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//! Turns a LaTeX string into a sequence of tokens
use crate::error::Error;
/// A single token
#[derive(Debug, Clone, PartialEq)]
pub enum Token {
/// A single letter, digit or symbol
Character(char),
/// The command name after a backslash (without the backslash)
Command(String),
/// The body of \text{...}, taken out as it is
Text(String),
LeftBrace,
RightBrace,
Caret,
Underscore,
/// &, the column separator in a matrix
Ampersand,
/// \\, the row separator in a matrix
RowBreak,
}
/// A token and its position in the source string
#[derive(Debug, Clone, PartialEq)]
pub struct Located {
pub token: Token,
pub position: usize,
}
/// Turns a string into a sequence of tokens
pub fn tokenize(source: &str) -> Result<Vec<Located>, Error> {
let characters: Vec<char> = source.chars().collect();
let mut tokens = Vec::new();
let mut index = 0;
while index < characters.len() {
let start = index;
let character = characters[index];
index += 1;
let token = match character {
c if c.is_whitespace() => continue,
'{' => Token::LeftBrace,
'}' => Token::RightBrace,
'^' => Token::Caret,
'_' => Token::Underscore,
'&' => Token::Ampersand,
'\\' => {
let (token, next) = read_command(&characters, index, start)?;
index = next;
token
}
c => Token::Character(c),
};
tokens.push(Located {token, position: start});
}
Ok(tokens)
}
/// Reads from the character after a backslash. Returns the command token and the next position
fn read_command(characters: &[char], index: usize, start: usize) -> Result<(Token, usize), Error> {
match characters.get(index) {
None => Err(Error::new("no command name after the backslash", start)),
Some('\\') => Ok((Token::RowBreak, index + 1)),
Some(first) if first.is_ascii_alphabetic() => {
let mut name = String::new();
let mut next = index;
while let Some(character) = characters.get(next) {
if !character.is_ascii_alphabetic() {
break;
}
name.push(*character);
next += 1;
}
if name == "text" {
let (body, after) = read_text(characters, next, start)?;
return Ok((Token::Text(body), after));
}
Ok((Token::Command(name), next))
}
Some(character) => Ok((Token::Command(character.to_string()), index + 1)),
}
}
/// Reads the {text} after \text, keeping its spaces
fn read_text(characters: &[char], index: usize, start: usize) -> Result<(String, usize), Error> {
let mut next = index;
while matches!(characters.get(next), Some(character) if character.is_whitespace()) {
next += 1;
}
if characters.get(next) != Some(&'{') {
return Err(Error::new("\\text must be followed by {text}", start));
}
next += 1;
let mut body = String::new();
let mut depth = 1;
while let Some(&character) = characters.get(next) {
next += 1;
match character {
'{' => {
depth += 1;
body.push(character);
}
'}' => {
depth -= 1;
if depth == 0 {
return Ok((body, next));
}
body.push(character);
}
_ => body.push(character),
}
}
Err(Error::new("\\text has an unclosed brace", start))
}
#[cfg(test)]
mod tests {
use super::*;
fn kinds(source: &str) -> Vec<Token> {
tokenize(source).unwrap().into_iter().map(|located| located.token).collect()
}
#[test]
fn reads_commands_and_characters() {
assert_eq!(
kinds(r"\frac{a}{2}"),
vec![
Token::Command("frac".to_string()),
Token::LeftBrace,
Token::Character('a'),
Token::RightBrace,
Token::LeftBrace,
Token::Character('2'),
Token::RightBrace,
]
);
}
#[test]
fn reads_row_break_and_ampersand() {
assert_eq!(kinds(r"a & b \\ c"), vec![
Token::Character('a'),
Token::Ampersand,
Token::Character('b'),
Token::RowBreak,
Token::Character('c'),
]);
}
#[test]
fn keeps_spaces_inside_text() {
assert_eq!(kinds(r"\text{a b}"), vec![Token::Text("a b".to_string())]);
assert_eq!(kinds(r"\text {x}"), vec![Token::Text("x".to_string())]);
}
#[test]
fn single_character_commands() {
assert_eq!(kinds(r"\, \; \{"), vec![
Token::Command(",".to_string()),
Token::Command(";".to_string()),
Token::Command("{".to_string()),
]);
}
#[test]
fn records_positions() {
let tokens = tokenize(r"x + \alpha").unwrap();
assert_eq!(tokens[0].position, 0);
assert_eq!(tokens[1].position, 2);
assert_eq!(tokens[2].position, 4);
}
#[test]
fn reports_a_lonely_backslash() {
assert!(tokenize("a\\").is_err());
assert!(tokenize(r"\text x").is_err());
}
}