day 25

Author: r0f1

README.md

@@ -26,4 +26,4 @@
 |[22](https://adventofcode.com/2020/day/22)|Crab Combat|[py](/day22/main.py)|
 |[23](https://adventofcode.com/2020/day/23)|Crab Cups|[py](/day23/main.py)|
 |[24](https://adventofcode.com/2020/day/24)|Lobby Layout|[py](/day24/main.py)|
-|[25](https://adventofcode.com/2020/day/25)|-|-|
+|[25](https://adventofcode.com/2020/day/25)|Combo Breaker|[py](/day25/main.py)|

day25/main.py

@@ -0,0 +1,22 @@
+with open("input.txt") as f:
+    public_key_card, public_key_door = [int(x.strip()) for x in f]
+
+def calc_loop_size(public_key):
+    subject_number = 7
+    n = 1
+    i = 0
+    while True:
+        n = (n * subject_number) % 20201227
+        if n == public_key:
+            return i+1
+        i += 1
+
+def calc_encryption_key(subject_number, loop_size):
+    n = 1
+    for i in range(loop_size):
+        n = (n * subject_number) % 20201227
+    return n
+
+loop_size = calc_loop_size(public_key_card)
+enc_key = calc_encryption_key(public_key_door, loop_size)
+print(enc_key)