1 files changed, 181 insertions, 0 deletions

diff --git a/2015/src/day21.cpp b/2015/src/day21.cpp
new file mode 100644
index 0000000..2a38c86
--- /dev/null
+++ b/2015/src/day21.cpp
@@ -0,0 +1,181 @@
+#include <array>
+#include <fstream>
+#include <iostream>
+#include <sstream>
+using namespace std;
+
+struct Item {
+        Item(string_view name, int cost, int damage, int armor)
+            : name_(name)
+            , cost_(cost)
+            , damage_(damage)
+            , armor_(armor)
+        {
+        }
+
+        const string name_;
+        const int    cost_;
+        const int    damage_;
+        const int    armor_;
+};
+
+struct Boss {
+        int hit_points_;
+        int damage_;
+        int armor_;
+};
+
+struct Player {
+        Player(const Item& weapon, const Item& armor, const tuple<Item, Item>& rings)
+            : damage_(weapon.damage_ + armor.damage_ + get<0>(rings).damage_ + get<1>(rings).damage_)
+            , armor_(weapon.armor_ + armor.armor_ + get<0>(rings).armor_ + get<1>(rings).armor_)
+        {
+        }
+
+        int hit_points_{ 100 };
+        int damage_;
+        int armor_;
+
+        bool would_win_against(Boss boss)
+        {
+                while ( true ) {
+                        auto player_damage = max(1, damage_ - boss.armor_);
+                        boss.hit_points_ -= player_damage;
+                        if ( boss.hit_points_ <= 0 ) {
+                                return true;
+                        }
+
+                        auto boss_damage = max(1, boss.damage_ - armor_);
+                        hit_points_ -= boss_damage;
+                        if ( hit_points_ <= 0 ) {
+                                return false;
+                        }
+                }
+        }
+};
+
+const array<Item, 5> weapons = {
+        Item("Dagger", 8, 4, 0),
+        Item("Shortsword", 10, 5, 0),
+        Item("Warhammer", 25, 6, 0),
+        Item("Longsword", 40, 7, 0),
+        Item("Greataxe", 74, 8, 0)
+};
+
+const array<Item, 6> armors = {
+        Item("Leather", 13, 0, 1),
+        Item("Chainmail", 31, 0, 2),
+        Item("Splintmail", 53, 0, 3),
+        Item("Bandedmail", 75, 0, 4),
+        Item("Platemail", 102, 0, 5),
+        Item("Noting", 0, 0, 0)
+};
+
+vector<tuple<Item, Item>>
+generate_ring_pairs()
+{
+        const array<Item, 7> rings = {
+                Item("Damage +1", 25, 1, 0),
+                Item("Damage +2", 50, 2, 0),
+                Item("Damage +3", 100, 3, 0),
+                Item("Defense +1", 20, 0, 1),
+                Item("Defense +2", 40, 0, 2),
+                Item("Defense +3", 80, 0, 3),
+                Item("Nothing 0", 0, 0, 0)
+        };
+
+        vector<tuple<Item, Item>> pairs;
+
+        for ( size_t i = 0; i < rings.size(); ++i ) {
+                for ( size_t j = i + 1; j < rings.size(); ++j ) {
+                        pairs.emplace_back(rings.at(i), rings.at(j));
+                }
+        }
+
+        pairs.emplace_back(rings.back(), rings.back());
+
+        return pairs;
+}
+
+auto
+split(const string& line, char sep = ' ')
+{
+        vector<string> parts;
+        stringstream   input{ line };
+
+        for ( string part; getline(input, part, sep); ) {
+                parts.emplace_back(part);
+        }
+
+        return parts;
+}
+
+Boss
+read_file(string_view filename)
+{
+        fstream input{ filename };
+        Boss    boss{};
+
+        for ( string line; getline(input, line); ) {
+                auto parts = split(line);
+
+                if ( parts[0] == "Hit" ) {
+                        boss.hit_points_ = stoi(parts[2]);
+                }
+                else if ( parts[0] == "Damage:" ) {
+                        boss.damage_ = stoi(parts[1]);
+                }
+                else if ( parts[0] == "Armor:" ) {
+                        boss.armor_ = stoi(parts[1]);
+                }
+        }
+        return boss;
+}
+
+void
+part1(const Boss& boss)
+{
+        auto rings = generate_ring_pairs();
+
+        auto min_cost = numeric_limits<int>::max();
+        for ( const auto& weapon: weapons ) {
+                for ( const auto& armor: armors ) {
+                        for ( const auto& ring_pair: rings ) {
+                                Player player(weapon, armor, ring_pair);
+
+                                if (player.would_win_against(boss)) {
+                                        min_cost = min(min_cost, weapon.cost_ + armor.cost_ + get<0>(ring_pair).cost_ + get<1>(ring_pair).cost_);
+                                }
+                        }
+                }
+        }
+        cout << min_cost << endl;
+}
+
+void
+part2(const Boss& boss)
+{
+        auto rings = generate_ring_pairs();
+
+        auto max_cost = numeric_limits<int>::min();
+        for ( const auto& weapon: weapons ) {
+                for ( const auto& armor: armors ) {
+                        for ( const auto& ring_pair: rings ) {
+                                Player player(weapon, armor, ring_pair);
+
+                                if (!player.would_win_against(boss)) {
+                                        max_cost = max(max_cost, weapon.cost_ + armor.cost_ + get<0>(ring_pair).cost_ + get<1>(ring_pair).cost_);
+                                }
+                        }
+                }
+        }
+        cout << max_cost << endl;
+}
+
+int
+main()
+{
+        auto boss = read_file("data/day21.txt");
+        part1(boss);
+        part2(boss);
+}