broadcast-common 12.0.0

Foundation for the rust-broadcast crates: symmetric Parse/Serialize traits, CRC-32/MPEG-2, BCD + MJD time codecs, bit I/O, 33-bit/PCR clock arithmetic, checked field widths, container-mux and Stage traits. no_std + alloc.
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//! ISO 639 -> BCP 47 (RFC 5646) language canonicalisation.
//!
//! A single home for the mapping shared by the HLS and DASH paths: an ISO
//! 639-2 (bibliographic or terminology) code is canonicalised to the BCP 47
//! primary language subtag (ISO 639-1 two-letter code where one exists, else
//! the ISO 639-2/T three-letter code). Backed by the `iso-639` crate; no
//! hand-rolled code table.

/// Canonicalise an ISO 639-2 language code into a BCP 47 (RFC 5646) primary
/// language subtag.
///
/// HLS `LANGUAGE` and DASH `AdaptationSet@lang` must be RFC 5646 language tags;
/// in BCP 47 the primary subtag is the ISO 639-1 two-letter code when one
/// exists, otherwise the ISO 639-2/T (== ISO 639-3) three-letter code. ISO
/// 639-2/B bibliographic codes (`fre`, `ger`, `dut`, `chi`, ...) are not valid
/// BCP 47 subtags and are mapped to their terminology (T) equivalents first.
///
/// `qaa`..`qtz` (private use), `und`, `mul`, `mis` and `zxx` pass through
/// unchanged. Unknown or malformed codes return `None`.
pub fn bcp47_language(iso639_2: &str) -> Option<String> {
    let code = iso639_2.trim().to_ascii_lowercase();
    if code.len() != 3 || !code.bytes().all(|b| b.is_ascii_lowercase()) {
        return None;
    }
    if code == "und" || code == "mul" || code == "mis" || code == "zxx" {
        return Some(code);
    }
    if code.starts_with('q') {
        let bytes = code.as_bytes();
        if (b'a'..=b't').contains(&bytes[1]) && bytes[2].is_ascii_lowercase() {
            return Some(code);
        }
    }
    let lang = iso_639::Language::from_str(&code)?;
    if let Some(a2) = lang.alpha2() {
        return Some(a2.to_string());
    }
    if let Some(t) = lang.alpha3_t() {
        return Some(t.to_string());
    }
    lang.alpha3_b().map(|b| b.to_string())
}

/// Whether a candidate language tag matches a preferred language by BCP 47
/// prefix, ASCII case-insensitive.
///
/// `preferred` is the desired language tag (e.g. from a user preference);
/// `language` is the candidate tag (e.g. from a manifest). A match holds when
/// the two are equal or `language` is a more specific subtag of `preferred`
/// (separated by `-`).
#[must_use]
pub fn language_matches(preferred: &str, language: &str) -> bool {
    let (p, l) = (
        preferred.to_ascii_lowercase(),
        language.to_ascii_lowercase(),
    );
    l == p || l.strip_prefix(&p).is_some_and(|rest| rest.starts_with('-'))
}

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

    #[test]
    fn exact_and_prefix() {
        assert!(language_matches("en", "en"));
        assert!(language_matches("en", "en-US"));
        assert!(!language_matches("en", "fr"));
        assert!(!language_matches("en", "english"));
    }
}