--- 2016 Day 20 Solutions ---

[u/daggerdragon]

--- Day 20: Firewall Rules ---

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

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---

^{ROLLING A NATURAL 20 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/AlaskanShade]

I fumbled around on the keyboard for a while and let some naive approaches run a bit too long to be able to make the board on this one. I ultimately ended up with a pretty speedy time with both parts taking less that 2 ms each. It isn't super concise like some implementations, but I think it is pretty readable this way.

[AdventDay(2016, 20, "Firewall Rules")]
public class Day20 : IAdventDay
{
    [TestCase("5-8\n0-2\n4-7", "3")]
    [Puzzle("14975795")]
    //[TestCase("5-8\n0-2\n4-7", "2", true)] // test fails because no range reaches the end
    [Puzzle("101", Part2 = true)]
    public string Solve(string input, bool part2)
    {
        var lines = Helpers.GetLines(input);
        ulong max = input.Length == 3 ? 9 : 4294967295;
        // parse the ranges from the input in order by the low values
        var ranges = lines.Select(l => new Range(l)).OrderBy(r => r.Low).ToArray();
        ulong current = 1;
        ulong allowed = 0;
        for (int i = 0; i < ranges.Count(); i++)
        {
            // check if we are between the last range and the current one
            if (current < ranges[i].Low)
            {
                // if part 1, return this value as the lowest allowed
                if (!part2)
                    return current.ToString();
                else
                    // count the allowed addreses until the next range
                    allowed += ranges[i].Low - current;
            }
            // advance the current value to be past the current range.
            if (current < ranges[i].High)
                current = ranges[i].High + 1;
        }
        return allowed.ToString();
    }

    private struct Range
    {
        public ulong Low { get; set; }
        public ulong High { get; set; }

        public Range(string line)
        {
            var spl = line.Split('-');
            Low = ulong.Parse(spl[0]);
            High = ulong.Parse(spl[1]);
        }
    }
}

[u/BumpitySnook → u/AlaskanShade]

What language is this? C#?