<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def Run(self, args):
 """Search the file for the pattern. This implements the grep algorithm used to scan files. It reads the data in chunks of BUFF_SIZE (10 MB currently) and can use different functions to search for matching patterns. In every step, a buffer that is a bit bigger than the block size is used in order to return all the requested results. Specifically, a preamble is used in order to not miss any patterns that start in one block of data and end in the next and also a postscript buffer is kept such that the algorithm can return bytes trailing the pattern even if the pattern is at the end of one block. One block: | Pre | Data | Post | Searching the pattern is done here: The following block is constructed like this: | Pre | Data | Post | | | Pre | Data | Post | The preamble is filled from Data so every hit that happens to fall entirely into the preamble has to be discarded since it has already been discovered in the step before. Grepping for memory If this action is used to grep the memory of a client machine using one of the GRR memory acquisition drivers, we have to be very careful not to have any hits in the GRR process memory space itself. Therefore, if the input is a literal, it is XOR encoded and only visible in memory when the pattern is matched. This is done using bytearrays which guarantees in place updates and no leaking patterns. Also the returned data is encoded using a different XOR 'key'. This should guarantee that there are no hits when the pattern is not present in memory. However, since the data will be copied to the preamble and the postscript, a single pattern might in some cases produce multiple hits. Args: args: A protobuf describing the grep request. Raises: RuntimeError: No search pattern has been given in the request. """

    fd = vfs.VFSOpen(args.target, progress_callback=self.Progress)
    fd.Seek(args.start_offset)
    base_offset = args.start_offset

    self.xor_in_key = args.xor_in_key
    self.xor_out_key = args.xor_out_key

    if args.regex:
      find_func = functools.partial(self.FindRegex, args.regex.AsBytes())
    elif args.literal:
      find_func = functools.partial(self.FindLiteral, args.literal.AsBytes())
    else:
      raise RuntimeError("Grep needs a regex or a literal.")

    preamble_size = 0
    postscript_size = 0
    hits = 0
    data = b""
    while fd.Tell() < args.start_offset + args.length:

      # Base size to read is at most the buffer size.
      to_read = min(args.length, self.BUFF_SIZE,
                    args.start_offset + args.length - fd.Tell())
      # Read some more data for the snippet.
      to_read += self.ENVELOPE_SIZE - postscript_size
      read_data = fd.Read(to_read)

      data = data[-postscript_size - self.ENVELOPE_SIZE:] + read_data

      postscript_size = max(0, self.ENVELOPE_SIZE - (to_read - len(read_data)))
      data_size = len(data) - preamble_size - postscript_size

      if data_size == 0 and postscript_size == 0:
        break

      for (start, end) in find_func(data):
        # Ignore hits in the preamble.
        if end <= preamble_size:
          continue

        # Ignore hits in the postscript.
        if end > preamble_size + data_size:
          continue

        # Offset of file in the end after length.
        if end + base_offset - preamble_size > args.start_offset + args.length:
          break

        data_start = max(0, start - args.bytes_before)
        data_end = min(len(data), end + args.bytes_after)
        out_data = utils.Xor(data[data_start:data_end], self.xor_out_key)

        hits += 1
        self.SendReply(
            rdf_client.BufferReference(
                offset=base_offset + start - preamble_size,
                data=out_data,
                length=len(out_data),
                pathspec=fd.pathspec))

        if args.mode == rdf_client_fs.GrepSpec.Mode.FIRST_HIT:
          return

        if hits >= self.HIT_LIMIT:
          msg = utils.Xor(
              b"This Grep has reached the maximum number of hits"
              b" (%d)." % self.HIT_LIMIT, self.xor_out_key)
          self.SendReply(
              rdf_client.BufferReference(offset=0, data=msg, length=len(msg)))
          return

      self.Progress()

      base_offset += data_size

      # Allow for overlap with previous matches.
      preamble_size = min(len(data), self.ENVELOPE_SIZE)