strawcore/src/youtube/js/lexer.rs

// JS lexer helpers. Mirrors NPE's utils/jsextractor/JavaScriptExtractor.java
// + Lexer.java + EcmaScriptTokenStream.java.
//
// NPE's lexer is vendored from Rhino 1.7.14 to handle regex-vs-division
// disambiguation. The `ress` crate is the direct Rust analog — same shape,
// pure-rust scanner. We delegate to it.
//
// Public surface mirrors NPE's static `matchToClosingBrace(src, start)`:
// given a substring `start` (e.g. `"xyz=function"`), find its first
// occurrence in `src`, then walk forward through tokens balancing braces
// (skipping braces inside strings, regex literals, comments) until the
// matching `}` of the function body is consumed. Returns the slice from
// the first `{` of the body through that closing `}`, inclusive.

use ress::tokens::{Punct, Token};
use ress::Scanner;

use crate::youtube::js::DeobfError;

/// Returns the substring of `src` starting immediately after `start` and
/// ending at the matching `}` of the first function body that follows
/// (inclusive). Everything between the anchor and the first `{` (e.g.
/// `(a)` for `xyz=function`) is included.
///
/// Mirrors `JavaScriptExtractor.matchToClosingBrace(src, start)` from NPE.
pub fn match_to_closing_brace(src: &str, start: &str) -> Result<String, DeobfError> {
    let prefix_idx = src
        .find(start)
        .ok_or_else(|| DeobfError::SigBodyParseFailed(format!("anchor not found: {start}")))?;
    let scan_from = prefix_idx + start.len();

    let scanner = Scanner::new(&src[scan_from..]);
    let mut depth = 0i32;
    let mut saw_open = false;
    let mut last_brace: Option<usize> = None;

    for item in scanner {
        let item = item.map_err(|e| {
            DeobfError::SigBodyParseFailed(format!("lexer error at byte {}: {e}", scan_from))
        })?;
        match item.token {
            Token::Punct(Punct::OpenBrace) => {
                saw_open = true;
                depth += 1;
            }
            Token::Punct(Punct::CloseBrace) => {
                depth -= 1;
                if depth == 0 && saw_open {
                    last_brace = Some(item.span.end);
                    break;
                }
            }
            _ => {}
        }
    }

    let last = last_brace.ok_or_else(|| {
        DeobfError::SigBodyParseFailed("no matching closing brace found".into())
    })?;
    Ok(src[scan_from..scan_from + last].to_string())
}

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

    #[test]
    fn balances_simple_function() {
        let src = "var a=1;xyz=function(b){return b;};var c=2;";
        let body = match_to_closing_brace(src, "xyz=function").unwrap();
        assert_eq!(body, "(b){return b;}");
    }

    #[test]
    fn skips_braces_inside_strings() {
        let src = r#"xyz=function(a){var x="}}";return a+x;}"#;
        let body = match_to_closing_brace(src, "xyz=function").unwrap();
        assert_eq!(body, r#"(a){var x="}}";return a+x;}"#);
    }

    #[test]
    fn skips_braces_inside_regex() {
        let src = r#"xyz=function(a){var re=/}{/;return a.replace(re,"");}"#;
        let body = match_to_closing_brace(src, "xyz=function").unwrap();
        assert!(body.starts_with("(a){"));
        assert!(body.ends_with("}"));
    }

    #[test]
    fn handles_nested_blocks() {
        let src = r#"
            xyz=function(a){
                if (a.length > 3) {
                    a = a.split("");
                    for (var i=0; i<a.length; i++) { a[i] = a[i]; }
                    return a.join("");
                }
                return a;
            };
        "#;
        let body = match_to_closing_brace(src, "xyz=function").unwrap();
        let opens = body.matches('{').count();
        let closes = body.matches('}').count();
        assert_eq!(opens, closes, "balanced");
        assert!(body.contains("(a)"));
        assert!(body.ends_with("}"));
    }

    #[test]
    fn anchor_not_found_returns_err() {
        let err = match_to_closing_brace("var a=1;", "qqq=function").unwrap_err();
        assert!(matches!(err, DeobfError::SigBodyParseFailed(_)));
    }
}