d21.swift

struct Food {
  let ingredient: Set<String>
  let allergens: Set<String>
}

func parse(_ input: String) -> [Food] {
  return input
    .components(separatedBy: .newlines)
    .map {
      let parts = $0.components(separatedBy: " (contains ")
      let ingredient = parts[0].components(separatedBy: .whitespaces)
      let allergens = Array(parts[1].components(separatedBy: .whitespaces))
        .map { String($0.dropLast()) }
      return Food(ingredient: Set(ingredient), allergens: Set(allergens))
    }
}

let parsed = parse(d21)

var all = parsed
  .reduce(into: Set<String>()) { result, food in
    result = result.union(food.allergens)
  }

var ingredients = parsed
  .reduce(into: Set<String>()) { result, food in
    result = result.union(food.ingredient)
  }

var dict: [String: String] = [:]

while !all.isEmpty {
  for a in all {
    let matching = parsed
      .filter { $0.allergens.contains(a) }
      .reduce(into: ingredients) { result, food in
        result = result.intersection(food.ingredient)
      }

    if matching.count == 1, let ingredient = matching.first {
      dict[a] = ingredient
      ingredients.remove(ingredient)
      all.remove(a)
    }
  }
}

let remaining = parsed
  .map { $0.ingredient.intersection(ingredients).count }
  .reduce(0, + )

print("Part 1:", remaining)

let list = dict
  .sorted(by: { $0.0 < $1.0 })
  .map { $0.value }
  .joined(separator: ",")

print("Part 2:", list)