String+splitLastWhitespace.swift

extension String {
    public func splitLastWhitespace(after offset: Int) -> [Substring] {
        guard let lastWS = self.prefix(offset).lastIndex(where: \.isWhitespace) else {
            return [self[self.startIndex...]]
        }
        let first = self[self.startIndex..<lastWS]
        let nextIndex = self.index(after: lastWS)
        guard nextIndex != self.endIndex else { return [first] }
        return [first, self[nextIndex...]]
    }
}