
#[derive(Clone, Copy, PartialEq, Debug, Default)]
struct IndentationLevel {
    tabs: u32,
    spaces: u32,
}

impl IndentationLevel {
    fn compare_strict(
        &self,
        other: &IndentationLevel,
        location: TextSize,
    ) -> Result<Ordering, LexicalError> {
        // We only know for sure that we're smaller or bigger if tabs
        // and spaces both differ in the same direction. Otherwise we're
        // dependent on the size of tabs.
        match self.tabs.cmp(&other.tabs) {
            Ordering::Less => {
                if self.spaces <= other.spaces {
                    Ok(Ordering::Less)
                } else {
                    Err(LexicalError {
                        location,
                        error: LexicalErrorType::TabError,
                    })
                }
            }
            Ordering::Greater => {
                if self.spaces >= other.spaces {
                    Ok(Ordering::Greater)
                } else {
                    Err(LexicalError {
                        location,
                        error: LexicalErrorType::TabError,
                    })
                }
            }
            Ordering::Equal => Ok(self.spaces.cmp(&other.spaces)),
        }
    }
}

// The indentations stack is used to keep track of the current indentation level.
// Similar to the CPython implementation, the Indentations stack always has at
// least one level which is never popped. See Reference 2.1.8.
#[derive(Debug)]
struct Indentations {
    indent_stack: Vec<IndentationLevel>,
}

impl Indentations {
    fn is_empty(&self) -> bool {
        self.indent_stack.len() == 1
    }

    fn push(&mut self, indent: IndentationLevel) {
        self.indent_stack.push(indent);
    }

    fn pop(&mut self) -> Option<IndentationLevel> {
        if self.is_empty() {
            return None;
        }
        self.indent_stack.pop()
    }

    fn current(&self) -> &IndentationLevel {
        self.indent_stack
            .last()
            .expect("Indentations must have at least one level")
    }
}

impl Default for Indentations {
    fn default() -> Self {
        Self {
            indent_stack: vec![IndentationLevel::default()],
        }
    }
}

// A CharWindow is a sliding window over an iterator of chars. It is used to
// allow for look-ahead when scanning tokens from the source code.
struct CharWindow<T: Iterator<Item = char>, const N: usize> {
    source: T,
    window: [Option<char>; N],
}

impl<T, const N: usize> CharWindow<T, N>
where
    T: Iterator<Item = char>,
{
    fn new(source: T) -> Self {
        Self {
            source,
            window: [None; N],
        }
    }

    fn slide(&mut self) -> Option<char> {
        self.window.rotate_left(1);
        let next = self.source.next();
        *self.window.last_mut().expect("never empty") = next;
        next
    }
}

impl<T, const N: usize, Idx> Index<Idx> for CharWindow<T, N>
where
    T: Iterator<Item = char>,
    Idx: SliceIndex<[Option<char>]>,
{
    type Output = Idx::Output;

    fn index(&self, index: Idx) -> &Self::Output {
        &self.window[index]
    }
}

/// A lexer for Python source code.
pub struct Lexer<T: Iterator<Item = char>> {
    // Contains the source code to be lexed.
    window: CharWindow<T, 3>,
    // Are we at the beginning of a line?
    at_begin_of_line: bool,
    // Amount of parenthesis.
    nesting: usize,
    // Indentation levels.
    indentations: Indentations,
    // Pending list of tokens to be returned.
    pending: Vec<Spanned>,
    // The current location.
    location: TextSize,
}

// generated in build.rs, in gen_phf()
/// A map of keywords to their tokens.
pub static KEYWORDS: phf::Map<&'static str, Tok> =
    include!(concat!(env!("OUT_DIR"), "/keywords.rs"));

/// Contains a Token along with its `range`.
pub type Spanned = (Tok, TextRange);
/// The result of lexing a token.
pub type LexResult = Result<Spanned, LexicalError>;

/// Create a new lexer from a source string.
///
/// # Examples
///
/// ```
/// use rustpython_parser::{Mode, lexer::lex};
///
/// let source = "def hello(): return 'world'";
/// let lexer = lex(source, Mode::Module);
///
/// for token in lexer {
///    println!("{:?}", token);
/// }
/// ```
#[inline]
pub fn lex(source: &str, mode: Mode) -> impl Iterator<Item = LexResult> + '_ {
    lex_starts_at(source, mode, TextSize::default())
}

/// Create a new lexer from a source string, starting at a given location.
/// You probably want to use [`lex`] instead.
pub fn lex_starts_at(
    source: &str,
    mode: Mode,
    start_offset: TextSize,
) -> SoftKeywordTransformer<Lexer<std::str::Chars<'_>>> {
    SoftKeywordTransformer::new(Lexer::new(source.chars(), start_offset), mode)
}

impl<T> Lexer<T>
where
    T: Iterator<Item = char>,
{
    /// Create a new lexer from T and a starting location. You probably want to use
    /// [`lex`] instead.
    pub fn new(input: T, start: TextSize) -> Self {
        let mut lxr = Lexer {
            at_begin_of_line: true,
            nesting: 0,
            indentations: Indentations::default(),
            // Usually we have less than 5 tokens pending.
            pending: Vec::with_capacity(5),
            location: start,
            window: CharWindow::new(input),
        };
        // Fill the window.
        lxr.window.slide();
        lxr.window.slide();
        lxr.window.slide();
        // TODO: Handle possible mismatch between BOM and explicit encoding declaration.
        // spell-checker:ignore feff
        if let Some('\u{feff}') = lxr.window[0] {
            lxr.window.slide();
            lxr.location += '\u{feff}'.text_len();
        }
        lxr
    }

    /// Lex an identifier. Also used for keywords and string/bytes literals with a prefix.
    fn lex_identifier(&mut self) -> LexResult {
        // Detect potential string like rb'' b'' f'' u'' r''
        match self.window[..3] {
            [Some(c), Some('"' | '\''), ..] => {
                if let Ok(kind) = StringKind::try_from(c) {
                    return self.lex_string(kind);
                }
            }
            [Some(c1), Some(c2), Some('"' | '\'')] => {
                if let Ok(kind) = StringKind::try_from([c1, c2]) {
                    return self.lex_string(kind);
                }
            }
            _ => {}
        };

        let start_pos = self.get_pos();
        let mut name = String::with_capacity(8);
        while self.is_identifier_continuation() {
            name.push(self.next_char().unwrap());
        }
        let end_pos = self.get_pos();

        if let Some(tok) = KEYWORDS.get(&name) {
            Ok((tok.clone(), TextRange::new(start_pos, end_pos)))
        } else {
            Ok((Tok::Name { name }, TextRange::new(start_pos, end_pos)))
        }
    }

    /// Numeric lexing. The feast can start!
    fn lex_number(&mut self) -> LexResult {
        let start_pos = self.get_pos();
        match self.window[..2] {
            [Some('0'), Some('x' | 'X')] => {
                // Hex! (0xdeadbeef)
                self.next_char();
                self.next_char();
                self.lex_number_radix(start_pos, 16)
            }
            [Some('0'), Some('o' | 'O')] => {
                // Octal style! (0o377)
                self.next_char();
                self.next_char();
                self.lex_number_radix(start_pos, 8)
            }
            [Some('0'), Some('b' | 'B')] => {
                // Binary! (0b_1110_0101)
                self.next_char();
                self.next_char();
                self.lex_number_radix(start_pos, 2)
            }
            _ => self.lex_normal_number(),
        }
    }

    /// Lex a hex/octal/decimal/binary number without a decimal point.
    fn lex_number_radix(&mut self, start_pos: TextSize, radix: u32) -> LexResult {
        let value_text = self.radix_run(radix);
        let end_pos = self.get_pos();
        let value = BigInt::from_str_radix(&value_text, radix).map_err(|e| LexicalError {
            error: LexicalErrorType::OtherError(format!("{e:?}")),
            location: start_pos,
        })?;
        Ok((Tok::Int { value }, TextRange::new(start_pos, end_pos)))
    }

    /// Lex a normal number, that is, no octal, hex or binary number.
    fn lex_normal_number(&mut self) -> LexResult {
        let start_pos = self.get_pos();
        let start_is_zero = self.window[0] == Some('0');
        // Normal number:
        let mut value_text = self.radix_run(10);

        // If float:
        if self.window[0] == Some('.') || self.at_exponent() {
            // Take '.':
            if self.window[0] == Some('.') {
                if self.window[1] == Some('_') {
                    return Err(LexicalError {
                        error: LexicalErrorType::OtherError("Invalid Syntax".to_owned()),
                        location: self.get_pos(),
                    });
                }
                value_text.push(self.next_char().unwrap());
                value_text.push_str(&self.radix_run(10));
            }

            // 1e6 for example:
            if let Some('e' | 'E') = self.window[0] {
                if self.window[1] == Some('_') {
                    return Err(LexicalError {
                        error: LexicalErrorType::OtherError("Invalid Syntax".to_owned()),
                        location: self.get_pos(),
                    });
                }
                value_text.push(self.next_char().unwrap().to_ascii_lowercase());
                // Optional +/-
                if matches!(self.window[0], Some('-' | '+')) {
                    if self.window[1] == Some('_') {
                        return Err(LexicalError {
                            error: LexicalErrorType::OtherError("Invalid Syntax".to_owned()),
                            location: self.get_pos(),
                        });
                    }
                    value_text.push(self.next_char().unwrap());
                }

                value_text.push_str(&self.radix_run(10));
            }

            let value = f64::from_str(&value_text).map_err(|_| LexicalError {
                error: LexicalErrorType::OtherError("Invalid decimal literal".to_owned()),
                location: self.get_pos(),
            })?;

            // Parse trailing 'j':
            if matches!(self.window[0], Some('j' | 'J')) {
                self.next_char();
                let end_pos = self.get_pos();
                Ok((
                    Tok::Complex {
                        real: 0.0,
                        imag: value,
                    },
                    TextRange::new(start_pos, end_pos),
                ))
            } else {
                let end_pos = self.get_pos();
                Ok((Tok::Float { value }, TextRange::new(start_pos, end_pos)))
            }
        } else {
            // Parse trailing 'j':
            if matches!(self.window[0], Some('j' | 'J')) {
                self.next_char();
                let end_pos = self.get_pos();
                let imag = f64::from_str(&value_text).unwrap();
                Ok((
                    Tok::Complex { real: 0.0, imag },
                    TextRange::new(start_pos, end_pos),
                ))
            } else {
                let end_pos = self.get_pos();
                let value = value_text.parse::<BigInt>().unwrap();
                if start_is_zero && !value.is_zero() {
                    // leading zeros in decimal integer literals are not permitted
                    return Err(LexicalError {
                        error: LexicalErrorType::OtherError("Invalid Token".to_owned()),
                        location: self.get_pos(),
                    });
                }
                Ok((Tok::Int { value }, TextRange::new(start_pos, end_pos)))
            }
        }
    }

    /// Consume a sequence of numbers with the given radix,
    /// the digits can be decorated with underscores
    /// like this: '1_2_3_4' == '1234'
    fn radix_run(&mut self, radix: u32) -> String {
        let mut value_text = String::new();

        loop {
            if let Some(c) = self.take_number(radix) {
                value_text.push(c);
            } else if self.window[0] == Some('_')
                && Lexer::<T>::is_digit_of_radix(self.window[1], radix)
            {
                self.next_char();
            } else {
                break;
            }
        }
        value_text
    }

    /// Consume a single character with the given radix.
    fn take_number(&mut self, radix: u32) -> Option<char> {
        let take_char = Lexer::<T>::is_digit_of_radix(self.window[0], radix);

        take_char.then(|| self.next_char().unwrap())
    }

    /// Test if a digit is of a certain radix.
    fn is_digit_of_radix(c: Option<char>, radix: u32) -> bool {
        match radix {
            2 => matches!(c, Some('0'..='1')),
            8 => matches!(c, Some('0'..='7')),
            10 => matches!(c, Some('0'..='9')),
            16 => matches!(c, Some('0'..='9') | Some('a'..='f') | Some('A'..='F')),
            other => unimplemented!("Radix not implemented: {}", other),
        }
    }

    /// Test if we face '[eE][-+]?[0-9]+'
    fn at_exponent(&self) -> bool {
        match self.window[..2] {
            [Some('e' | 'E'), Some('+' | '-')] => matches!(self.window[2], Some('0'..='9')),
            [Some('e' | 'E'), Some('0'..='9')] => true,
            _ => false,
        }
    }

    /// Lex a single comment.
    #[cfg(feature = "full-lexer")]
    fn lex_comment(&mut self) -> LexResult {
        let start_pos = self.get_pos();
        let mut value = String::new();
        loop {
            match self.window[0] {
                Some('\n' | '\r') | None => {
                    let end_pos = self.get_pos();
                    return Ok((Tok::Comment(value), TextRange::new(start_pos, end_pos)));
                }
                Some(_) => {}
            }
            value.push(self.next_char().unwrap());
        }
    }

    #[cfg(feature = "full-lexer")]
    fn lex_and_emit_comment(&mut self) -> Result<(), LexicalError> {
        let comment = self.lex_comment()?;
        self.emit(comment);
        Ok(())
    }

    /// Discard comment if full-lexer is not enabled.
    #[cfg(not(feature = "full-lexer"))]
    fn lex_comment(&mut self) {
        loop {
            match self.window[0] {
                Some('\n' | '\r') | None => {
                    return;
                }
                Some(_) => {}
            }
            self.next_char().unwrap();
        }
    }

    #[cfg(not(feature = "full-lexer"))]
    #[inline]
    fn lex_and_emit_comment(&mut self) -> Result<(), LexicalError> {
        self.lex_comment();
        Ok(())
    }

    /// Lex a string literal.
    fn lex_string(&mut self, kind: StringKind) -> LexResult {
        let start_pos = self.get_pos();
        for _ in 0..u32::from(kind.prefix_len()) {
            self.next_char();
        }
        let quote_char = self.next_char().unwrap();
        let mut string_content = String::with_capacity(5);

        // If the next two characters are also the quote character, then we have a triple-quoted
        // string; consume those two characters and ensure that we require a triple-quote to close
        let triple_quoted = if self.window[..2] == [Some(quote_char); 2] {
            self.next_char();
            self.next_char();
            true
        } else {
            false
        };

        loop {
            match self.next_char() {
                Some(c) => {
                    if c == '\\' {
                        if let Some(next_c) = self.next_char() {
                            string_content.push('\\');
                            string_content.push(next_c);
                            continue;
                        }
                    }
                    if c == '\n' && !triple_quoted {
                        return Err(LexicalError {
                            error: LexicalErrorType::OtherError(
                                "EOL while scanning string literal".to_owned(),
                            ),
                            location: self.get_pos(),
                        });
                    }

                    if c == quote_char {
                        if triple_quoted {
                            // Look ahead at the next two characters; if we have two more
                            // quote_chars, it's the end of the string; consume the remaining
                            // closing quotes and break the loop
                            if self.window[..2] == [Some(quote_char); 2] {
                                self.next_char();
                                self.next_char();
                                break;
                            }
                        } else {
                            break;
                        }
                    }
                    string_content.push(c);
                }
                None => {
                    return Err(LexicalError {
                        error: if triple_quoted {
                            LexicalErrorType::Eof
                        } else {
                            LexicalErrorType::StringError
                        },
                        location: self.get_pos(),
                    });
                }
            }
        }
        let end_pos = self.get_pos();
        let tok = Tok::String {
            value: string_content,
            kind,
            triple_quoted,
        };
        Ok((tok, TextRange::new(start_pos, end_pos)))
    }

    // Checks if the character c is a valid starting character as described
    // in https://docs.python.org/3/reference/lexical_analysis.html#identifiers
    fn is_identifier_start(&self, c: char) -> bool {
        match c {
            'a'..='z' | 'A'..='Z' | '_' => true,
            _ => is_xid_start(c),
        }
    }

    // Checks if the character c is a valid continuation character as described
    // in https://docs.python.org/3/reference/lexical_analysis.html#identifiers
    fn is_identifier_continuation(&self) -> bool {
        match self.window[0] {
            Some('a'..='z' | 'A'..='Z' | '_' | '0'..='9') => true,
            Some(c) => is_xid_continue(c),
            _ => false,
        }
    }

    // This is the main entry point. Call this function to retrieve the next token.
    // This function is used by the iterator implementation.
    fn inner_next(&mut self) -> LexResult {
        // top loop, keep on processing, until we have something pending.
        while self.pending.is_empty() {
            // Detect indentation levels
            if self.at_begin_of_line {
                self.handle_indentations()?;
            }

            self.consume_normal()?;
        }

        Ok(self.pending.remove(0))
    }

    // Given we are at the start of a line, count the number of spaces and/or tabs until the first character.
    fn eat_indentation(&mut self) -> Result<IndentationLevel, LexicalError> {
        // Determine indentation:
        let mut spaces: u32 = 0;
        let mut tabs: u32 = 0;
        loop {
            match self.window[0] {
                Some(' ') => {
                    /*
                    if tabs != 0 {
                        // Don't allow spaces after tabs as part of indentation.
                        // This is technically stricter than python3 but spaces after
                        // tabs is even more insane than mixing spaces and tabs.
                        return Some(Err(LexicalError {
                            error: LexicalErrorType::OtherError("Spaces not allowed as part of indentation after tabs".to_owned()),
                            location: self.get_pos(),
                        }));
                    }
                    */
                    self.next_char();
                    spaces += 1;
                }
                Some('\t') => {
                    if spaces != 0 {
                        // Don't allow tabs after spaces as part of indentation.
                        // This is technically stricter than python3 but spaces before
                        // tabs is even more insane than mixing spaces and tabs.
                        return Err(LexicalError {
                            error: LexicalErrorType::TabsAfterSpaces,
                            location: self.get_pos(),
                        });
                    }
                    self.next_char();
                    tabs += 1;
                }
                Some('#') => {
                    self.lex_and_emit_comment()?;
                    spaces = 0;
                    tabs = 0;
                }
                Some('\x0C') => {
                    // Form feed character!
                    // Reset indentation for the Emacs user.
                    self.next_char();
                    spaces = 0;
                    tabs = 0;
                }
                Some('\n' | '\r') => {
                    // Empty line!
                    #[cfg(feature = "full-lexer")]
                    let tok_start = self.get_pos();
                    self.next_char();
                    #[cfg(feature = "full-lexer")]
                    let tok_end = self.get_pos();
                    #[cfg(feature = "full-lexer")]
                    self.emit((Tok::NonLogicalNewline, TextRange::new(tok_start, tok_end)));
                    spaces = 0;
                    tabs = 0;
                }
                None => {
                    spaces = 0;
                    tabs = 0;
                    break;
                }
                _ => {
                    self.at_begin_of_line = false;
                    break;
                }
            }
        }

        Ok(IndentationLevel { tabs, spaces })
    }

    // Push/pop indents/dedents based on the current indentation level.
    fn handle_indentations(&mut self) -> Result<(), LexicalError> {
        let indentation_level = self.eat_indentation()?;

        if self.nesting != 0 {
            return Ok(());
        }

        // Determine indent or dedent:
        let current_indentation = self.indentations.current();
        let ordering = indentation_level.compare_strict(current_indentation, self.get_pos())?;
        match ordering {
            Ordering::Equal => {
                // Same same
            }
            Ordering::Greater => {
                // New indentation level:
                self.indentations.push(indentation_level);
                let tok_pos = self.get_pos();
                self.emit((
                    Tok::Indent,
                    TextRange::new(
                        tok_pos
                            - TextSize::new(indentation_level.spaces)
                            - TextSize::new(indentation_level.tabs),
                        tok_pos,
                    ),
                ));
            }
            Ordering::Less => {
                // One or more dedentations
                // Pop off other levels until col is found:

                loop {
                    let current_indentation = self.indentations.current();
                    let ordering =
                        indentation_level.compare_strict(current_indentation, self.get_pos())?;
                    match ordering {
                        Ordering::Less => {
                            self.indentations.pop();
                            let tok_pos = self.get_pos();
                            self.emit((Tok::Dedent, TextRange::empty(tok_pos)));
                        }
                        Ordering::Equal => {
                            // We arrived at proper level of indentation.
                            break;
                        }
                        Ordering::Greater => {
                            return Err(LexicalError {
                                error: LexicalErrorType::IndentationError,
                                location: self.get_pos(),
                            });
                        }
                    }
                }
            }
        }

        Ok(())
    }

    // Take a look at the next character, if any, and decide upon the next steps.
    fn consume_normal(&mut self) -> Result<(), LexicalError> {
        if let Some(c) = self.window[0] {
            // Identifiers are the most common case.
            if self.is_identifier_start(c) {
                let identifier = self.lex_identifier()?;
                self.emit(identifier);
            } else {
                self.consume_character(c)?;
            }
        } else {
            // We reached end of file.
            let tok_pos = self.get_pos();

            // First of all, we need all nestings to be finished.
            if self.nesting > 0 {
                return Err(LexicalError {
                    error: LexicalErrorType::Eof,
                    location: tok_pos,
                });
            }

            // Next, insert a trailing newline, if required.
            if !self.at_begin_of_line {
                self.at_begin_of_line = true;
                self.emit((Tok::Newline, TextRange::empty(tok_pos)));
            }

            // Next, flush the indentation stack to zero.
            while !self.indentations.is_empty() {
                self.indentations.pop();
                self.emit((Tok::Dedent, TextRange::empty(tok_pos)));
            }

            self.emit((Tok::EndOfFile, TextRange::empty(tok_pos)));
        }

        Ok(())
    }

    // Dispatch based on the given character.
    fn consume_character(&mut self, c: char) -> Result<(), LexicalError> {
        match c {
            '0'..='9' => {
                let number = self.lex_number()?;
                self.emit(number);
            }
            '#' => {
                self.lex_and_emit_comment()?;
            }
            '"' | '\'' => {
                let string = self.lex_string(StringKind::String)?;
                self.emit(string);
            }
            '=' => {
                let tok_start = self.get_pos();
                self.next_char();
                match self.window[0] {
                    Some('=') => {
                        self.next_char();
                        let tok_end = self.get_pos();
                        self.emit((Tok::EqEqual, TextRange::new(tok_start, tok_end)));
                    }
                    _ => {
                        let tok_end = self.get_pos();
                        self.emit((Tok::Equal, TextRange::new(tok_start, tok_end)));
                    }
                }
            }
            '+' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::PlusEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::Plus, TextRange::new(tok_start, tok_end)));
                }
            }
            '*' => {
                let tok_start = self.get_pos();
                self.next_char();
                match self.window[0] {
                    Some('=') => {
                        self.next_char();
                        let tok_end = self.get_pos();
                        self.emit((Tok::StarEqual, TextRange::new(tok_start, tok_end)));
                    }
                    Some('*') => {
                        self.next_char();
                        match self.window[0] {
                            Some('=') => {
                                self.next_char();
                                let tok_end = self.get_pos();
                                self.emit((
                                    Tok::DoubleStarEqual,
                                    TextRange::new(tok_start, tok_end),
                                ));
                            }
                            _ => {
                                let tok_end = self.get_pos();
                                self.emit((Tok::DoubleStar, TextRange::new(tok_start, tok_end)));
                            }
                        }
                    }
                    _ => {
                        let tok_end = self.get_pos();
                        self.emit((Tok::Star, TextRange::new(tok_start, tok_end)));
                    }
                }
            }
            '/' => {
                let tok_start = self.get_pos();
                self.next_char();
                match self.window[0] {
                    Some('=') => {
                        self.next_char();
                        let tok_end = self.get_pos();
                        self.emit((Tok::SlashEqual, TextRange::new(tok_start, tok_end)));
                    }
                    Some('/') => {
                        self.next_char();
                        match self.window[0] {
                            Some('=') => {
                                self.next_char();
                                let tok_end = self.get_pos();
                                self.emit((
                                    Tok::DoubleSlashEqual,
                                    TextRange::new(tok_start, tok_end),
                                ));
                            }
                            _ => {
                                let tok_end = self.get_pos();
                                self.emit((Tok::DoubleSlash, TextRange::new(tok_start, tok_end)));
                            }
                        }
                    }
                    _ => {
                        let tok_end = self.get_pos();
                        self.emit((Tok::Slash, TextRange::new(tok_start, tok_end)));
                    }
                }
            }
            '%' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::PercentEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::Percent, TextRange::new(tok_start, tok_end)));
                }
            }
            '|' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::VbarEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::Vbar, TextRange::new(tok_start, tok_end)));
                }
            }
            '^' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::CircumflexEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::CircumFlex, TextRange::new(tok_start, tok_end)));
                }
            }
            '&' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::AmperEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::Amper, TextRange::new(tok_start, tok_end)));
                }
            }
            '-' => {
                let tok_start = self.get_pos();
                self.next_char();
                match self.window[0] {
                    Some('=') => {
                        self.next_char();
                        let tok_end = self.get_pos();
                        self.emit((Tok::MinusEqual, TextRange::new(tok_start, tok_end)));
                    }
                    Some('>') => {
                        self.next_char();
                        let tok_end = self.get_pos();
                        self.emit((Tok::Rarrow, TextRange::new(tok_start, tok_end)));
                    }
                    _ => {
                        let tok_end = self.get_pos();
                        self.emit((Tok::Minus, TextRange::new(tok_start, tok_end)));
                    }
                }
            }
            '@' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::AtEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    let tok_end = self.get_pos();
                    self.emit((Tok::At, TextRange::new(tok_start, tok_end)));
                }
            }
            '!' => {
                let tok_start = self.get_pos();
                self.next_char();
                if let Some('=') = self.window[0] {
                    self.next_char();
                    let tok_end = self.get_pos();
                    self.emit((Tok::NotEqual, TextRange::new(tok_start, tok_end)));
                } else {
                    return Err(LexicalError {
                        error: LexicalErrorType::UnrecognizedToken { tok: '!' },
                        location: tok_start,
                    });
                }
            }
            '~' => {
                self.eat_single_char(Tok::Tilde);
            }
            '(' => {
                self.eat_single_char(Tok::Lpar);
                self.nesting += 1;
            }
            ')' => {
                self.eat_single_char(Tok::Rpar);
                if self.nesting == 0 {
                    return Err(LexicalError {
                        error: LexicalErrorType::NestingError,
                        location: self.get_pos(),
                    });
                }
                self.nesting -= 1;
            }
            '[' => {
                self.eat_single_char(Tok::Lsqb);
                self.nesting += 1;