parseAcceptLanguage function

List<LanguageRange> parseAcceptLanguage(String header)

Parses header into language ranges ordered most-preferred first.

Returns an empty list for an empty/whitespace header. Malformed entries (empty tag, unparseable or out-of-range q) are skipped rather than failing the whole header, since a single bad client value should not deny content negotiation.

Example:

parseAcceptLanguage('en-US,en;q=0.9,fr;q=0.8')
  .map((r) => r.tag); // ('en-us', 'en', 'fr')

Audited: 2026-06-12 11:26 EDT

Implementation

List<LanguageRange> parseAcceptLanguage(String header) {
  final List<LanguageRange> ranges = <LanguageRange>[];
  if (header.trim().isEmpty) {
    return ranges;
  }

  for (final String raw in header.split(',')) {
    final LanguageRange? range = _parseEntry(raw);
    // Drop unparseable entries and explicit q=0 (not acceptable).
    if (range != null && range.quality > 0) {
      ranges.add(range);
    }
  }

  // Stable sort by quality descending: equal-weight tags keep header order,
  // which is the documented tie-break clients rely on.
  final List<int> order = List<int>.generate(ranges.length, (int i) => i);
  order.sort((int i, int j) {
    final int byQuality = ranges[j].quality.compareTo(ranges[i].quality);
    return byQuality != 0 ? byQuality : i.compareTo(j);
  });
  return <LanguageRange>[for (final int i in order) ranges[i]];
}