code: func getu4(s []byte) rune {
  	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
  		return -1
  	}
  	var val rune
  	for i := 2; i < len(s) && i < 6; i++ {
  		var v byte
  		c := s[i]
  		switch c {
  		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
  			v = c - '0'
  		case 'a', 'b', 'c', 'd', 'e', 'f':
  			v = c - 'a' + 10
  		case 'A', 'B', 'C', 'D', 'E', 'F':
  			v = c - 'A' + 10
  		default:
  			return -1
  		}
  
  		val <<= 4
  		val |= rune(v)
  	}
  	return val
  }
docs: // getu4 decodes \uXXXX from the beginning of s, returning the hex value,
  // or it returns -1.