Syplheed-Reborn/crates/sylpheed-formats/src/ratc.rs

//! `RATC` — a nested resource bundle.
//!
//! After a small header, a RATC holds named children — `foo.t32` (T8aD
//! textures), `foo.rat` (nested RATC), fonts, PNG — each immediately preceded by
//! its ASCII name string. The children are self-locating by their 4-char magic,
//! so we list them by scanning for those magics and pairing each with the name
//! run that precedes it. This is reliable for *listing* (names / types / sizes
//! are literal bytes); decoding a child's pixels is delegated to that child's
//! own parser ([`crate::t8ad`]).

/// A child resource inside a RATC bundle.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct RatcChild {
    /// Name string preceding the child (e.g. `prselectbtn_g04b.t32`), or empty.
    pub name: String,
    /// Child magic tag: `T8aD`, `RATC`, `ttcf`, `png`, …
    pub kind: String,
    /// Byte offset of the child (its magic) within the RATC payload.
    pub offset: usize,
    /// Byte length of the child, up to the next child (or end of payload).
    pub size: usize,
}

/// Magic at the start of a RATC bundle.
pub const RATC_MAGIC: [u8; 4] = *b"RATC";

/// Whether `bytes` is a RATC bundle.
pub fn is_ratc(bytes: &[u8]) -> bool {
    bytes.len() >= 4 && bytes[0..4] == RATC_MAGIC
}

/// Recognized child magics and their display tag.
fn child_kind(m: &[u8]) -> Option<&'static str> {
    match m {
        b"T8aD" => Some("T8aD"),
        b"RATC" => Some("RATC"),
        b"ttcf" => Some("ttc"),
        b"\x89PNG" => Some("png"),
        _ => None,
    }
}

/// List the children of a RATC bundle (the self-magic at offset 0 is skipped).
pub fn parse(bytes: &[u8]) -> Option<Vec<RatcChild>> {
    if !is_ratc(bytes) {
        return None;
    }

    // Offsets of every recognized child magic (skip the self RATC at 0).
    let mut offs: Vec<(usize, &'static str)> = Vec::new();
    let mut i = 4;
    while i + 4 <= bytes.len() {
        if let Some(kind) = child_kind(&bytes[i..i + 4]) {
            offs.push((i, kind));
            i += 4;
        } else {
            i += 1;
        }
    }

    let mut children = Vec::with_capacity(offs.len());
    for (idx, &(off, kind)) in offs.iter().enumerate() {
        let next = offs.get(idx + 1).map(|&(o, _)| o).unwrap_or(bytes.len());
        children.push(RatcChild {
            name: name_before(bytes, off),
            kind: kind.to_string(),
            offset: off,
            size: next.saturating_sub(off),
        });
    }
    Some(children)
}

/// The nearest name string preceding `off`: the *last* printable run (len ≥ 3)
/// in the 96 bytes before the child magic. A few record-header bytes usually sit
/// between the name and the magic, so an exact-adjacency scan isn't enough.
fn name_before(bytes: &[u8], off: usize) -> String {
    let start = off.saturating_sub(96);
    let window = &bytes[start..off];
    let mut best = String::new();
    let mut run_start: Option<usize> = None;
    let flush = |from: usize, to: usize, best: &mut String| {
        if to - from >= 3 {
            *best = String::from_utf8_lossy(&window[from..to]).trim().to_string();
        }
    };
    for (i, &c) in window.iter().enumerate() {
        if (0x20..=0x7e).contains(&c) {
            run_start.get_or_insert(i);
        } else if let Some(s) = run_start.take() {
            flush(s, i, &mut best);
        }
    }
    if let Some(s) = run_start {
        flush(s, window.len(), &mut best);
    }
    best
}

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

    #[test]
    fn lists_named_children() {
        let mut b = RATC_MAGIC.to_vec();
        b.extend_from_slice(&[0u8; 28]); // header padding
        b.extend_from_slice(b"logo.t32");
        let t8_off = b.len();
        b.extend_from_slice(b"T8aD");
        b.extend_from_slice(&[0u8; 40]); // some child bytes
        b.extend_from_slice(b"sub.rat");
        let ratc_off = b.len();
        b.extend_from_slice(b"RATC");
        b.extend_from_slice(&[0u8; 8]);

        let kids = parse(&b).unwrap();
        assert_eq!(kids.len(), 2);
        assert_eq!(kids[0].kind, "T8aD");
        assert_eq!(kids[0].name, "logo.t32");
        assert_eq!(kids[0].offset, t8_off);
        assert_eq!(kids[0].size, ratc_off - t8_off);
        assert_eq!(kids[1].kind, "RATC");
        assert_eq!(kids[1].name, "sub.rat");
    }

    #[test]
    fn rejects_non_ratc() {
        assert!(parse(b"T8aD....").is_none());
    }
}