day 21

aoc2020.test/DayTests.cs

@@ -27,7 +27,7 @@ public class DayTests
     [DataRow(typeof(Day18), "12918250417632", "171259538712010")]
     [DataRow(typeof(Day19), "160", "357")]
     [DataRow(typeof(Day20), "21599955909991", "")]
-    [DataRow(typeof(Day21), "", "")]
+    [DataRow(typeof(Day21), "2436", "dhfng,pgblcd,xhkdc,ghlzj,dstct,nqbnmzx,ntggc,znrzgs")]
     [DataRow(typeof(Day22), "", "")]
     [DataRow(typeof(Day23), "", "")]
     [DataRow(typeof(Day24), "", "")]

aoc2020/Day21.cs

@@ -5,17 +5,49 @@ namespace aoc2020;
 /// </summary>
 public sealed class Day21 : Day
 {
+    private readonly IEnumerable<(string[] Allergens, string[] Ingredients)> _parsedFoods;
+    private readonly IEnumerable<(string Allergen, string Ingredient)> _dangerousFoods;
+
     public Day21() : base(21, "Allergen Assessment")
     {
+        _parsedFoods = Input.Select(line => line.TrimEnd(')').Split(" (contains "))
+            .Select(split => (Allergens: split[1].Split(", "), Ingredients: split[0].Split(' ')));
+        
+        _dangerousFoods = _parsedFoods
+            .SelectMany(i => i.Allergens.Select(Allergen => (Allergen, i.Ingredients)))
+            .GroupBy(
+                pair => pair.Allergen,
+                pair => pair.Ingredients.Select(i => i),
+                // group by intersection of ingredients
+                (Allergen, collection) =>
+                    (Allergen, Ingredients: collection.Aggregate((acc, it) => acc.Intersect(it)))
+            )
+            .OrderBy(food => food.Ingredients.Count())
+            .Aggregate(
+                Enumerable.Empty<(string Allergen, string Ingredient)>(),
+                (poisons, pair) =>
+                    poisons.Concat(new[] {(
+                        allergen: pair.Allergen,
+                        ingredient: pair.Ingredients.Except(poisons.Select(i => i.Ingredient)).First()
+                    )})
+            );
     }
 
     public override string Part1()
     {
-        return "";
+        var part1 = _parsedFoods
+            .SelectMany(i => i.Ingredients)
+            .Count(i => !_dangerousFoods.Select(t => t.Ingredient).Contains(i));
+        
+        return $"{part1}";
     }
 
     public override string Part2()
     {
-        return "";
+        var part2 = _dangerousFoods
+            .OrderBy(i => i.Allergen)
+            .Select(i => i.Ingredient);
+        
+        return $"{string.Join(',', part2)}";
     }
 }