excerptAround function

String excerptAround(String text, String query, { int contextChars = 60, String ellipsis = '...', })

Returns an excerpt of text around the first occurrence of query, with contextChars characters on each side and ellipsis at truncation points.

Implementation

String excerptAround(String text, String query, {int contextChars = 60, String ellipsis = '...'}) {
  if (text.isEmpty) return '';
  final String q = query.trim();
  if (q.isEmpty) {
    if (text.length <= contextChars * 2 + 2) return text;
    final int takeFirst = contextChars.clamp(0, text.length);
    final int fromEnd = (text.length - contextChars).clamp(0, text.length);
    return '${text.substringSafe(0, takeFirst)}$ellipsis${text.substringSafe(fromEnd)}';
  }
  // Match case-insensitively but slice the original-case text: lowercase only
  // for locating the index so the returned excerpt preserves the source casing.
  final int idx = text.toLowerCase().indexOf(q.toLowerCase());
  if (idx < 0) {
    if (text.length <= contextChars * 2 + 2) return text;
    final int takeFirst = contextChars.clamp(0, text.length);
    final int fromEnd = (text.length - contextChars).clamp(0, text.length);
    return '${text.substringSafe(0, takeFirst)}$ellipsis${text.substringSafe(fromEnd)}';
  }
  final int start = (idx - contextChars).clamp(0, text.length);
  final int end = (idx + q.length + contextChars).clamp(0, text.length);
  final int safeStart = start.clamp(0, text.length);
  final int safeEnd = end.clamp(0, text.length);
  // Add a leading/trailing ellipsis only when the window is actually clipped at
  // that edge; an excerpt that reaches the real start or end gets no ellipsis.
  final String head = start > 0 ? '$ellipsis ' : '';
  final String tail = end < text.length ? ' $ellipsis' : '';
  return '$head${text.substringSafe(safeStart, safeEnd)}$tail';
}