paint like BWBWWBWWWB ......

how to generate two or more W ???
Sol : After painting by W leave some space without paint. Again paint with W.

Consider 8 sectors on the disk. Paint 4 consecutive sectors with one paint, say White. Then paint one adjacent sector with Black paint. Then the next adjacent sector to this one with White paint and the remaining two sectors with Black paint.

So the disk now looks like ....W W W W B W B B... in a circular fashion.

Once you can see a sequence having 4 consecutive whites, you can tell which direction the disk is spinning.

After 4 whites, if you get one black and then one white, it is spinning in the clockwise direction.

After 4 whites, if you get 2 blacks and one white, it is spinning in the anti-clockwise direction.

I dont know but I assume to color an arrow like structure having a tail and a head. Color the head ">" in black and tail ">" in white. Now while the CD is revolving, if the tail follows head its Clockwise and if Head follows Tail its anti-clockwise.

changing the icons like Head will be like a tilted triangle and tail like an greater than sign ">"

hi ck
m sorry .... i think your answer is not correct because how would a sensor come to know whether color sign is > or <. sensor would give just give output same in both the cases.
VP's answer would work definitely.

I've verified that WWBBBWBW is one of the correct answer.
If we use binary, the equivalent question is:
" find an eight digital binary string where the circular left shift and the circular left shift for the inverse string are different."

This brings me to think:
" Is there an invariant while doing the left shifting? (no matter how we do circular left shift on this string, we can apply some logical operation to that binary string and get an invariant) while inverting that string changes this invariant? "

I have another idea to add

you can have painting pattern like this.
B W B&W
the last one is the mix of black and white.Even if the sensor cant sense that color,the direction can be found.
If clock wise B W _ B W _
if anti clock wise W B _ W B _
Is this ok?

Assign uniques codes to clockwise and counter-clockwise. Whichever one appears will indicate direction of spin.

This problem is analogous to finding Endianess.

may be we can have a sequence like bwbwwbwwwbwwwwbwwwww.......
so if see W's increasing then we can have it as one direction
and if we see them decreasing then another direction

IMHO VPs answer is the best one and simple to understand as well...thanx vp..

One possible solution can be WBBWWWB with the most optimised approach.
Please validate, if posible...

Cheers!!

Blanks are unnecessary. I found that the easiest way is to paint it as BWBWWB.
If the sensor detect BWWBWB or BWBBWW, the CD runs the opposite direction.