Arduino bitwise runninglight with a single delay - c

Question

I have the assignment to create a running light on my Arduino with the following requirements/constraints
Requirements
Implement a socalled “runninglight” with 3 leds.
The on time of each led is the same, being 0.5 sec.
After the last Led is lit, the first starts again.
Constraints
The leds are connected to pin 8,9 and 10 of the Arduino. (don’t forget the resistors)
The running led must be implemented in the loop of the Arduino
The setup() function may contain any Arduino function/macro.
Only the Arduino function delay() is allowed in the loop.
No use of any other Arduino function/macro etc. in the loop.
No use of for/while in the loop() function
No use of your own defined functions
Your loop function must contain not more then 10 statements.
So in the loop you may only use
o Delay function
o Your variables
o The following characters/digits: * ( ) & ~ | ; = < > ! + - 0 1 2 3 4 5 6 7 8 9
I have tried the following code
#include <Arduino.h>
//mask for setting correct bits in register to setup leds to 1
#define DDBMASK 0x07
//masks for setting correct bits in register for led control to 1
#define LEDMASK 0x01
byte *ddBPtr; //point to ddB register (data direction)
byte *portBPtr; //point to portB resiger (data register)
void setup() {
//setup data direction register to set pins to output
ddBPtr = (byte *) 0x24;
*ddBPtr |= DDBMASK; // 0b0000 0111
//assign pointer to right register
portBPtr = (byte *) 0x25;
}
void loop() {
//use data register (portB) for controlling leds
*portBPtr ^= LEDMASK;
delay(500);
*portBPtr ^= LEDMASK;
*portBPtr ^= LEDMASK << 1;
delay(500);
*portBPtr ^= LEDMASK << 1;
*portBPtr ^= LEDMASK << 2;
delay(500);
*portBPtr ^= LEDMASK << 2;
}
Apparently this is possible with only one delay function and following the requirements and constrains. I have been stuck on this for hours and cant figure it out.
I tried the following which also does not work because I'm unable to reset my counter back to zero
void loop() {
//use data register (portB) for controlling leds
*portBPtr ^= (LEDMASK << ledCount);
delay(500);
*portBPtr ^= (LEDMASK << ledCount);
ledCount++;
//cant figure out a way to reset ledCount back to 0 after 3 itterations
//running light only runs once so first led does not light after third led is lit
}
What am I missing?

Answer 1

Perhaps this:
*portBPtr |= (1 << ledCount);
delay(500);
*portBPtr &= ~(1 << ledCount);
ledCount = (9 >> (ledCount << 1)) & 3;
This also avoids ^ which is not in your list of allowed symbols.
The line ledCount = (9 >> (ledCount << 1)) & 3 is like a lookup table with each entry taking up two bits. It maps 0 to 1, 1 to 2, 2 to 0. The normal way to write this code is ledCount = (1 + ledCount) % 3;
This might also work, being a little simpler:
delay(500);
*portBPtr = (1 << ledCount);
ledCount = (9 >> (ledCount << 1)) & 3;
By using assignment rather than | or & or ^, all the other bits will be zero, presumably causing the non-current leds to turn off.