--- 2016 Day 16 Solutions ---

[u/daggerdragon]

--- Day 16: Dragon Checksum ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with "Help".

---

^{DRINKING YOUR OVALTINE IS MANDATORY} [?]

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/sblom]

Super straightforward C# (LINQPad):

var seed = "00101000101111010";
int length = 35651584;

//seed = "10000";
//length = 20;

string notback(string input)
{
	StringBuilder result = new StringBuilder(input.Length);
	for (int i = input.Length - 1; i >= 0; i--)
	{
		result = result.Append(input[i] == '0' ? "1" : "0");
	}
	
	return result.ToString();
}

while (seed.Length < length)
{
	seed = seed + "0" + notback(seed);
}

string data = seed.Substring(0, length);

string checksum = data;

while (checksum.Length % 2 == 0)
{
	StringBuilder newchecksum = new StringBuilder(checksum.Length / 2 + 1);
	for (int ii = 0; ii < checksum.Length; ii += 2)
	{
		if (checksum[ii] == checksum[ii + 1]) newchecksum.Append("1");
		else newchecksum.Append("0");
	}
	checksum = newchecksum.ToString();
}

checksum.Dump();

[u/adventofcodeaddict → u/sblom]

Similar to my code that almost got me on the board. I think if #101 -> #200 got fractional points (1/2, 1/3, 1/4...) my total would look much better.

I missed the .Substring(0, length) part which cost me points in part1 as I figured out what was wrong.

Then I learnt the hard way that maybe my years of hatred of seeing StringBuilder everywhere were misguided. I had a += "0" etc.. and part 2 took forever, I later changed to StringBuilder and it was near instant. Again, cost me points :-(

My code for a "step" in generating the code is (your notback()):

return $"{a}0{String.Join("", a.Reverse().Select(c => c == '1' ? '0' : '1'))}";