Day 12

Day11.cs

@@ -1,4 +1,4 @@
-#define Day11
+// #define Day11
 #if Day11
 using AoC2021;
 

Day12.cs

@@ -0,0 +1,68 @@
+#define Day12
+#if Day12
+using System.Collections.Immutable;
+using AoC2021;
+const string START = "start";
+const string END = "end";
+var paths = new Dictionary<string, ImmutableHashSet<string>>();
+
+File.ReadAllLines("day12/input")
+    .Select(l => l.Split("-"))
+    .ForEachAsList(x => {
+        if (x[0] != END && x[1] != START) {
+            if (paths.ContainsKey(x[0])) {
+                paths[x[0]] = paths[x[0]].Add(x[1]);
+            } else {
+                paths.Add(x[0], ImmutableHashSet.Create(x[1]));
+            }
+        }
+
+        if (x[1] != END && x[0] != START) {
+            if (paths.ContainsKey(x[1])) {
+                paths[x[1]] = paths[x[1]].Add(x[0]);
+            } else {
+                paths.Add(x[1], ImmutableHashSet.Create(x[0]));
+            }
+        }
+    });
+
+Console.WriteLine(PathCountPart1(paths, ImmutableHashSet.Create(START), START));
+Console.WriteLine(PathCountPart2(paths, ImmutableHashSet.Create(START), START, false));
+
+long PathCountPart1(Dictionary<string, ImmutableHashSet<string>> paths, ImmutableHashSet<string> visitedSmallCaves, string current) {
+    long count = paths[current].Contains(END) ? 1 : 0;
+    ImmutableHashSet<string> adjacent;
+    adjacent = paths[current].Except(visitedSmallCaves).Remove(END);
+    foreach (var a in adjacent) {
+        if (a.ToLower() == a) {
+            count += PathCountPart1(paths, visitedSmallCaves.Add(a), a);
+        }
+        else {
+            count += PathCountPart1(paths, visitedSmallCaves, a);
+        }
+    }
+
+    return count;
+}
+
+long PathCountPart2(Dictionary<string, ImmutableHashSet<string>> paths, ImmutableHashSet<string> visitedSmallCaves, string current, bool twice) {
+    long count = paths[current].Contains(END) ? 1 : 0;
+    ImmutableHashSet<string> adjacent;
+    if (twice) {
+        adjacent = paths[current].Except(visitedSmallCaves).Remove(END);
+    } else {
+        adjacent = paths[current].Remove(END);
+    }
+
+    foreach (var a in adjacent) {
+        if (a.ToLower() == a) {
+            count += PathCountPart2(paths, visitedSmallCaves.Add(a), a, twice || visitedSmallCaves.Contains(a));
+        }
+        else {
+            count += PathCountPart2(paths, visitedSmallCaves, a, twice);
+        }
+    }
+
+    return count;
+}
+#endif

day12/input

@@ -0,0 +1,23 @@
+start-YA
+ps-yq
+zt-mu
+JS-yi
+yq-VJ
+QT-ps
+start-yq
+YA-yi
+start-nf
+nf-YA
+nf-JS
+JS-ez
+yq-JS
+ps-JS
+ps-yi
+yq-nf
+QT-yi
+end-QT
+nf-yi
+zt-QT
+end-ez
+yq-YA
+end-JS