hat_splitter/

split.rs

use std::sync::LazyLock;

use icu_segmenter::WordSegmenter;
use once_cell::sync::Lazy;
use regex::Regex;

enum Token {
    Word(String),
    Punctuation(String),
    Whitespace(String),
    Space(String),
}

impl Token {
    fn inner(self) -> String {
        match self {
            Token::Word(s) | Token::Punctuation(s) | Token::Whitespace(s) | Token::Space(s) => s,
        }
    }
}

pub trait Splitter {
    // At some point it would be great to do this without allocations...
    //fn split<'a>(&self, input: &'a str) -> Vec<&'a str>;

    /// Splits a string into words.
    fn split(&self, input: &str) -> Vec<String>;

    /// Splits a string into words and limits the size of each word to `max_bytes`. As this
    /// function enforces a byte limit, it may split unicode characters. That is, this function
    /// does not guarantee that the resulting byte arrays are valid UTF-8.
    fn split_with_limit(&self, input: &str, max_bytes_per_word: usize) -> Vec<Vec<u8>>;
}

pub struct HATSplitter;

impl Default for HATSplitter {
    fn default() -> Self {
        Self::new()
    }
}

impl HATSplitter {
    pub fn new() -> Self {
        Self
    }

    fn unicode_word_split(input: &str) -> Vec<&str> {
        // Note: we could also try `new_auto` which uses a LSTM (we should figure out which is better)
        static WORD_SEGMENTER: LazyLock<WordSegmenter> =
            LazyLock::new(WordSegmenter::new_dictionary);
        let breakpoints: Vec<usize> = WORD_SEGMENTER.segment_str(input).collect();
        breakpoints.windows(2).map(|w| &input[w[0]..w[1]]).collect()
    }

    fn split_at_matches<'a>(s: &'a str, re: &Regex) -> Vec<&'a str> {
        let mut result = Vec::new();
        let mut match_start = 0;

        for regex_match in re.find_iter(s) {
            let match_end = regex_match.start() + 1;
            result.push(&s[match_start..match_end]);
            match_start = match_end;
        }

        if match_start < s.len() {
            result.push(&s[match_start..s.len()]);
        }

        result
    }

    fn split_camel_case(s: &str) -> Vec<&str> {
        static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(\p{Ll})(\p{Lu})").unwrap());
        Self::split_at_matches(s, &RE)
    }

    fn split_snake_case(s: &str) -> Vec<&str> {
        static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"_").unwrap());
        Self::split_at_matches(s, &RE)
    }

    fn combine_spaces(strings: Vec<&str>) -> Vec<String> {
        strings.into_iter().fold(Vec::new(), |mut acc, s| {
            if s == " " {
                // If we have a space and the last element is also spaces, append to it
                if let Some(last) = acc.last_mut() {
                    if last.chars().all(|c| c == ' ') {
                        last.push(' ');
                        return acc;
                    }
                }
            }
            // Otherwise add as a new element
            acc.push(s.to_string());
            acc
        })
    }

    // This function does its best to avoid splitting unicode characters, but in some cases it has
    // no choice (e.g., if max_bytes < 4 and an emoji comes in).
    fn split_long_words(strings: Vec<String>, max_bytes: usize) -> Vec<Vec<u8>> {
        if max_bytes == 0 {
            panic!("max_bytes must be greater than 0");
        }
        strings.into_iter().fold(Vec::new(), |mut result, string| {
            let bytes = string.as_bytes();
            if bytes.len() <= max_bytes {
                result.push(bytes.to_vec());
                return result;
            }

            let mut start_byte = 0;
            while start_byte < bytes.len() {
                let end_byte = std::cmp::min(start_byte + max_bytes, bytes.len());

                // Backtrack to find a valid UTF-8 boundary
                let end = (start_byte + 1..=end_byte)
                    .rev()
                    .find(|&i| string.is_char_boundary(i))
                    .unwrap_or(end_byte); // Fall back to end_byte if no boundary found

                result.push(bytes[start_byte..end].to_vec());
                start_byte = end;
            }
            result
        })
    }

    /// The Lexer takes a string and splits it into logical tokens.
    fn lex(s: &str) -> Vec<Token> {
        static WHITESPACE_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"^\s+$").unwrap());
        static PUNCTUATION_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"^\p{P}$").unwrap());

        let words = Self::unicode_word_split(s);

        let words = words
            .iter()
            .flat_map(|s| Self::split_camel_case(s))
            .flat_map(|s| Self::split_snake_case(s))
            .collect::<Vec<&str>>();

        let words = Self::combine_spaces(words.clone());

        words
            .into_iter()
            .map(|s| {
                if s == " " {
                    Token::Space(s)
                } else if WHITESPACE_RE.is_match(s.as_str()) {
                    Token::Whitespace(s)
                } else if PUNCTUATION_RE.is_match(s.as_str()) {
                    Token::Punctuation(s)
                } else {
                    Token::Word(s)
                }
            })
            .collect()
    }

    /// The Parser takes tokens and groups them into a string split.
    fn parse(tokens: Vec<Token>) -> Vec<String> {
        let groups = tokens
            .into_iter()
            .fold(Vec::<Vec<Token>>::new(), |mut groups, token| {
                let should_append_to_last_group = |last_group: &Vec<Token>, token: &Token| {
                    matches!(
                        (last_group.last(), token),
                        (Some(Token::Space(_)), Token::Word(_))
                            | (
                                Some(Token::Space(_) | Token::Word(_) | Token::Punctuation(_)),
                                Token::Punctuation(_),
                            )
                    )
                };

                if let Some(last_group) = groups.last_mut() {
                    if should_append_to_last_group(last_group, &token) {
                        last_group.push(token);
                        return groups;
                    }
                }

                groups.push(vec![token]);
                groups
            });

        // Concatenate groups
        groups
            .into_iter()
            .map(|group| group.into_iter().map(Token::inner).collect())
            .collect()
    }
}

impl Splitter for HATSplitter {
    fn split(&self, input: &str) -> Vec<String> {
        Self::parse(Self::lex(input))
    }

    fn split_with_limit(&self, input: &str, max_bytes: usize) -> Vec<Vec<u8>> {
        Self::split_long_words(Self::parse(Self::lex(input)), max_bytes)
    }
}

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

    #[test]
    fn it_works() {
        let result = HATSplitter::new().split("Hello, world!");

        assert_eq!(result, vec!["Hello,", " world!"]);
    }

    #[test]
    fn it_handles_empty_input() {
        let result = HATSplitter::new().split("");

        assert!(result.is_empty());
    }

    #[test]
    fn it_splits_camel_case() {
        let result = HATSplitter::new().split("howAreYou");

        assert_eq!(result, vec!["how", "Are", "You"]);
    }

    #[test]
    fn it_splits_snake_case() {
        let result = HATSplitter::new().split("how_are_you");

        assert_eq!(result, vec!["how_", "are_", "you"]);
    }

    #[test]
    fn it_limits_word_size() {
        let result = HATSplitter::new().split_with_limit("verylongword", 10);

        assert_eq!(result, vec![b"verylongwo".to_vec(), b"rd".to_vec()]);
    }

    #[test]
    fn it_splits_large_unicode_characters() {
        let result = HATSplitter::new().split_with_limit("🌝", 2);

        assert_eq!(result.len(), 2);
    }

    #[test]
    fn it_does_not_split_unicode_where_possible() {
        // This is one word with a 2-byte 'ü' starting at byte offset 1. We hope that the splitter
        // preserves this character by splitting into three parts instead of two.
        let result = HATSplitter::new().split_with_limit("für", 2);

        assert_eq!(
            result,
            vec![b"f".to_vec(), "ü".as_bytes().to_vec(), b"r".to_vec()]
        );
    }

    #[test]
    #[should_panic]
    fn it_handles_zero_max_bytes() {
        HATSplitter::new().split_with_limit("abc", 0);
    }
}