Most in need of transparency

Judges' comments:

To use:

make

./prog sample_input.txt a b c d

cat a

cat b

cat c

cat d

cat a b c d | ./prog

Try:

./prog prog.c a b c d e

cat a

cat b

cat c

cat d

cat e

cat a b c d e | ./prog

cat a b c d e | ./prog | diff - prog.c

Selected Judges Remarks:

Do you understand how this program works? You may have to go over and over the output again.

Author’s comments:

Summary

Scramble/unscramble to single file:

./prog original.txt > scrambled.txt

./prog < scrambled.txt > unscrambled.txt

Scramble/unscramble to multiple files:

./prog input.txt output1.txt output2.txt ...

cat output1.txt output2.txt ... | ./prog > unscrambled.txt

Re-scramble:

./prog original.txt | ./prog - > scrambled1.txt
./prog original.txt | ./prog - | ./prog - > scrambled2.txt
./prog original.txt | ./prog - | ./prog - | ... > scrambled3.txt

Scramble slowly:

./prog input.txt | ruby prog.c

Input must be in UTF-8 encoding.

Details

This is a text scrambling utility. To scramble text, specify input file name as the first argument:

./prog input.txt > output.txt

If original input file was small enough to fit in your terminal, you can simply use cat to unscramble the output text.

cat output.txt

It might seem like nothing was scrambled, but you can try passing the output through strings(1) and see that most of the original strings are gone. What happened is that the original text has been shuffled with ANSI cursor positioning codes inserted such that characters are printed in random order. This effect is more visible if there is a delay after each character is printed. For convenience, an utility is included in the source to output characters slowly:

./prog input.txt | ruby prog.c

Scrambling order is deterministic, but you can reshuffle the output by piping it through the same utility again, by specifying - as the first argument:

./prog input.txt | ./prog - | ./prog - | ...

You can also split the input to multiple files, each getting a subset of the original input characters:

./prog input.txt output1.txt output2.txt ...
cat output1.txt output2.txt ... > unscrambled.txt

You can only unscramble files with cat if the input fits in a single terminal screen. If you don’t know what will be the terminal size of the recipient, note that prog.c will fit in a 80x25 terminal, so a good guideline is to pick files that have fewer rows and columns than prog.c.

For files that are larger than your terminal window, cat will not be sufficient for unscrambling because any scrolling causes the character positions to be misaligned. But fear not, this utility also includes unscrambling functionality. If no argument is specified, input will be read through stdin and unscrambled to stdout:

./prog < scrambled.txt > unscrambled.txt

Where this is useful is that if you scramble the input to multiple files and unscramble each output files individually, those output files can be printed on transparencies and distributed separately.

./prog input.txt output1.txt output2.txt output3.txt
./prog < output1.txt > layer1.txt
./prog < output2.txt > layer2.txt
./prog < output3.txt > layer3.txt

In other words, this is a utility for facilitating visual cryptography.

This utility can also be used just for removing trailing whitespaces and expanding tabs to 8 spaces.

./prog input.txt | ./prog

Features

UTF-8 support

A simple text scrambling utility might not have required a screenful of code, but this particular one natively supports UTF-8. Specifically: individual UTF-8 byte sequences will not be split into broken ones, and the utility is aware of full-width versus half-width differences such that cursor is positioned correctly for most CJK input. sample_input.txt demonstrates this feature with a mix of Basic Latin and Japanese characters.

Output cursor positions will most definitely be wrong for inputs containing combining characters and right-to-left text, but it should be possible to unscramble those text back to the original using this same utility, since the utility is self-consistent in how cursor positions are handled.

Error handling

If any file can not be opened for reading or writing, an error is printed to stderr, and the utility will exit with nonzero status.

If there isn’t enough memory, the utility will likely crash.

If input contains CR-LF end of line sequences, those CRs are silently dropped. In fact, most control codes are silently ignored except line feeds (preserved) and tabs (expanded to 8 spaces).

Compatibility

Verified to work and compiles without warnings on these environments:

gcc 7.3.0 on Linux
gcc 7.4.0 on Cygwin
clang 4.0.1-10 on Linux
clang 5.0.1 on Cygwin
gcc 4.8.4 on Linux

Full version

I tried really hard to make prog.c fit in a 80x25 terminal without wrapping. If I were not constrained by space, I would have made an ASCII art out of it. violet.c is that program, and also comes with the minor benefit of supporting lines longer than a million columns depending on sizeof(int).

Violet is named after “Violet Evergarden”, the character who is highly skilled in writing letters.