1 files changed, 155 insertions, 0 deletions
diff --git a/21/main.py b/21/main.py
new file mode 100755
index 0000000..2c75798
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+++ b/21/main.py
@@ -0,0 +1,155 @@
+#!/usr/bin/python
+
+from copy import copy
+
+
+
+class Entity:
+    pass
+
+class Item:
+    pass
+
+class Shop:
+    pass
+
+
+
+# Annoying boilerplate to initialize the shop with the items
+# given in the text. This is as compact as I could make it.
+def init_shop():
+    table_weapons = [
+        "Dagger        8     4       0",
+        "Shortsword   10     5       0",
+        "Warhammer    25     6       0",
+        "Longsword    40     7       0",
+        "Greataxe     74     8       0"
+    ]
+    table_armour = [
+        "Leather      13     0       1",
+        "Chainmail    31     0       2",
+        "Splintmail   53     0       3",
+        "Bandedmail   75     0       4",
+        "Platemail   102     0       5",
+        "NONE          0     0       0"  # Because it's optional
+    ]
+    table_rings = [
+        "Damage+1     25     1       0",
+        "Damage+2     50     2       0",
+        "Damage+3    100     3       0",
+        "Defense+1    20     0       1",
+        "Defense+2    40     0       2",
+        "Defense+3    80     0       3",
+        "NONE          0     0       0"  # Because it's optional
+    ]
+    tables = [
+        (table_weapons, "weapons"),
+        (table_armour, "armour"),
+        (table_rings, "rings")
+    ]
+
+    shop = Shop()
+    shop.weapons = []
+    shop.armour  = []
+    shop.rings   = []
+
+    for table, attr in tables:
+        for row in table:
+            fields = row.split()
+
+            item = Item()
+            item.name   = fields[0]
+            item.cost   = int(fields[1])
+            item.damage = int(fields[2])
+            item.armour = int(fields[3])
+
+            # Sometimes I love you, Python. But usually I don't.
+            shop.__dict__[attr].append(item)
+
+    return shop
+
+
+
+def will_win(player, boss):
+    turn = 0
+    while boss.health > 0 and player.health > 0:
+        # Player's turn to attack
+        if turn % 2 == 0:
+            boss.health   -= max(1, player.damage -   boss.armour)
+        # Boss' turn to attack
+        else: # turn % 2 == 1
+            player.health -= max(1,   boss.damage - player.armour)
+        turn += 1
+
+    # Did the player win?
+    return boss.health <= 0
+
+
+
+def solve_partn(partn, player_stats, boss_stats):
+    shop = init_shop()
+
+    min_cost = 666
+    max_cost =   0
+    for w in shop.weapons:
+        for a in shop.armour:
+            for r1 in shop.rings:
+                for r2 in shop.rings:
+                    # No more than one of each item
+                    if r1 == r2:
+                        continue
+
+                    boss   = copy(  boss_stats)
+                    player = copy(player_stats)
+
+                    cost = w.cost + a.cost + r1.cost + r2.cost
+                    player.damage +=  w.damage
+                    player.damage += r1.damage
+                    player.damage += r2.damage
+                    player.armour +=  a.armour
+                    player.armour += r1.armour
+                    player.armour += r2.armour
+
+                    # Part 1: find lowest cost where player wins
+                    if will_win(player, boss):
+                        if cost < min_cost:
+                            min_cost = cost
+                    # Part 2: find highest cost where player loses
+                    else:
+                        if cost > max_cost:
+                            max_cost = cost
+
+    if partn == 1:
+        return min_cost
+    else: # partn == 2
+        return max_cost
+
+
+
+def main():
+    # Set up player's stats given in puzzle text
+    player = Entity()
+    player.health = 100
+    player.damage =   0
+    player.armour =   0
+
+    # Read boss' stats from input text file
+    boss = Entity()
+    with open("input.txt", "r") as f:
+        lines = f.read().splitlines()
+        for i in range(len(lines)): # len(lines) = 3
+            words = lines[i].split()
+            if i == 0:
+                boss.health = int(words[-1])
+            if i == 1:
+                boss.damage = int(words[-1])
+            if i == 2:
+                boss.armour = int(words[-1])
+
+    print("Part 1 solution:", solve_partn(1, player, boss)) # 121 for me
+    print("Part 2 solution:", solve_partn(2, player, boss)) # 201 for me
+
+
+
+if __name__ == "__main__":
+    main()