4 files changed, 101 insertions, 0 deletions
diff --git a/2015/21/.gitignore b/2015/21/.gitignore
new file mode 100644
index 0000000..ffc46fe
--- /dev/null
+++ b/2015/21/.gitignore
@@ -0,0 +1 @@
+puzzle-[12].py
diff --git a/2015/21/Makefile b/2015/21/Makefile
new file mode 100644
index 0000000..9f0c9fb
--- /dev/null
+++ b/2015/21/Makefile
@@ -0,0 +1,8 @@
+all:
+	sed '/# START PART 2/,/# END PART 2/d' puzzles.py >puzzle-1.py
+	sed '/# START PART 1/,/# END PART 1/d' puzzles.py >puzzle-2.py
+	chmod +x puzzle-[12].py
+
+.PHONY: clean
+clean:
+	rm -f puzzle-[12].py
diff --git a/2015/21/input b/2015/21/input
new file mode 100644
index 0000000..e749051
--- /dev/null
+++ b/2015/21/input
@@ -0,0 +1,3 @@
+Hit Points: 103
+Damage: 9
+Armor: 2
diff --git a/2015/21/puzzles.py b/2015/21/puzzles.py
new file mode 100644
index 0000000..48e8c4e
--- /dev/null
+++ b/2015/21/puzzles.py
@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+
+
+import itertools
+import math
+
+SHOP = [
+	("Weapon", 8, 4, 0),
+	("Weapon", 10, 5, 0),
+	("Weapon", 25, 6, 0),
+	("Weapon", 40, 7, 0),
+	("Weapon", 74, 8, 0),
+	("Armor", 0, 0, 0),
+	("Armor", 13, 0, 1),
+	("Armor", 31, 0, 2),
+	("Armor", 53, 0, 3),
+	("Armor", 75, 0, 4),
+	("Armor", 102, 0, 5),
+	("Ring", 0, 0, 0),
+	("Ring", 0, 0, 0),
+	("Ring", 25, 1, 0),
+	("Ring", 50, 2, 0),
+	("Ring", 100, 3, 0),
+	("Ring", 20, 0, 1),
+	("Ring", 40, 0, 2),
+	("Ring", 80, 0, 3),
+]
+
+
+def wins(d: int, hp: int) -> bool:
+	turns = hp / d
+	if turns > int(turns):
+		turns = math.ceil(turns)
+
+	return (turns - 1) * d < 100
+
+
+def valid(e: tuple[tuple[str, int, int, int], ...]) -> bool:
+	w = a = r = 0
+	for i in e:
+		if i[0] == "Weapon":
+			w += 1
+		elif i[0] == "Armor":
+			a += 1
+		elif i[0] == "Ring":
+			r += 1
+	return (w == 1) and (a <= 1) and (r <= 2)
+
+
+def main() -> None:
+	with open("input", "r", encoding="utf-8") as f:
+		b_health, b_damage, b_armor = f.readlines()
+	b_health = int(b_health.strip().split(": ")[1])
+	b_damage = int(b_damage.strip().split(": ")[1])
+	b_armor = int(b_armor.strip().split(": ")[1])
+
+	# START PART 1
+	min_cost = 999999999999999999999999999999999999999999999999999999999
+	# END PART 1
+	# START PART 2
+	max_cost = 0
+	# END PART 2
+
+	for combs in [itertools.combinations(SHOP, i) for i in range(1, 5)]:
+		for comb in list(filter(valid, combs)):
+			cost = sum(x[1] for x in comb)
+			damage = sum(x[2] for x in comb)
+			armor = sum(x[3] for x in comb)
+			delta = (damage - b_armor) - (b_damage - armor)
+
+			# START PART 1
+			if (delta > 0) or ((delta == 0) and wins(damage - b_armor, b_health)):
+				min_cost = min(cost, min_cost)
+			# END PART 1
+			# START PART 2
+			if (delta < 0) or ((delta == 0) and not wins(damage - b_armor, b_health)):
+				max_cost = max(cost, max_cost)
+			# END PART 2
+
+	# START PART 1
+	print(min_cost)
+	# END PART 1
+	# START PART 2
+	print(max_cost)
+	# END PART 2
+
+
+if __name__ == "__main__":
+	main()