Day 04

The elves have stacked the toilet rolls (@) amongst other empty slots (.) in a storage room as shown below:

..@@.@@@@.
@@@.@.@.@@
@@@@@.@.@@
@.@@@@..@.
@@.@@@@.@@
.@@@@@@@.@
.@.@.@.@@@
@.@@@.@@@@
.@@@@@@@@.
@.@.@@@.@.

To get to them, they need space around them (<4 neighbouring rolls @)

Toilet roll neighbour count from 0 (cold) to 8 (hot)

Tips

parse the file into a text array and then convert (replace) all the positions with . by 0 and all the positions with @ by 1 to get a binary matrix
this is a classic “convolution” problem - you can use conv2() with a 3x3 kernel of ones to count the number of neighbouring rolls for each position
be careful on how you construct the neighbourhood kernel… and finally
remember that the neighbour count is only relevant at positions where there is a roll (@ / 1)
so you will have to combine the neighbourhood information with the current layout of rolls.

The second problem is similar, but now you have to iterate the process described in step 1 until no more rolls can be removed.

there are no do... while loops in Matlab, but you can start with a while true and then break when the condition (e.g. no_more_rolls_removed) is met

Animated gif showing the iterative removal of toilet rolls by elves ;)

Matlab solution

Matlab code / solution for the first part of that problem.