Best one-liner

Dave Burton
http://snox.net/ioccc
Twitter: @lv2jmp

The code for this entry can be found in prog.c

Judges' comments:

To use:

make
./prog

Try:

./prog 128

./prog_be 128

./prog 170

./prog 85

Selected Judges Remarks:

This tiny one-liner is a bit-twiddlers 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 XOR'ed.

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?

Author’s comments:

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 SEGV on zero arguments, and display strange results if 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