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Cody Boone Ferguson 0c6b533363 Rename demo.sh scripts to try.sh

This is because they are under the try section. I'm aware that there's a
script called try.me.sh but that's not been decided on a name change. It
probably should be changed to try.sh to be consistent and probably will
be but it's not been yet.

The manifest files in tmp/ have been updated as well, including the
macOS Numbers file.

2023-10-29 01:48:16 -07:00

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Best One Liner

Thomas A. Fine
US
@thomasafine@social.linux.pizza

To build:

make all

To use:

echo text | ./fine

Try:

./try.sh

Make sure that you read the last line backwards! Credit goes to Merlyn LeRoy (Brian Westley) for that. See his 22 August 1992 rec.puzzles post. To those who have a harder time reading backwards it says: Gary lent the rug, rotated from the rug gary lent!

Judges' remarks:

This filter, 80 chars plus a newline, fits into a single line on most terminals (unless your terminal has a line wrap mis-feature :-)).

Note in 2023: fixing this entry to work with modern systems increased the size, originally to 106 characters but dropped down to 85 with clever use of the Makefile and removal of a cast that was not strictly necessary.

This entry is likely one of the smallest C implementations of this filter, excluding programs that resort to command line or include file tricks.

How does this program work? Which 3 bytes of C code can be changed into 2 bytes, allowing the program to still work, but also stripping the high bit off some input?

Author's remarks:

The author wishes to thank J Greely for the last 6 bytes.

Here's about how it does it: [rot13 to read]

NOTE: Bits are referred to as 76543210, high to low.

Get the character in variable a. Note the fun way we check for EOF. (thanks J).
Reverse bit 6, and strip bit 5. Bit 6 determines (mostly) whether or not this character is a letter. Bit 5 would determine case if we cared. Assign this value to variable b (We need to save it, since we need to make 2 comparisons and maybe one operation with this value). We can't abuse our first variable, since it saves the bits we're changing.
Check this value to see if it is a letter. It must be non-zero - if it is zero, then we have either the @ (64) or the ` (96). It also must be less than 27 - this checks the range for letters as well as implicitly checking the value of bit 6 (if it was originally clear, it is set now, so b would be too big).
If it is not a letter, the conditional expression inside of the putchar() returns the original character (a). Otherwise, it returns a really messy expression that performs the actual rotation and fixes the fifth and sixth bits:
- A. Subtract 1, to get into the range 0-25 (for the modulo operation).
- B. Add 13 to the character. Steps A and B are combined.
- C. Take this modulo 26 to make the addition wrap around.
- D. Fix bits 5 and 6, based on variable a.
- E. Add 1 back in (we took it out in step A.) This is okay to do after step D, because we know the addition won't trickle up past the fourth bit (b would have to be 31 for this to happen).

Notice that this implementation operates only on characters with the high bit clear. The high bit can be ignored without changing the size of the program. Also, if you know you are only dealing with seven bit data, the size can be reduced by one byte. These are left as exercises for the student.

Copyright and CC BY-SA 4.0 License:

Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)

For more information, see: https://creativecommons.org/licenses/by-sa/4.0/

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