Advent of Code in Unreal Engine Day 20

My day 20 solution only works for Part 1. I am fairly confident the generally approach is okay, but something is getting me mixed up with the math for wrapping around.

The relevant code (which is wrong, somehow):

void AD20Actor::MixStep(TArray<FD20Mix>& Mixed, int Step)
{
	ensure(Step>=0 && Step<Mixed.Num());
	
	const int64 End = Mixed.Num();
	const int64 From = Mixed[Step].Location;

	// handle positive wrap around
	int64 Too = From + Mixed[Step].Value;
	if (Too>=End)
	{
		// WORKS for PART 1?
		Too = Too % (End-1);
		
		// if (Too>=From) Too++;
		// Too = (Too % End);		
	}

	// handle negative wrap around 
	// WORKS for PART 1
	else if (Too<=0)
	{
		auto Steps = -Too / (End-1);
		Too += (Steps+1) * (End-1);
	}

	if (Too==From) return;
	
	// shuffle everything AFTER the slot the moving item is moving from DOWN
    // shuffle everything AFTER the moving items destination UP
	for(int i=0;i!=Mixed.Num();++i)
	{
		int D = 0;		
		if (Mixed[i].Location > From)
			D -= 1;
		if (Mixed[i].Location+D >= Too)
			D += 1;
		Mixed[i].Location = (Mixed[i].Location + D) % End;
	}

	Mixed[Step].Location = Too;
}

I think it’s probably the wrapping that’s wrong, but I don’t really understand why.

For the last step, “4 moves between -3 and 0” – 4 starts in the 5th position, add 4 is 9, 2 past the end.

But 2 past the end puts it between 2 and -3. So what gives? Well when we “remove” the item, the array becomes 1 smaller, so I thought the wrap is at 6 not 7 and that puts this in the right location. My tests pass.

But Part 2 has us wrapping around not once, but thousands of times around in an array thousands of elements long. When I do that, my results don’t match the part 2 example results, and I’ve tried for far to long now at figuring it out.

So that’s it for me. My 2nd day I’ve failed to solve Part 2 in time for bed. Can you see what’s wrong? Please let me know if you can explain what I’ve missed… post a comments or reply on to me on Mastodon.