day 8 part 1

AdventOfCode/Day08.cs

@@ -0,0 +1,90 @@
+namespace AdventOfCode;
+
+internal class Day08 : BaseDay
+{
+    char[][] _input;
+
+    public Day08()
+    {
+        _input = File.ReadAllLines(InputFilePath).Select(x => x.ToCharArray()).ToArray();
+    }
+
+    public override ValueTask<string> Solve_1()
+    {
+        Dictionary<char, List<Point>> antennas = GetAntennas();
+
+        HashSet<Point> antinodes = new HashSet<Point>();
+        foreach (var kv in antennas)
+        {
+            antinodes.UnionWith(GetAntinodes(kv.Value));
+        }
+
+        return new(antinodes.Count.ToString());
+    }
+
+    public override ValueTask<string> Solve_2()
+    {
+        return new("");
+    }
+
+    private Dictionary<char, List<Point>> GetAntennas()
+    {
+        Dictionary<char, List<Point>> antennas = new Dictionary<char, List<Point>>();
+        for (int i = 0; i < _input.Length; i++)
+        {
+            for (int j = 0; j < _input[i].Length; j++)
+            {
+                char ch = _input[i][j];
+                if (ch != '.')
+                {
+                    if (!antennas.TryAdd(ch, [new Point(i, j)]))
+                    {
+                        antennas[ch].Add(new Point(i, j));
+                    }
+                }
+            }
+        }
+        return antennas;
+    }
+
+    private HashSet<Point> GetAntinodes(List<Point> antennas)
+    {
+        HashSet<Point> antinodes = new HashSet<Point>();
+
+        for (int i = 0; i < antennas.Count - 1; ++i)
+        {
+            for (int j = i + 1; j < antennas.Count; ++j)
+            {
+                (Point? antinodeOne, Point? antinodeTwo) = GetAntinodes(antennas[i], antennas[j]);
+
+                if (antinodeOne != null)
+                {
+                    antinodes.Add(antinodeOne);
+                }
+                if (antinodeTwo != null)
+                {
+                    antinodes.Add(antinodeTwo);
+                }
+            }
+        }
+
+        return antinodes;
+    }
+
+    private (Point? antinodeOne, Point? antinodeTwo) GetAntinodes(Point a, Point b)
+    {
+        var antinodeOne = new Point(2 * b.I - a.I, 2 * b.J - a.J);
+        var antinodeTwo = new Point(2 * a.I - b.I, 2 * a.J - b.J);
+
+        return (IsWithinBounds(antinodeOne) ? antinodeOne : null, 
+            IsWithinBounds(antinodeTwo) ? antinodeTwo : null);
+    }
+
+    private bool IsWithinBounds(Point point)
+    { 
+        return point.I >= 0 && point.I < _input.Length
+            && point.J >= 0 && point.J < _input[0].Length;
+    }
+
+    private record Point(int I, int J);
+}