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+# --- Day 13: Knights of the Dinner Table ---
+
+# In years past, the holiday feast with your family hasn't gone so well. Not
+# everyone gets along! This year, you resolve, will be different. You're going
+# to find the optimal seating arrangement and avoid all those awkward
+# conversations.
+
+# You start by writing up a list of everyone invited and the amount their
+# happiness would increase or decrease if they were to find themselves sitting
+# next to each other person. You have a circular table that will be just big
+# enough to fit everyone comfortably, and so each person will have exactly two
+# neighbors.
+
+# For example, suppose you have only four attendees planned, and you calculate
+# their potential happiness as follows:
+
+# Alice would gain 54 happiness units by sitting next to Bob.
+# Alice would lose 79 happiness units by sitting next to Carol.
+# Alice would lose 2 happiness units by sitting next to David.
+# Bob would gain 83 happiness units by sitting next to Alice.
+# Bob would lose 7 happiness units by sitting next to Carol.
+# Bob would lose 63 happiness units by sitting next to David.
+# Carol would lose 62 happiness units by sitting next to Alice.
+# Carol would gain 60 happiness units by sitting next to Bob.
+# Carol would gain 55 happiness units by sitting next to David.
+# David would gain 46 happiness units by sitting next to Alice.
+# David would lose 7 happiness units by sitting next to Bob.
+# David would gain 41 happiness units by sitting next to Carol.
+
+# Then, if you seat Alice next to David, Alice would lose 2 happiness units
+# (because David talks so much), but David would gain 46 happiness units
+# (because Alice is such a good listener), for a total change of 44.
+
+# If you continue around the table, you could then seat Bob next to Alice (Bob
+# gains 83, Alice gains 54). Finally, seat Carol, who sits next to Bob (Carol
+# gains 60, Bob loses 7) and David (Carol gains 55, David gains 41). The
+# arrangement looks like this:
+
+#      +41 +46
+# +55   David    -2
+# Carol       Alice
+# +60    Bob    +54
+#      -7  +83
+
+# After trying every other seating arrangement in this hypothetical scenario,
+# you find that this one is the most optimal, with a total change in happiness
+# of 330.
+
+# What is the total change in happiness for the optimal seating arrangement of
+# the actual guest list?
+
+import re
+from collections import defaultdict
+from itertools import permutations
+from typing import DefaultDict
+
+with open("files/P13.txt") as f:
+    sittings = [line for line in f.read().strip().split("\n")]
+
+
+arrengements: DefaultDict = defaultdict(dict)
+for happiness in sittings:
+    pattern = r"(?P<p1>\S+) would (?P<op>lose|gain) (?P<val>\d+) happiness units by sitting next to (?P<p2>\S+)."
+    matches = re.match(pattern, happiness)
+    p1, op, val, p2 = re.match(pattern, happiness).group(
+        "p1", "op", "val", "p2"
+    )
+    arrengements[p1][p2] = -int(val) if op == "lose" else int(val)
+
+
+def part_1() -> None:
+    max_happiness = 0
+    for ordering in permutations(arrengements.keys()):
+        happiness = sum(
+            arrengements[a][b] + arrengements[b][a]
+            for a, b in zip(ordering, ordering[1:])
+        )
+        happiness += (
+            arrengements[ordering[0]][ordering[-1]]
+            + arrengements[ordering[-1]][ordering[0]]
+        )
+        max_happiness = max(max_happiness, happiness)
+    print(f"The total change in happiness is {max_happiness}")
+
+
+# --- Part Two ---
+
+# In all the commotion, you realize that you forgot to seat yourself. At this
+# point, you're pretty apathetic toward the whole thing, and your happiness
+# wouldn't really go up or down regardless of who you sit next to. You assume
+# everyone else would be just as ambivalent about sitting next to you, too.
+
+# So, add yourself to the list, and give all happiness relationships that
+# involve you a score of 0.
+
+# What is the total change in happiness for the optimal seating arrangement
+# that actually includes yourself?
+
+
+def part_2() -> None:
+    for somebody in list(arrengements.keys()):
+        arrengements["me"][somebody] = 0
+        arrengements[somebody]["me"] = 0
+
+    max_happiness = 0
+    for ordering in permutations(arrengements.keys()):
+        happiness = sum(
+            arrengements[a][b] + arrengements[b][a]
+            for a, b in zip(ordering, ordering[1:])
+        )
+        happiness += (
+            arrengements[ordering[0]][ordering[-1]]
+            + arrengements[ordering[-1]][ordering[0]]
+        )
+        max_happiness = max(max_happiness, happiness)
+    print(
+        f"The total change in happiness is {max_happiness}, including myself"
+    )
+
+
+if __name__ == "__main__":
+    part_1()
+    part_2()