Question

This may be a really silly question, but it just crossed my mind and I thought it would be interesting to know for sure….

So here is the scenario:

Users have 3 options for each day of the week: AM, PM, and OFF. These are mutually exclusive choices, so there is no option to work both an AM and PM on same day.

So if I wanted to store their AM shifts and PM shifts as separate bitmasks, and User1 chooses the following:

 S    M   T   W   Th   F   Sa
 A    P   X   A   X    P   A

I would have the following:

 $shifts['User1']['AM'] = 73;  //  1001001
 $shifts['User1']['PM'] = 34;  //  0100010

Now, if I just wanted to know which days User1 worked, I could obviously just do:

 $shifts['User1']['All'] = $shifts['User1']['AM'] | $shifts['User1']['PM'];

Or even just:

 $shifts['User1']['All'] = $shifts['User1']['AM'] + $shifts['User1']['PM'];

But what if I wanted the final result to distinguish AM from PM, something to the effect of:

 $shifts['User1']['AM'] = A00A00A;
 $shifts['User1']['PM'] = 0P000P0;

So that the A’s and P’s are both considered set, but that

 A00A00A | 0P000P0 = AP0A0PA;

Is there a common way of doing this, or am I thinking about this totally wrong?

Answer 1

To represent three states in a binary fashion you need 2 bits. For instance, you could say that:

PM = 01

AM = 10

OFF = 00

So now you have this:

A00A00A translates to 10 00 00 10 00 00 10

0P000P0 translates to 00 01 00 00 00 01 00

Applying bitwise OR operation:

10 00 00 10 00 00 10
00 01 00 00 00 01 00
--------------------
10 01 00 10 00 01 10
A  P  0  A  0  P  A

You get AP0A0PA, your desired result.