string[] lines = """
                 7 6 4 2 1
                 1 2 7 8 9
                 9 7 6 2 1
                 1 3 2 4 5
                 8 6 4 4 1
                 1 3 6 7 9
                 """.Split('\n', StringSplitOptions.RemoveEmptyEntries);
// string[] lines = System.IO.File.ReadAllLines("input.txt");

int levelsSafe = 0;
foreach (string input in lines)
{
    LinkedList<int> connected = new LinkedList<int>();
    var levels = input.Split(' ', StringSplitOptions.RemoveEmptyEntries).Select(int.Parse);
    foreach (var paredLevel in levels)
    {
        connected.AddLast(paredLevel);
    }


    Console.Write($"Testing {input} = ");
    var safe = TestLevel(connected);
    Console.WriteLine($"{(safe ? "Safe" : "Unsafe")}");
    levelsSafe += safe ? 1 : 0;
}

Console.WriteLine(levelsSafe);

bool TestLevel(LinkedList<int> linkedList)
{
    var node = linkedList.First;
    List<LevelDirection> directions = new List<LevelDirection>();
    while (node != null)
    {
        bool safeRange = node switch
        {
            { Previous: not null, Next: not null }
                => InsideRange2(node.Previous.Value, node.Value, node.Next.Value),
            { Previous: not null, Next: null }
                => InsideRange(node.Previous.Value, node.Value),
            { Previous: null, Next: not null }
                => InsideRange(node.Next.Value, node.Value),
            _ => throw new InvalidOperationException()
        };

        if(safeRange is false)
        {
            return false;
        }
        
        if (node.Next is not null)
        {
            directions.Add(node.Value > node.Next.Value ? LevelDirection.Increasing : LevelDirection.Decreasing);
        }
        
        node = node.Next;
    }

    bool allIncreasing = directions.All(x => x == LevelDirection.Increasing);
    bool allDecreasing = directions.All(x => x == LevelDirection.Decreasing);
    
    return allIncreasing || allDecreasing;

    bool InsideRange(int current, int target)
    {
        var abs = Math.Abs(current - target);
        return abs is >= 1 and <= 3;
    }
    
    bool InsideRange2(int prev, int current, int next)
    {
        var p = Math.Abs(prev - current);
        var n = Math.Abs(next - current);
        return p is >= 1 and <= 3 && n is >= 1 and <= 3;
    }
}

enum LevelDirection
{
    Increasing,
    Decreasing
}