snipped from FTS4 for HYPERTRACK pod

FTS4.swift

            let view:String.UTF8View = string.utf8

            if let from = range.lowerBound.samePosition(in: view),
               let to = range.upperBound.samePosition(in: view) {
                offset.pointee += Int32(string[string.startIndex..<range.lowerBound].utf8.count)
                length.pointee = Int32(view.distance(from: from, to: to))
                return token
            } else {
                return nil
            }