Add Day 12 Part 1

Day12/Program.cs

@@ -0,0 +1,125 @@
+var lines = File.ReadAllLines("input.txt");
+var grid = CreateGrid(lines);
+
+int[][] directions = new int[][] 
+{
+    [-1, 0],
+    [0, -1],
+    [0, 1],
+    [1, 0]
+};
+
+var part1 = Part1(grid, directions);
+
+Console.WriteLine($"Part1: {part1}\n");
+
+int Part1(char[,] grid, int[][] directions)
+{
+    var rows = grid.GetLength(0);
+    var columns = grid.GetLength(1);
+    var neighbourDictionary = new Dictionary<(int, int), HashSet<(int, int)>>();
+
+    for (var i = 0; i < rows; i++)
+    {
+        for (int j = 0; j < columns; j++)
+        {
+            neighbourDictionary[(i, j)] = new HashSet<(int, int)>();
+
+            foreach (var dir in directions)
+            {
+                var di = dir[0];
+                var dj = dir[1];
+
+                if (i + di >= 0 && i + di < rows && j + dj >= 0 && j + dj < columns)
+                {
+                    if (grid[i, j] == grid[i + di, j + dj])
+                    {
+                        neighbourDictionary[(i, j)].Add((i + di, j + dj));
+                    }
+                }
+            }
+        }
+    }
+    
+    var regions = new List<HashSet<(int, int)>>();
+    var remainingPoints = new HashSet<(int, int)>();
+
+    for (var i = 0; i < rows; i++)
+    {
+        for (var j = 0; j < columns; j++)
+        {
+            remainingPoints.Add((i, j));
+        }
+    }
+
+    while (remainingPoints.Count > 0)
+    {
+        var point = remainingPoints.First();
+        remainingPoints.Remove(point);
+
+        var region = GetRegion(point, neighbourDictionary);
+        regions.Add(region);
+        remainingPoints.ExceptWith(region);
+    }
+    
+    return regions.Sum(region => Perimeter(region, neighbourDictionary) * region.Count);
+}
+
+int CountSplitSegments(List<(int, int, int, string)> lines)
+{
+    if (lines.Count == 0) return 0;
+
+    var segmentCount = 1;
+    for (var i = 1; i < lines.Count; i++)
+    {
+        var prev = lines[i - 1];
+        var curr = lines[i];
+
+        var isContiguous = 
+            (prev.Item1 == curr.Item1 && curr.Item2 - prev.Item2 == 1 && curr.Item3 == prev.Item3);
+
+        if (!isContiguous)
+        {
+            segmentCount++;
+        }
+    }
+    return segmentCount;
+}
+
+HashSet<(int, int)> GetRegion((int, int) point, Dictionary<(int, int), HashSet<(int, int)>> neighbourDict)
+{
+    var region = new HashSet<(int, int)>();
+    var remaining = new HashSet<(int, int)> { point };
+
+    while (remaining.Count > 0)
+    {
+        var curPoint = remaining.First();
+        remaining.Remove(curPoint);
+        region.Add(curPoint);
+
+        var newRemaining = neighbourDict[curPoint].Except(region);
+        remaining.UnionWith(newRemaining);
+    }
+
+    return region;
+}
+
+int Perimeter(HashSet<(int, int)> region, Dictionary<(int, int), HashSet<(int, int)>> neighbourDict)
+{
+    return region.Sum(point => 4 - neighbourDict[point].Count);
+}
+
+char[,] CreateGrid(string[] lines)
+{
+    var grid = new char[lines.Length, lines[0].Length];
+
+    for (int i = 0; i < lines.Length; i++)
+    {
+        for (int j = 0; j < lines[i].Length; j++)
+        {
+            grid[i, j] = lines[i][j];
+        }
+    }
+
+    return grid;
+}