Day 21

I'm sure this could be optimized plenty, but it works.

Q21.cs

@@ -0,0 +1,144 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+
+namespace _2020
+{
+   class Q21
+   {
+      static List<Tuple<List<string>, List<string>>> list = new List<Tuple<List<string>, List<string>>>();
+      static Dictionary<string, int> ingredientCounts = new Dictionary<string, int>();
+      static IEnumerable<string> allAllergens = new List<string>();
+      static IEnumerable<string> allIngredients = new List<string>();
+
+      public static void Go()
+      {
+         var start = DateTime.Now;
+         MakeList();
+         Util.Log($"Q21 MakeList took {(DateTime.Now - start).TotalMilliseconds}ms");
+         var p1start = DateTime.Now;
+         Part1();
+         Util.Log($"Q21 part1 took {(DateTime.Now - p1start).TotalMilliseconds}ms");
+         var p2start = DateTime.Now;
+         Part2();
+         Util.Log($"Q21 part2 took {(DateTime.Now - p2start).TotalMilliseconds}ms");
+
+         Util.Log($"Q21 took {(DateTime.Now - start).TotalMilliseconds}ms");
+      }
+
+      static void MakeList()
+      {
+         foreach (var line in File.ReadAllLines("21input.txt"))
+         {
+            var chunks = line.Split(" (contains ");
+            var ingredients = new List<string>(chunks[0].Split(' '));
+            var allergens = new List<string>(chunks[1][..^1].Split(", "));
+            list.Add(new Tuple<List<string>, List<string>>(ingredients, allergens));
+         }
+
+         list.ForEach(entry =>
+         {
+            allIngredients = allIngredients.Union(entry.Item1);
+            allAllergens = allAllergens.Union(entry.Item2);
+
+            foreach (var item in entry.Item1)
+            {
+               if (!ingredientCounts.ContainsKey(item))
+               {
+                  ingredientCounts.Add(item, 0);
+               }
+
+               ingredientCounts[item]++;
+            }
+         });
+      }
+
+      static void Part1()
+      {
+         IEnumerable<string> possibleAllergenIngredients = new List<string>();
+         foreach (var a in allAllergens)
+         {
+            var possibleIngredients = new List<List<string>>();
+            foreach (var item in list)
+            {
+               if (item.Item2.Contains(a))
+               {
+                  possibleIngredients.Add(item.Item1);
+               }
+            }
+
+            var intersection = possibleIngredients.Aggregate(possibleIngredients[0], (commonIngredients, item) => commonIngredients = new List<string>(commonIngredients.Intersect(item)));
+            possibleAllergenIngredients = possibleAllergenIngredients.Union(intersection);
+         }
+
+         var nonAllergenWords = allIngredients.Except(possibleAllergenIngredients);
+         var count = 0;
+         foreach (var word in nonAllergenWords)
+         {
+            count += ingredientCounts[word];
+         }
+
+         Util.Log($"Q21Part1: count={count}");
+      }
+
+      static void Part2()
+      {
+         IEnumerable<string> possibleAllergenIngredients = new List<string>();
+         var allergenIngredientMap = new Dictionary<string, IEnumerable<string>>();
+         foreach (var a in allAllergens)
+         {
+            var possibleIngredients = new List<List<string>>();
+            foreach (var item in list)
+            {
+               if (item.Item2.Contains(a))
+               {
+                  possibleIngredients.Add(item.Item1);
+               }
+            }
+
+            var intersection = possibleIngredients.Aggregate(possibleIngredients[0], (commonIngredients, item) => commonIngredients = new List<string>(commonIngredients.Intersect(item)));
+            allergenIngredientMap.Add(a, intersection);
+            possibleAllergenIngredients = possibleAllergenIngredients.Union(intersection);
+         }
+
+         bool foundAll = false;
+         while (!foundAll)
+         {
+            foundAll = true;
+            foreach (var i in allergenIngredientMap)
+            {
+               if (i.Value.Count() > 1)
+               {
+                  var possibleIngredients = i.Value;
+                  var filtered = allergenIngredientMap.Where(x => x.Key != i.Key).Aggregate(possibleIngredients, (commonIngredients, item) =>
+                  {
+                     if (item.Value.Count() == 1)
+                     {
+                        possibleIngredients = possibleIngredients.Except(item.Value);
+                     }
+
+                     return possibleIngredients;
+                  });
+
+                  if (filtered.Count() == 1)
+                  {
+                     allergenIngredientMap[i.Key] = filtered;
+                  }
+                  else
+                  {
+                     foundAll = false;
+                  }
+               }
+            }
+         }
+
+         var orderedAllergenMap = allergenIngredientMap.OrderBy(x => x.Key);
+         var items = orderedAllergenMap.Aggregate(new List<string>(), (lst, itm) => { lst.Add(itm.Value.ElementAt(0)); return lst; });
+         var joined = string.Join(',', items);
+
+         Util.Log($"Q21Part2: avoid={joined}");
+      }
+   }
+}