Author:

• Name: Dave Burton
  Location: US - United States of America (United States)

To build:

    make

Bugs and (Mis)features:

The current status of this entry is:

STATUS: INABIAF - please DO NOT fix

For more detailed information see 2020/burton in bugs.html.

To use:

    ./prog arg ...

Try:

    ./try.sh

Alternate code:

By default, the program works for Little Endian machines. This alternate build is for Big Endian machines (that works whether you have LE or BE).

Alternate build:

Running make builds the alternate code by default.

Alternate use:

Use prog_be as you would prog above.

Judges’ remarks:

Can you determine why this shows strange output for ./prog 128 128 and ./prog 128 128 128 128 but not ./prog 128 128 128?

Can you find a pattern here?

What happens if you specify different numbers?

This tiny one-liner is a bit-twiddler’s delight! It does all of the work without looping.

To understand how it works, a good place to start is to look at the hex value that results when the two magic constants are XORed.

Once you’ve figured out what happens with one set of constants, how can choosing new constants, but using the same computation generate a reversed result?

This code may be tested by:

    make test

Author’s remarks:

Invoke this with a single non-negative integer less than 256, and a useful transformation occurs.

Rule 2-ish:

Exporting the constants B,I,T,S to the Makefile allows the code to be compiled for either LE (little-endian) or BE (big-endian) machines(!!), so they are truly configuration parameters, not logic.

Notes:

One ping argument only. It will segfault on zero arguments, and display strange results with more than one argument.

Small, non-negative integers only. Useful range is 0 .. 511, sorta, for LE; 0 .. 255 for BE. Bonus points if you can explain why the limits work this way.

ASCII character set, 64-bit long longs are required.

v,a,r,i,a,b,l,e,s_NAME the operation.

No loops, no branches?!?

Compilation:

Compiles cleanly under clang -Wall -Weverything -pedantic, but you need to add --include stdio.h --include stdlib.h for the printf(3) and atoi(3).

On a little-endian machine:

    clang -include stdio.h -include stdlib.h -Wall -Weverything -pedantic -DB=6945503773712347754LL -DI=5859838231191962459LL -DT=0 -DS=7 -o prog prog.c

On a big-endian machine:

    clang -include stdio.h -include stdlib.h -Wall -Weverything -pedantic -DB=7091606191627001958LL -DI=6006468689561538903LL -DT=1 -DS=0 -o prog_be prog.c

You might be interested to compile it both ways on the same host, and try:

    ./prog 1; ./prog_be 1

Primary files

• prog.c - entry source code
• Makefile - entry Makefile
• prog.orig.c - original source code
• try.sh - script to try entry
• check_be.sh - test suite support file
• check_le.sh - test suite support file
• correct - test suite comparison data

Secondary files

• 2020_burton.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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