Add Day12 implementation with puzzle-solving logic and tests

AoC_2025/Day12.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+using System.Data;
+using System.Data.Common;
+using System.IO;
+using System.Linq;
+using AdvenOfCode.Contracts;
+
+namespace AoC_2025;
+
+public class Day12 : IPuzzleSolver<long>
+{
+    private (int[] presentSizes, (int area, int[] presentCounts)[] regions) ParsePuzzleInput(string path)
+    {
+        var puzzleInput = File.ReadAllText(path)
+            .Split($"{Environment.NewLine}{Environment.NewLine}", StringSplitOptions.TrimEntries)
+            .ToList();
+        var presentSizes = puzzleInput[..^1]
+            .Where(s => !string.IsNullOrWhiteSpace(s))
+            .Select(s => s.Count(c => c == '#'))
+            .ToArray();
+        var regions = puzzleInput[^1]
+            .Split(Environment.NewLine, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Select(s => s.Split(':', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+            .Select(s => 
+                (s[0].Split('x').Select(int.Parse).Aggregate((x, y) => x * y),
+                 s[1].Split(' ').Select(int.Parse).ToArray()))
+            .ToArray();
+        return (presentSizes, regions);
+    }
+
+    public long SolvePart1(string pathToPuzzleInput)
+    {
+        var (presentSizes, regions) = ParsePuzzleInput(pathToPuzzleInput);
+        var everythingFits =
+            regions.Count(region => region.area >= GetMinTotalPresentArea(region.presentCounts, presentSizes));
+        return everythingFits;
+    }
+
+    private static int GetMinTotalPresentArea(int[] presentCounts, int[] presentSizes)
+    {
+        return presentCounts
+            .Select((presentCount, index) => presentCount * presentSizes[index])
+            .Sum();
+    }
+
+    public long SolvePart2(string pathToPuzzleInput)
+    {
+        return 0;
+    }
+}