This is it: the bottom of the ocean trench, the last place the sleigh keys could be. Your submarine's experimental antenna still isn't boosted enough to detect the keys, but they must be here. All you need to do is reach the seafloor and find them.

At least, you'd touch down on the seafloor if you could; unfortunately, it's completely covered by two large herds of sea cucumbers, and there isn't an open space large enough for your submarine.

Read the full puzzle.

using System.Collections.Generic;

namespace AdventOfCode.Y2021.Day25;

[ProblemName("Sea Cucumber")]
class Solution : Solver {

    public object PartOne(string input) {

        var map = input.Split('\n');
        var (ccol, crow) = (map[0].Length, map.Length);

        int right(int icol) => (icol + 1) % ccol;
        int left(int icol) => (icol - 1 + ccol) % ccol;
        int up(int irow) => (irow - 1 + crow) % crow;
        int down(int irow) => (irow + 1) % crow;

        bool movesRight(int irow, int icol) =>
            map[irow][icol] == '>' && map[irow][right(icol)] == '.';
        bool movesDown(int irow, int icol) =>
            map[irow][icol] == 'v' && map[down(irow)][icol] == '.';

        for(var steps = 1;; steps++) {
            var anyMoves = false;

            var newMap = new List<string>();
            for (var irow = 0; irow < crow; irow++) {
                var st = "";
                for (var icol = 0; icol < ccol; icol++) {
                    anyMoves |= movesRight(irow, icol);
                    st +=
                        movesRight(irow, icol) ? '.' :
                        movesRight(irow, left(icol)) ? '>' :
                        map[irow][icol];
                    
                }
                newMap.Add(st);
            }

            map = newMap.ToArray();
            newMap.Clear();

            for (var irow = 0; irow < crow; irow++) {
                var st = "";
                for (var icol = 0; icol < ccol; icol++) {
                    anyMoves |= movesDown(irow, icol);
                    st +=
                        movesDown(irow, icol) ? '.' :
                        movesDown(up(irow), icol) ? 'v' :
                        map[irow][icol];
                }
                newMap.Add(st);
            }

            map = newMap.ToArray();

            if (!anyMoves) {
                return steps;
            } 
        }
    }
}

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