Move to subdirectory

2020/day21/main.swift

@@ -0,0 +1,91 @@
+import Foundation
+let input = loadData(day: 21)
+
+let scanner = Scanner(string: input)
+scanner.charactersToBeSkipped = .whitespaces
+
+extension Scanner {
+    func ingredient() -> String? {
+        scanUpToCharacters(from: .whitespaces)
+    }
+
+    func allergens() -> Set<String>? {
+        guard string("(contains") else { return nil }
+        var result: Set<String> = []
+        let end = CharacterSet(charactersIn: ",)")
+        repeat {
+            guard let allergen = scanUpToCharacters(from: end) else { return nil }
+            result.insert(allergen)
+        } while string(",")
+        guard string(")") else { return nil }
+        return result
+    }
+
+    func end() -> Set<String>? {
+        let allergens = self.allergens()
+        guard string("\n") else { return nil }
+        return allergens ?? []
+    }
+
+    typealias Food = (ingredients: Set<String>, allergens: Set<String>)
+
+    func food() -> Food? {
+        var ingredients = Set<String>()
+        while true {
+            guard let ingredient = self.ingredient() else { return nil }
+            ingredients.insert(ingredient)
+
+            if let end = self.end() {
+                return (ingredients, end)
+            }
+        }
+    }
+
+    func foods() -> [Food] {
+        var result: [Food] = []
+        while !isAtEnd, let food = self.food() {
+            result.append(food)
+        }
+        return result
+    }
+}
+
+let data = scanner.foods()
+
+var possibleIngredientsByAllergen: [String: Set<String>] = [:]
+var allIngredients: Set<String> = []
+var allergens: [(String, String)] = []
+
+for (ingredients, allergens) in data {
+    allIngredients.formUnion(ingredients)
+    for allergen in allergens {
+        if let possible = possibleIngredientsByAllergen[allergen] {
+            possibleIngredientsByAllergen[allergen] = possible.intersection(ingredients)
+        } else {
+            possibleIngredientsByAllergen[allergen] = ingredients
+        }
+    }
+}
+
+while !possibleIngredientsByAllergen.isEmpty {
+    for (allergen, possible) in possibleIngredientsByAllergen where possible.count == 1 {
+        let ingredient = possible.first!
+        allIngredients.remove(ingredient)
+        possibleIngredientsByAllergen.removeValue(forKey: allergen)
+        allergens.append((allergen, ingredient))
+        for key in possibleIngredientsByAllergen.keys {
+            possibleIngredientsByAllergen[key]?.remove(ingredient)
+        }
+    }
+}
+
+let count = data.reduce(0) { $0 + $1.ingredients.intersection(allIngredients).count }
+
+print("part 1:", count)
+
+let canonicalDangerousList = allergens
+    .sorted { $0.0 < $1.0 }
+    .map { $0.1 }
+    .joined(separator: ",")
+
+print("part 2:", canonicalDangerousList)