Day 12 (Part 2)

Day12_2/Day12_2.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Remove="input.txt" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Tools\Tools.csproj" />
+  </ItemGroup>
+
+</Project>

Day12_2/Program.cs

@@ -0,0 +1,88 @@
+using System.Numerics;
+
+var input = File.ReadAllLines("input.txt");
+
+BigInteger runningTotal = 0;
+foreach (var line in input)
+{
+    var parts = line.Split(' ');
+    var pattern = parts[0];
+    pattern = string.Join("?", Enumerable.Repeat(pattern, 5));
+    var groups = parts[1].Split(',').Select(int.Parse).ToArray();
+    var totalGroups = new List<int>();
+    for (int i = 0; i < 5; i++)
+    {
+        totalGroups.AddRange(groups);
+    }
+    runningTotal += GetArrangmentsCount(pattern, totalGroups.ToArray());
+}
+
+Console.WriteLine(runningTotal);
+
+long GetArrangmentsCount(string pattern, int[] groups)
+{
+    var arrangements = new long[pattern.Length + 1, groups.Length + 1];
+    arrangements[0, 0] = 1;
+
+    for (int patternLength = 1; patternLength <= pattern.Length; patternLength++)
+    {
+        var patternIndex = patternLength - 1;
+        for (int groupCount = 0; groupCount <= groups.Length; groupCount++)
+        {
+            var character = pattern[patternIndex];
+            if (character == '.' || character == '?')
+            {
+                arrangements[patternLength, groupCount] += arrangements[patternLength - 1, groupCount];
+            }
+
+            if (character == '#' || character == '?')
+            {
+                if (groupCount == 0)
+                {
+                    continue;
+                }
+
+                // The last group must end on this character
+                var groupSize = groups[groupCount - 1];
+                if (patternLength < groupSize)
+                {
+                    continue;
+                }
+
+                bool canPlaceGroup = true;
+                for (int endIndex = patternIndex; endIndex >= patternIndex - groupSize + 1; endIndex--)
+                {
+                    if (pattern[endIndex] == '.')
+                    {
+                        canPlaceGroup = false;
+                        break;
+                    }
+                }
+
+                // There must be a non-# before the group
+                if (patternIndex - groupSize >= 0 && pattern[patternIndex - groupSize] == '#')
+                {
+                    canPlaceGroup = false;
+                }
+
+                if (canPlaceGroup)
+                {
+                    if (patternLength == groupSize)
+                    {
+                        if (groupCount == 1)
+                        {
+                            arrangements[patternLength, groupCount] += 1;
+                        }
+                    }
+                    else
+                    {
+                        arrangements[patternLength, groupCount] +=
+                            arrangements[patternLength - groupSize - 1, groupCount - 1];
+                    }
+                }
+            }
+        }
+    }
+
+    return arrangements[pattern.Length, groups.Length];
+}

Day12_2/Properties/launchSettings.json

@@ -0,0 +1,7 @@
+{
+  "profiles": {
+    "Run": {
+      "commandName": "Project"
+    }
+  }
+}