Finally, you come across an information kiosk with a list of rooms. Of course, the list is encrypted and full of decoy data, but the instructions to decode the list are barely hidden nearby. Better remove the decoy data first.

Each room consists of an encrypted name (lowercase letters separated by dashes) followed by a dash, a sector ID, and a checksum in square brackets.

Read the full puzzle.

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace AdventOfCode.Y2016.Day04;

[ProblemName("Security Through Obscurity")]
class Solution : Solver {

    public object PartOne(string input) => (
        from i in Parse(input)
        let name = i.name.Replace("-", "")
        let computedChecksum = string.Join("", (from ch in name group ch by ch into g orderby -g.Count(), g.Key select g.Key).Take(5))
        where computedChecksum == i.checksum
        select i.sectorid
    ).Sum();

    public object PartTwo(string input) => (
        from i in Parse(input)
        let name = string.Join("", from ch in i.name select ch == '-' ? ' ' : (char)('a' + (ch - 'a' + i.sectorid) % 26))
        where name.Contains("northpole")
        select i.sectorid
    ).Single();

    IEnumerable<(string name, int sectorid, string checksum)> Parse(string input){
        var rx = new Regex(@"([^\d]+)\-(\d+)\[(.*)\]");
        
        return from line in input.Split('\n')
            let m = rx.Match(line)
            select (m.Groups[1].Value, int.Parse(m.Groups[2].Value), m.Groups[3].Value);
         
    }
}

Please ☆ my repo if you like it!