code: func getu4(s []byte) rune {
  	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
  		return -1
  	}
  
  	// logic taken from:
  	// github.com/buger/jsonparser/blob/master/escape.go#L20
  	var h [4]int
  	for i := range h {
  		c := s[2+i]
  		switch {
  		case c >= '0' && c <= '9':
  			h[i] = int(c - '0')
  		case c >= 'A' && c <= 'F':
  			h[i] = int(c - 'A' + 10)
  		case c >= 'a' && c <= 'f':
  			h[i] = int(c - 'a' + 10)
  		default:
  			return -1
  		}
  	}
  	return rune(h[0]<<12 + h[1]<<8 + h[2]<<4 + h[3])
  }
docs: // getu4 decodes \uXXXX from the beginning of s, returning the hex value,
  // or it returns -1.