String relative()

in lib/src/context.dart [467:546]
49 lines of code
24 McCabe index (conditional complexity)

  String relative(String path, {String? from}) {
    // Avoid expensive computation if the path is already relative.
    if (from == null && isRelative(path)) return normalize(path);

    from = from == null ? current : absolute(from);

    // We can't determine the path from a relative path to an absolute path.
    if (isRelative(from) && isAbsolute(path)) {
      return normalize(path);
    }

    // If the given path is relative, resolve it relative to the context's
    // current directory.
    if (isRelative(path) || isRootRelative(path)) {
      path = absolute(path);
    }

    // If the path is still relative and `from` is absolute, we're unable to
    // find a path from `from` to `path`.
    if (isRelative(path) && isAbsolute(from)) {
      throw PathException('Unable to find a path to "$path" from "$from".');
    }

    final fromParsed = _parse(from)..normalize();
    final pathParsed = _parse(path)..normalize();

    if (fromParsed.parts.isNotEmpty && fromParsed.parts[0] == '.') {
      return pathParsed.toString();
    }

    // If the root prefixes don't match (for example, different drive letters
    // on Windows), then there is no relative path, so just return the absolute
    // one. In Windows, drive letters are case-insenstive and we allow
    // calculation of relative paths, even if a path has not been normalized.
    if (fromParsed.root != pathParsed.root &&
        ((fromParsed.root == null || pathParsed.root == null) ||
            !style.pathsEqual(fromParsed.root!, pathParsed.root!))) {
      return pathParsed.toString();
    }

    // Strip off their common prefix.
    while (fromParsed.parts.isNotEmpty &&
        pathParsed.parts.isNotEmpty &&
        style.pathsEqual(fromParsed.parts[0], pathParsed.parts[0])) {
      fromParsed.parts.removeAt(0);
      fromParsed.separators.removeAt(1);
      pathParsed.parts.removeAt(0);
      pathParsed.separators.removeAt(1);
    }

    // If there are any directories left in the from path, we need to walk up
    // out of them. If a directory left in the from path is '..', it cannot
    // be cancelled by adding a '..'.
    if (fromParsed.parts.isNotEmpty && fromParsed.parts[0] == '..') {
      throw PathException('Unable to find a path to "$path" from "$from".');
    }
    pathParsed.parts.insertAll(0, List.filled(fromParsed.parts.length, '..'));
    pathParsed.separators[0] = '';
    pathParsed.separators
        .insertAll(1, List.filled(fromParsed.parts.length, style.separator));

    // Corner case: the paths completely collapsed.
    if (pathParsed.parts.isEmpty) return '.';

    // Corner case: path was '.' and some '..' directories were added in front.
    // Don't add a final '/.' in that case.
    if (pathParsed.parts.length > 1 && pathParsed.parts.last == '.') {
      pathParsed.parts.removeLast();
      pathParsed.separators
        ..removeLast()
        ..removeLast()
        ..add('');
    }

    // Make it relative.
    pathParsed.root = '';
    pathParsed.removeTrailingSeparators();

    return pathParsed.toString();
  }