def read1(self, n):
        """Read up to n bytes with at most one read() system call."""

        # Simplify algorithm (branching) by transforming negative n to large n.
        if n < 0 or n is None:
            n = self.MAX_N

        # Bytes available in read buffer.
        len_readbuffer = len(self._readbuffer) - self._offset

        # Read from file.
        if self._compress_left > 0 and n > len_readbuffer + len(self._unconsumed):
            nbytes = n - len_readbuffer - len(self._unconsumed)
            nbytes = max(nbytes, self.MIN_READ_SIZE)
            nbytes = min(nbytes, self._compress_left)

            data = self._fileobj.read(nbytes)
            self._compress_left -= len(data)

            if data and self._decrypter is not None:
                data = ''.join(map(self._decrypter, data))

            if self._compress_type == ZIP_STORED:
                self._update_crc(data, eof=(self._compress_left==0))
                self._readbuffer = self._readbuffer[self._offset:] + data
                self._offset = 0
            else:
                # Prepare deflated bytes for decompression.
                self._unconsumed += data

        # Handle unconsumed data.
        if (len(self._unconsumed) > 0 and n > len_readbuffer and
            self._compress_type == ZIP_DEFLATED):
            data = self._decompressor.decompress(
                self._unconsumed,
                max(n - len_readbuffer, self.MIN_READ_SIZE)
            )

            self._unconsumed = self._decompressor.unconsumed_tail
            eof = len(self._unconsumed) == 0 and self._compress_left == 0
            if eof:
                data += self._decompressor.flush()

            self._update_crc(data, eof=eof)
            self._readbuffer = self._readbuffer[self._offset:] + data
            self._offset = 0

        # Read from buffer.
        data = self._readbuffer[self._offset: self._offset + n]
        self._offset += len(data)
        return data
Read up to n bytes with at most one read() system call.