Somehow, a network packet got lost and ended up here. It's trying to follow a routing diagram (your puzzle input), but it's confused about where to go.

Its starting point is just off the top of the diagram. Lines (drawn with |, -, and +) show the path it needs to take, starting by going down onto the only line connected to the top of the diagram. It needs to follow this path until it reaches the end (located somewhere within the diagram) and stop there.

Read the full puzzle.

using System.Linq;

namespace AdventOfCode.Y2017.Day19;

[ProblemName("A Series of Tubes")]
class Solution : Solver {

    public object PartOne(string input) => FollowPath(input).msg;

    public object PartTwo(string input) => FollowPath(input).steps;

    (string msg, int steps) FollowPath(string input){
        var map = input.Split('\n');
        var (ccol, crow) = (map[0].Length, map.Length);
        var (icol, irow) = (map[0].IndexOf('|'), 0);
        var (dcol, drow) = (0, 1);

        var msg = "";
        var steps = 0;

        while (true) {
            irow += drow;
            icol += dcol;
            steps++;

            if (icol < 0 || icol >= ccol || irow < 0 || irow >= crow || map[irow][icol] == ' ') {
                break;
            }

            switch (map[irow][icol]) {
                case '+':
                    (dcol, drow) = (
                            from q in new[] { (drow: dcol, dcol: -drow), (drow: -dcol, dcol: drow)}
                            let icolT = icol + q.dcol
                            let irowT = irow + q.drow
                            where icolT >= 0 && icolT < ccol && irowT >= 0 && irowT < crow && map[irowT][icolT] != ' '
                            select (q.dcol, q.drow)
                        ).Single();
                    break;
                case char ch when (ch >= 'A' && ch <= 'Z'):
                    msg += ch;
                    break;
            }
        }
        return (msg, steps);
    }
}

Please ☆ my repo if you like it!