Solution to problem 4 in Python

src/Year_2016/P4.py

@@ -24,18 +24,23 @@
 # What is the sum of the sector IDs of the real rooms?
 
 from collections import Counter
-from typing import Tuple
+from typing import Tuple, Union
 
 with open("files/P4.txt") as f:
     instructions = [line for line in f.read().strip().split()]
 
 
-def split_instruction(instr: str) -> Tuple[str, str, str]:
-    *name_, sector_id_checksum = str(instr).split("-")
-    name = "".join(name_)
+def split_instruction(
+    instr: str, split_name: bool = True
+) -> Union[Tuple[str, int, str], Tuple[list[str], int, str]]:
+    *name, sector_id_checksum = str(instr).split("-")
+    if split_name:
+        name_: str = "".join(name)
+    else:
+        name_: list[str] = name
     sector_id, checksum = sector_id_checksum[:3], sector_id_checksum[4:-1]
 
-    return name, sector_id, checksum
+    return name_, int(sector_id), checksum
 
 
 def part_1() -> None:
@@ -46,10 +51,49 @@ def part_1() -> None:
         chars = Counter(sorted(name))
         checksum_calculated = "".join(n[0] for n in chars.most_common(5))
         if checksum_calculated == checksum:
-            count += int(sector_id)
+            count += sector_id
 
     print(f"The sum is {count}")
 
 
+# --- Part Two ---
+
+# With all the decoy data out of the way, it's time to decrypt this list and
+# get moving.
+
+# The room names are encrypted by a state-of-the-art shift cipher, which is
+# nearly unbreakable without the right software. However, the information kiosk
+# designers at Easter Bunny HQ were not expecting to deal with a master
+# cryptographer like yourself.
+
+# To decrypt a room name, rotate each letter forward through the alphabet a
+# number of times equal to the room's sector ID. A becomes B, B becomes C, Z
+# becomes A, and so on. Dashes become spaces.
+
+# For example, the real name for qzmt-zixmtkozy-ivhz-343 is very encrypted
+# name.
+
+# What is the sector ID of the room where North Pole objects are stored?
+
+
+def caeser_shift(string: str, rotation: int) -> str:
+    return "".join(
+        chr(((ord(char) - ord("a") + rotation) % 26) + ord("a"))
+        for char in string
+    )
+
+
+def part_2() -> None:
+    for instruction in instructions:
+        name, sector_id, checksum = split_instruction(
+            instruction, split_name=False
+        )
+        _decrypted = [caeser_shift(string, sector_id % 26) for string in name]
+        decrypted: str = " ".join(["".join(word) for word in _decrypted])
+        if decrypted.startswith("northpole"):
+            print(f"The sector ID is {sector_id}")
+
+
 if __name__ == "__main__":
     part_1()
+    part_2()