CS2105 Assignment 0 Solved

0. Accessing the sunfire Server
   To test your programs, please use your SoC UNIX ID and password to log on to sunfirefirst:
   • If you don’t have your SoC UNIX account, please create it here:

          https://mysoc.nus.edu.sg/~newacct
     

   • If you forget your SoC password, please reset it here using your NUSNET ID and password:

          https://mysoc.nus.edu.sg/~myacct/resetpass.cgi
     

   • If you are using a UNIX-like system (e.g. Linux, Mac, or Cygwin on Windows), SSH should be available from the command line and you may simply type:ssh <SoC-UNIX-ID>@sunfire.comp.nus.edu.sg
   • To copy files to sunfire on command line, use scp. The usage is as following:scp -r <source-folder> <SoC-UNIX-ID>@sunfire.comp.nus.edu.sg:<target-folder>

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• After logging on to sunfire, you can run a Python 3 script with the following command:/usr/local/Python-3.7/bin/python3 <path-to-script>For convenience, you can set a shortcut to python3 as follows:

       alias python3=/usr/local/Python-3.7/bin/python3
  

  This shortcut is temporary, and it lasts for the current SSH session only. To make it permanent, you can run the following command once to store the shortcut into the configuration file:

  echo alias python3=/usr/local/Python-3.7/bin/python3 >> ~/.bash_profile

  With the shortcut, you can then run a Python 3 script simply with

  python3 <path-to-script>

• If you are using a Windows machine, you may need to install an SSH client (e.g. “SSH Secure Shell Client”, available in LumiNUS Files -> “SSH Secure Shell” folder) if your machine does not have one installed. You may use the “SSH Secure File Transfer Client” software (bundled with “SSH Secure Shell Client”) to upload your programs to sunfire for testing.Testing Your Programs

• Upload your programs along with the test folder from the package to sunfire.
• Make sure that your programs and the test folder are in the same directory.
• Run bash test/<Exercise Name>.sh for testing.– Forexample,totestyourprogramforExercise1,runthefollowingcommand: bash test/Checksum.sh– By default, the script runs through all test cases. You can also choose to run a certain test case by specifying the case number in the command:

           bash test/Checksum.sh 3
  

• To stop a test, press and optionally hold Ctrl-c if pressing once does not exit thetest.
• If you use Java or C/C++, we will compile your program each time you run the testing script. The compiled code is stored in a temporary folder and removed after testing, in order to avoid affecting files under your own folders.
• You may assume that all input data to your programs are valid. Hence, there is no need to perform input data validation in your programs.If you have any question or encounter any problem with the steps above, please post your questions on LumiNUS forum or consult the teaching team.

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Program Submission

• Please create a zip file containing your source files (only) and submit it to the Assignment_0 folder of LumiNUS Files.
• The file name should be <Matric Number>.zip where <Matric Number> is your matriculation number which starts with letter A. An example file name would be A0165432X.zip.
• All file names, including the zip file and all source files within the zip, are case- sensitive.
• In addition, your zip file should not contain any folders or subfolders or irrelevant files such as test.jpg.
• Although we always try to find your actual source files during grading. You are encouraged to submit only once, or otherwise only your latest submission will be graded.You are not allowed to post your solutions to any publicly accessible site on the Internet.Plagiarism Warning

  You are free to discuss this assignment with your friends. But, ultimately, you should write your own program. We employ zero-tolerance policy against plagiarism. If a sus- picious case is found, student would be asked to explain his/her code to the evaluator in face. Confirmed breach may result in zero mark for the assignment and further dis- ciplinary action from the school.

  Question & Answer

  If you have any doubts on this assignment, please post your questions on Piazza. We are not supposed to debug programs for you, and we provide support for programming queries on a best-effort basis only. The intention of Q&A is to help clarify misconcep- tions and give you necessary directions.

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Exercise 1 – Checksum (1 mark)

Checksum can be used to detect if data is corrupted during network transmission (e.g. a bit flips from 0 to 1). Write a program Checksum.py to calculate the CRC-32 checksum for a file <src> entered as command-line argument. File <src> should be placed in the same folder as Checksum.py.

You may use the crc32() function from the zlib library to calculate the CRC-32 check- sum (see https://docs.python.org/3/library/zlib.html#zlib.crc32 for details. Remember to import this library before use!). Firstly, you will need to read all the bytes from a file and store them into a bytes object. You should open the file with binary reading mode by using “rb” as the mode argument of open(). Then, you can call the read() method of the file object and get the entire file content in a bytes object. Now you can get the checksum by directly calling crc32() with the file data, and finally print this unsigned 32-bit checksum.

An example code snippet is given below.

with open(“test.jpg”, “rb”) as f: bytes = f.read()

checksum = zlib.crc32(bytes)

Note: in practice, you may encounter an out-of-memory error if you attempt to load a huge file entirely into the memory. In this exercise, however, we limit the maximum file size to 10MB so that you can do so safely without triggering such errors.

If you use Java, you may want to import the java.util.zip.CRC32 class. If you use C/C++, you can call the crc32() function of zlib as well, and we will link your program to zlib using -lz during grading.

Sample run:

$ python3 Checksum.py test/test.jpg
3237218320

Exercise 2 – PacketExtr (1 mark)

In this exercise, you are going to write a program, PacketExtr.py, to read consecutive “packets” from the stdin stream, extract their data payloads in a responsive manner and output to stdout.

We define a custom format of packets for this exercise. A packet consists of a text- based header and a binary data payload following the header. The header is a string formatted as “Size:␣<size>B”, where ␣ represents a white-space character, and <size> is a decimal integer representing the number of bytes of the binary data following the header. The header is ended with the “B” character, and the payload immediately fol- lows the header without any byte (such as “\n”) in between. For example, “Size:␣2105B” is a complete and valid header.

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The binary payload can contain any byte, not limited to printable characters. Therefore, the payload should not be treated as string data in your program. This packet format clearly defines the boundary between header and binary data within a packet, and also the boundary between consecutive packets. It ensures that all information can be parsed correctly over a data stream.

Packets are fed to your program sequentially through stdin, until End-Of-File (EOF) is encountered. It is guaranteed that all packets have correct formats and correct pay- load sizes. Your program should be responsive to the input in the sense that upon receiving a full packet, it outputs the payload of the packet to stdout without any extra characters including newline. Unlike Exercise 1, the program cannot read all data once and process them in batch.

It is recommended that your program reads and writes data in binary mode instead of the default text mode. For interactive I/O, read1() should be used instead of the ordinary read(), and flush() should be called after each write() to avoid delays caused by buffering. Both read() and read1() accept one argument which is the max- imum number of bytes to read. The main difference is that, read() returns only when the specified number of bytes are read or End Of File (EOF) is encountered, while read1() returns immediately upon new data stream in and may return fewer bytes than specified. You can learn more about the details at https://docs.python.org/ 3/library/io.html#io.BufferedReader and https://docs.python.org/3/library/ io.html#io.BufferedWriter. The following code shows how to do binary I/O on stdin and stdout:

import sys
# read **at most** 5 bytes from stdin
data = sys.stdin.buffer.read1(5)
# write data to stdout and flush immediately sys.stdout.buffer.write(data) sys.stdout.buffer.flush()

Here, the data object is of bytes class. Python 3 programs can operate on binary data using bytes objects. This class is very similar to str, the string class. For example, both classes have functions find(), split(), and also slice operators for range access (e.g. a[0:10]). Details about this class can be found at https://docs.python.org/3/ library/stdtypes.html#bytes-and-bytearray-operations.

The following shows how to manipulate bytes objects:

# prepend b to the ’…’ expression to form a bytes object # instead of str
pos = data.find(b’x’)
if pos >= 0:

# if byte ’x’ is found in data
part1 = data [0: pos +1] # this is similar to str slicing part2 = data[pos+1:] # slice until the end of data

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To convert a bytes object to str, your program can call the decode() method of bytes. In this exercise, there is no need to worry about text encoding, as we only use basic ASCII characters in headers.

Finally, to detect EOF on stdin, your program can check the length of the bytes object read from stdin. If your program expects to read more data from stdin but receives a zero-length bytes object, it means that there is no more data on stdin and EOF is encountered.

When testing your program on command line, you can feed the contents of a file to stdin of your program using file redirection (<), instead of typing into the terminal. You can also use another type of redirection (>) to save the output of your program to a file rather than let it print to the terminal. For example, the following line feeds the file input.data to the program and save its output to output.data:

python3 PacketExtr.py < input.data > output.data
You can then compare binary contents of output.data and the given reference output

by running:

cmp output.data ref-output.data
By default, cmp outputs all differences found between the two files. Hence, no output

means that the two files are identical.

Note that above command line testing can only test your program’s correctness. Dur- ing grading, in addition to testing correctness, we will test responsiveness by setting a one-second timeout after feeding your program a full packet. That is, if your program does not output the packet payload on time, we will deem your program as unrespon- sive. This timeout is sufficient for our test data sizes. We reiterate that your program should not do batch processing, as interactive processing is one of the key points of this exercise. We limit the size of each packet to 1MB and the size of all packets to 10MB.

Sample run:

$ python3 PacketExtr.py < test/packets-a.in > run-a.out
$ cmp test/packets-a.out run-a.out

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