Lab13. String Encoding

EE231002 Introduction to Programming
Lab13. String Encoding

Traditional English characters are encoded using 7-bit ASCII format. Thus, some existing
network communication protocols assume 7-bit per byte streams. Other bytes longer than 7
bits are used for network control. For non-ASCII characters, such Chinese, these protocols
have troubles handling their transmission. This limitation was recognized in the early days,
and some encoding schemes were developed. In essence, these encoding schemes convert nonASCII characters into ASCII streams making them transmittable using the standard protocols.
Then, decoders are invoked to translate those streams back to the original format. In this way,
non-ASCII streams can be easily transmitted using the existing framework.
In this assignment we will implement a special encoding scheme. A short encoded file is
shown below:
begin
4@f5d2P00
end
The first line is a header. The encoded file always starts with the begin keyword. The
encoded text starts from the next line. Each encoded line begins with a line lengths character,
then followed by the encoded string. Depends on the original text, the number of encoded line
varies and can be as large as needed. The tail of the encoded file is a single line, end.
The line length character is formed by adding 48 to the length and then treat the sum as
a single ASCII character. For example, the line length of the original text is 4, adding 48 it
becomes 52, which is 0x34. And the ASCII code is ’4’.
The rest of encoded string is formed by taking 3 original characters, which have total of 24
bits, split them into 4 groups, each group then has 6 bits. Two 0’s are added to the left to form
8-bit char again. And then, add 48 (or 0x30) to each byte. These 4 char’s are then the encoded
text for the 3 original characters.
Original text: C a t
ASCII code: 0100,0011 0110,0001 0111,0100
Splitting: 0001,0000 0011,0110 0000,0101 0011,0100
Add 0x30: 0100,0000 0110,0110 0011,0101 0110,0100
Encoded text: @ f 5 h
The encoding process continues by taking the next 3 characters as input and producing 4
characters output. This process is repeated until all the input characters are processed. In the
case that we have less than 3 characters left to be encoded. NULL characters ('\0') are added to
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form 3 characters such that the preceding encoding process can be carried out. The line length
character, however, does not count these added NULL characters, and in decoding, they should
not be printed out. Also note that all control characters, such as \t, \n, \0, are also encoded.
Your assignment is to write a C program to decode an encoded file. The program should
read the header line and ignore it. Then convert the encoded string back to the original form by
revering the process as shown in the figure above. Of course, it needs to detect the tail section
and stop the conversion process. Two encoded files are provided for you to test your program:
etext.cd and ctext.cd. You can use the standard Unix command line to redirect file as the
standard input to the program as the following:
$ ./a.out < etext.cd
Of course, you can also save the decoded strings to a file using the following command as an
example. By doing so, you can detect if any non-printable characters have been accidentally
printed out.
$ ./a.out < etext.cd > etext
Notes.
1. Create a directory lab13 and use it as the working directory.
2. Name your program source file as lab13.c.
3. The first few lines of your program should be comments as the following.
/* EE231002 Lab13. String Encoding
ID, Name
Date:
*/
4. After you finish verifying your program, you can submit your source code by
$ ∼ee2310/bin/submit lab13 lab13.c
If you see a ”submitted successfully” message, then you are done. In case you want to
check which file and at what time you submitted your labs, you can type in the following
command:
$ ∼ee2310/bin/subrec lab13
It will show the submission records of lab13.
5. You should try to write the program as efficient as possible. The format of your program
should be compact and easy to understand. These are part of the grading criteria.
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