docs: int nextIndexOf(CharSequence seq) {
          bufferUp();
          // doesn't handle scanning for surrogates
          char startChar = seq.charAt(0);
          for (int offset = bufPos; offset < bufLength; offset++) {
              // scan to first instance of startchar:
              if (startChar != charBuf[offset])
                  while(++offset < bufLength && startChar != charBuf[offset]) { /* empty */ }
              int i = offset + 1;
              int last = i + seq.length()-1;
              if (offset < bufLength && last <= bufLength) {
                  for (int j = 1; i < last && seq.charAt(j) == charBuf[i]; i++, j++) { /* empty */ }
                  if (i == last) // found full sequence
                      return offset - bufPos;
              }
          }
          return -1;
      }