Day21.java

import java.util.ArrayList;
import java.util.List;

public class Day21 {

	static final int BOSS_HIT_POINTS = 104;
	static final int BOSS_DAMAGE = 8;
	static final int BOSS_ARMOR = 1;

	List<Item> weapons = new ArrayList<>();
	List<Item> armors = new ArrayList<>();
	List<Item> rings = new ArrayList<>();

	class Item {
		String name;
		int cost;
		int damage;
		int armor;

		public Item(String name, int cost, int damage, int armor) {
			this.name = name;
			this.cost = cost;
			this.damage = damage;
			this.armor = armor;
		}

		public Item(String data) {

			this.name = data.substring(0, 12).trim();
			this.cost = Integer.parseInt(data.substring(12, 15).trim());
			this.damage = Integer.parseInt(data.substring(15, 21).trim());
			this.armor = Integer.parseInt(data.substring(21).trim());
		}

	}

	class Player {
		String name;
		int hitPoints;
		int damage;
		int armor;
		int cost;

		public Player(String name, int hitPoints, int damage, int armor) {
			this.name = name;
			this.hitPoints = hitPoints;
			this.damage = damage;
			this.armor = armor;
			cost = 0;
		}

		public void equip(Item item) {
			this.armor += item.armor;
			this.damage += item.damage;
			this.cost += item.cost;
		}

		public void fight(Player other) {
			int hit = Math.max(damage - other.armor, 1); // deal at least 1 hit
															// Point
			other.hitPoints -= hit; // adjust other's hit points
		}

		public boolean isDead() {
			return hitPoints <= 0;
		}

		@Override
		public String toString() {
			return String.format("Player: %s\tHit Points: %3d\tDamage: %2d\tArmor: %2d\n", name, hitPoints, damage,
					armor);
		}
	}

	class Game {

		Player[] players = new Player[2];

		public Game() {

			players[0] = new Player("Henry", 100, 0, 0);
			players[1] = new Player("Boss", Day21.BOSS_HIT_POINTS, Day21.BOSS_DAMAGE, Day21.BOSS_ARMOR);
		}

		public void equipPlayer(Item item) {
			players[0].equip(item);
		}

		public int tick(int player) { // one fight round either Human -> Boss or
										// Boss -> Human
			players[player].fight(players[1 - player]);
			if (players[1 - player].isDead()) {
				return player;
			}
			return -1; // no one dies - no winner
		}

		public int runGame() {
			while (!players[0].isDead() && !players[1].isDead()) {
				for (int i = 0; i < 2; i++) {
					tick(i);
					if (players[1].isDead()) {
						return 0;
					}
					if (players[0].isDead()) {
						return 1;
					}
				}
			}
			if (players[0].isDead()) {
				return 1;
			}
			return 0;
		}
	}

	public Day21() {

		weapons.add(new Item("Dagger        8     4       0"));
		weapons.add(new Item("Shortsword   10     5       0"));
		weapons.add(new Item("Warhammer    25     6       0"));
		weapons.add(new Item("Longsword    40     7       0"));
		weapons.add(new Item("Greataxe     74     8       0"));

		armors.add(new Item("None          0     0       0"));
		armors.add(new Item("Leather      13     0       1"));
		armors.add(new Item("Chainmail    31     0       2"));
		armors.add(new Item("Splintmail   53     0       3"));
		armors.add(new Item("Bandedmail   75     0       4"));
		armors.add(new Item("Platemail   102     0       5"));

		rings.add(new Item("None          0     0       0"));
		rings.add(new Item("None          0     0       0"));
		rings.add(new Item("Damage +1    25     1       0"));
		rings.add(new Item("Damage +2    50     2       0"));
		rings.add(new Item("Damage +3   100     3       0"));
		rings.add(new Item("Defense +1   20     0       1"));
		rings.add(new Item("Defense +2   40     0       2"));
		rings.add(new Item("Defense +3   80     0       3"));

	}

	public void solve() {
		int min = Integer.MAX_VALUE;
		int max = Integer.MIN_VALUE;
		int cost = 0;
		// Brute force attack:

		// Run through all weapons
		for (Item weapon : weapons) {

			// run through all armors
			for (Item armor : armors) {

				// run through all the rings (0..2)
				for (int ring1 = 0; ring1 < rings.size() - 1; ring1++) {
					for (int ring2 = ring1 + 1; ring2 < rings.size(); ring2++) {
						// Start new game round
						Game game = new Game();
						// equip human
						game.players[0].equip(weapon);
						game.players[0].equip(armor);
						game.players[0].equip(rings.get(ring1));
						game.players[0].equip(rings.get(ring2));
						cost = game.players[0].cost;

						int winner = game.runGame();

						// If human has won
						if (winner == 0) {
							if (min >= cost) {
								min = cost;
							}
						}

						// Part 2 - Max Gold to lose
						if (winner == 1) {
							if (max <= cost) {
								max = cost;
							}
						}
					}
				}

			}
		}

		System.out.println("Part1: Minimum cost is: " + min);
		System.out.println("Part2: Maximum cost is: " + max);

	}

	public static void main(String[] args) {
		Day21 day21 = new Day21();
		day21.solve();
	}
}