Initial Days

Day2/Day2.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net9.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <None Update="input.txt">
+        <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      </None>
+    </ItemGroup>
+
+</Project>

Day2/Program.cs

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+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)
+{
+    int unsafeCount = 0;
+    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)
+        {
+            unsafeCount++;
+        }
+        
+        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);
+    
+    Console.Write($" {unsafeCount}|{allIncreasing}|{allDecreasing} ");
+    return ((allIncreasing || allDecreasing) && unsafeCount == 0) || (allIncreasing == false && allDecreasing == false);
+
+    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
+}