That's awesome news, congrats on solving it!

I thought it only destroy itself if it LANDS on a gem of its color

That would be rainbow gem. Crash gems trigger around it too. In your given board, it would destroy RR on left, and if B was R, it would clear the entire board at once.

S,1,2 are all red gems, so they cannot merge with Green gems. So maybe 1 expand left one, 2 cannot expand?

Yeah, seems correct.

can crash gems be part of a power gem?

No, because as soon as it touches another gem of its color (if another gem lands next to it), no matter if powergem or not, everything around it gets destroyed (including itself ofc).

Btw, there's 1 tip that I can give you, and it's that the tests are very thorough and you have to account for every possibility. You may think you could take some shortcuts, or avoid certain situations, but it won't work. Soon enough, you can end up with a lot of spaghetti code :P

It's been some time since I solved it, but from what I remember, first all non-powergem pieces form gems, if possible (top to bottom, left to right), then merging single pieces with gems (horizontally first, then vertically), then merging gems with other gems.

I'm not sure how to interpret that first board, but if I understood correctly, then 1 would expand left once, then 2 would merge with G (horizontal growth priority)

And yes, merging can occur many times: in your second board, 1 would merge with S on left AND right side, then it would merge with gem2 to form 1 massive powergem.

At least that's how I remember these interactions, I could be wrong :D

Edit: for the first board, let's say those S weren't there. I think 2 would merge first, even though 1 is higher, because of horizontal growth first, as mentioned previously.

In your given board, none of them will merge. For power gems to merge, they should have same exact height (if you want to merge horizontally), and start at same elevation. (similar with vertical merging)

For example: 0 - empty place, 1,2 - power gems (let's assume all non-0s are red)

0000000
0000000
0000000
1122222
1122222

1 would merge with 2. But in case of:

0000000
0000000
0022222
1122222
1122222

No merge would happen, because heights are different. Here's an example of successful vertical powergem merge:

0000000
0002200
0002200
0001100
0001100

Last example of singles merging with a powergem: (S - non-powergem pieces)

0000000
0002200
0002200
00011S0
00011S0

Singles will merge with powergem 1, and that's it (no merge with 2)

Does that make it any clearer? TBH you have to read the rules many times to understand exactly how it works.