Merge pull request #21 from MartinZikmund/feature/aoc2024-day20

AoC 2024 Day 20 Solutions

src/AdventOfCode.Puzzles/2024/20/Part1/Part1.cs

@@ -0,0 +1,117 @@
+namespace AdventOfCode.Puzzles._2024._20.Part1;
+
+public partial class Part1 : IPuzzleSolution
+{
+    private int _width;
+    private int _height;
+    private int[,] _map;
+    private Point _start;
+    private Point _end;
+
+    private const int WantToSave = 100;
+    private const int CheatTime = 2;
+
+    public async Task<string> SolveAsync(StreamReader inputReader)
+    {
+        var lines = await inputReader.ReadAllLinesAsync();
+        _height = lines.Count;
+        _width = lines[0].Length;
+
+        _map = new int[_width, _height];
+        for (int y = 0; y < _height; y++)
+        {
+            for (int x = 0; x < _width; x++)
+            {
+                _map[x, y] = lines[y][x] == '#' ? -1 : 0;
+                if (lines[y][x] == 'S')
+                {
+                    _start = new Point(x, y);
+                }
+                else if (lines[y][x] == 'E')
+                {
+                    _end = new Point(x, y);
+                }
+            }
+        }
+
+        FindDistances(_start, _end);
+
+        return CountCheats().ToString();
+    }
+
+    private int CountCheats()
+    {
+        var cheats = 0;
+        for (int y = 0; y < _height; y++)
+        {
+            for (int x = 0; x < _width; x++)
+            {
+                if (_map[x, y] == -1)
+                {
+                    continue;
+                }
+                var start = new Point(x, y);
+                cheats += CountCheats(start);
+            }
+        }
+
+        return cheats;
+    }
+
+    private int CountCheats(Point start)
+    {
+        HashSet<Point> targets = new HashSet<Point>();
+        Queue<(Point position, int steps)> queue = new();
+        queue.Enqueue((start, 0));
+        while (queue.Count > 0)
+        {
+            var (position, steps) = queue.Dequeue();
+
+            if (steps == CheatTime && _map[position.X, position.Y] != -1)
+            {
+                var saved = _map[position.X, position.Y] - _map[start.X, start.Y] - CheatTime;
+                if (saved >= WantToSave)
+                {
+                    targets.Add(position);
+                }
+            }
+
+            if (steps >= CheatTime)
+            {
+                continue;
+            }
+
+            foreach (var direction in Directions.WithoutDiagonals)
+            {
+                var next = position + direction;
+                if ((next.X < 0 || next.X >= _width || next.Y < 0 || next.Y >= _height))
+                {
+                    continue;
+                }
+
+                queue.Enqueue((next, steps + 1));
+            }
+        }
+
+        return targets.Count;
+    }
+
+    private void FindDistances(Point start, Point end)
+    {
+        var currentPoint = start;
+
+        while (currentPoint != end)
+        {
+            foreach (var direction in Directions.WithoutDiagonals)
+            {
+                var next = currentPoint + direction;
+                if (_map[next.X, next.Y] == 0 && next != start)
+                {
+                    _map[next.X, next.Y] = _map[currentPoint.X, currentPoint.Y] + 1;
+                    currentPoint = next;
+                    break;
+                }
+            }
+        }
+    }
+}

src/AdventOfCode.Puzzles/2024/20/Part2/Part2.cs

@@ -0,0 +1,106 @@
+using System.Reflection.Metadata.Ecma335;
+
+namespace AdventOfCode.Puzzles._2024._20.Part2;
+
+public partial class Part2 : IPuzzleSolution
+{
+    private int _width;
+    private int _height;
+    private int[,] _map;
+    private Point _start;
+    private Point _end;
+
+    private const int WantToSave = 100;
+    private const int CheatTime = 20;
+
+    public async Task<string> SolveAsync(StreamReader inputReader)
+    {
+        var lines = await inputReader.ReadAllLinesAsync();
+        _height = lines.Count;
+        _width = lines[0].Length;
+
+        _map = new int[_width, _height];
+        for (int y = 0; y < _height; y++)
+        {
+            for (int x = 0; x < _width; x++)
+            {
+                _map[x, y] = lines[y][x] == '#' ? -1 : 0;
+                if (lines[y][x] == 'S')
+                {
+                    _start = new Point(x, y);
+                }
+                else if (lines[y][x] == 'E')
+                {
+                    _end = new Point(x, y);
+                }
+            }
+        }
+
+        FindDistances(_start, _end);
+
+        return CountCheats().ToString();
+    }
+
+    private int CountCheats()
+    {
+        var cheats = 0;
+        for (int y = 0; y < _height; y++)
+        {
+            for (int x = 0; x < _width; x++)
+            {
+                if (_map[x, y] == -1)
+                {
+                    continue;
+                }
+                var start = new Point(x, y);
+                cheats += CountCheats(start);
+            }
+        }
+
+        return cheats;
+    }
+
+    private int CountCheats(Point start)
+    {
+        HashSet<Point> targets = new HashSet<Point>();
+
+        for (int y = 0; y < _height; y++)
+        {
+            for (int x = 0; x < _width; x++)
+            {
+                var distance = Math.Abs(start.X - x) + Math.Abs(start.Y - y);
+                if (distance > CheatTime)
+                {
+                    continue;
+                }
+
+                var saved = _map[x, y] - _map[start.X, start.Y] - distance;
+                if (saved >= WantToSave)
+                {
+                    targets.Add(new Point(x, y));
+                }
+            }
+        }
+
+        return targets.Count;
+    }
+
+    private void FindDistances(Point start, Point end)
+    {
+        var currentPoint = start;
+
+        while (currentPoint != end)
+        {
+            foreach (var direction in Directions.WithoutDiagonals)
+            {
+                var next = currentPoint + direction;
+                if (_map[next.X, next.Y] == 0 && next != start)
+                {
+                    _map[next.X, next.Y] = _map[currentPoint.X, currentPoint.Y] + 1;
+                    currentPoint = next;
+                    break;
+                }
+            }
+        }
+    }
+}

src/AdventOfCode.Puzzles/2024/20/TestData.txt

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+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############