You open the door and find yourself on the roof. The city sprawls away from you for miles and miles.

There's not much time now - it's already Christmas, but you're nowhere near the North Pole, much too far to deliver these stars to the sleigh in time.

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2016.Day25;

[ProblemName("Clock Signal")]
class Solution : Solver {

    public object PartOne(string input) {
        for (int a = 0; ; a++) {
            var length = 0;
            var expectedBit = 0;
            foreach (var actualBit in Run(input, a).Take(100)) {
                if (actualBit == expectedBit) {
                    expectedBit = 1 - expectedBit;
                    length++;
                } else {
                    break;
                }
            }
            if (length == 100) {
                return a;
            }
        }
    }
    
    IEnumerable<int> Run(string input, int a) {
        var prg = Parse(input);
        var regs = new Dictionary<string, int>();
        var ip = 0;
        int getReg(string reg) {
            return int.TryParse(reg, out var n) ? n
                : regs.ContainsKey(reg) ? regs[reg]
                : 0;
        }
        void setReg(string reg, int value) {
            if (!int.TryParse(reg, out var _)) {
                regs[reg] = value;
            }
        }

        setReg("a", a);

        while (ip < prg.Length) {
            var stm = prg[ip];
            switch (stm[0]) {
                case "cpy":
                    setReg(stm[2], getReg(stm[1]));
                    ip++;
                    break;
                case "inc":
                    setReg(stm[1], getReg(stm[1]) + 1);
                    ip++;
                    break;
                case "out":
                    yield return getReg(stm[1]);
                    ip++;
                    break;
                case "dec":
                    setReg(stm[1], getReg(stm[1]) - 1);
                    ip++;
                    break;
                case "jnz":
                    ip += getReg(stm[1]) != 0 ? getReg(stm[2]) : 1;
                    break;
                default:
                    throw new Exception("Cannot parse " + string.Join(" ", stm));
            }
        }
    }
    
    string[][] Parse(string input) =>
        input.Split('\n').Select(line => line.Split(' ')).ToArray();
}

Please ☆ my repo if you like it!