Solution to problem 11 in Python

src/P11.py

@@ -0,0 +1,176 @@
+# --- Day 11: Seating System ---
+
+# Your plane lands with plenty of time to spare. The final leg of your journey
+# is a ferry that goes directly to the tropical island where you can finally
+# start your vacation. As you reach the waiting area to board the ferry, you
+# realize you're so early, nobody else has even arrived yet!
+
+# By modeling the process people use to choose (or abandon) their seat in the
+# waiting area, you're pretty sure you can predict the best place to sit. You
+# make a quick map of the seat layout (your puzzle input).
+
+# The seat layout fits neatly on a grid. Each position is either floor (.), an
+# empty seat (L), or an occupied seat (#). For example, the initial seat layout
+# might look like this:
+
+# L.LL.LL.LL
+# LLLLLLL.LL
+# L.L.L..L..
+# LLLL.LL.LL
+# L.LL.LL.LL
+# L.LLLLL.LL
+# ..L.L.....
+# LLLLLLLLLL
+# L.LLLLLL.L
+# L.LLLLL.LL
+
+# Now, you just need to model the people who will be arriving shortly.
+# Fortunately, people are entirely predictable and always follow a simple set
+# of rules. All decisions are based on the number of occupied seats adjacent to
+# a given seat (one of the eight positions immediately up, down, left, right,
+# or diagonal from the seat). The following rules are applied to every seat
+# simultaneously:
+
+#     If a seat is empty (L) and there are no occupied seats adjacent to it,
+# the seat becomes occupied.
+#     If a seat is occupied (#) and four or more seats adjacent to it are also
+# occupied, the seat becomes empty.
+#     Otherwise, the seat's state does not change.
+
+# Floor (.) never changes; seats don't move, and nobody sits on the floor.
+
+# After one round of these rules, every seat in the example layout becomes
+# occupied:
+
+# #.##.##.##
+# #######.##
+# #.#.#..#..
+# ####.##.##
+# #.##.##.##
+# #.#####.##
+# ..#.#.....
+# ##########
+# #.######.#
+# #.#####.##
+
+# After a second round, the seats with four or more occupied adjacent seats
+# become empty again:
+
+# #.LL.L#.##
+# #LLLLLL.L#
+# L.L.L..L..
+# #LLL.LL.L#
+# #.LL.LL.LL
+# #.LLLL#.##
+# ..L.L.....
+# #LLLLLLLL#
+# #.LLLLLL.L
+# #.#LLLL.##
+
+# This process continues for three more rounds:
+
+# #.##.L#.##
+# #L###LL.L#
+# L.#.#..#..
+# #L##.##.L#
+# #.##.LL.LL
+# #.###L#.##
+# ..#.#.....
+# #L######L#
+# #.LL###L.L
+# #.#L###.##
+
+# #.#L.L#.##
+# #LLL#LL.L#
+# L.L.L..#..
+# #LLL.##.L#
+# #.LL.LL.LL
+# #.LL#L#.##
+# ..L.L.....
+# #L#LLLL#L#
+# #.LLLLLL.L
+# #.#L#L#.##
+
+# #.#L.L#.##
+# #LLL#LL.L#
+# L.#.L..#..
+# #L##.##.L#
+# #.#L.LL.LL
+# #.#L#L#.##
+# ..L.L.....
+# #L#L##L#L#
+# #.LLLLLL.L
+# #.#L#L#.##
+
+# At this point, something interesting happens: the chaos stabilizes and
+# further applications of these rules cause no seats to change state! Once
+# people stop moving around, you count 37 occupied seats.
+
+# Simulate your seating area by applying the seating rules repeatedly until no
+# seats change state. How many seats end up occupied?
+
+with open("files/P11.txt", "r") as f:
+    seats = [line for line in f.read().strip().split("\n")]
+
+
+def seats_around(seats: list[str], r: int, c: int) -> int:
+    total = 0
+    around_me = [
+        (-1, -1),
+        (-1, 0),
+        (-1, 1),
+        (0, -1),
+        (0, 1),
+        (1, -1),
+        (1, 0),
+        (1, 1),
+    ]
+    for row, col in around_me:
+        _row = r + row
+        _col = c + col
+        if (
+            _row >= 0
+            and _row < len(seats)
+            and _col >= 0
+            and _col < len(seats[c])
+        ):
+            total += seats[_row][_col] == "#"
+    return total
+
+
+def free_seats(v: list[str]) -> int:
+    total = 0
+    for row in v:
+        total += row.count("#")
+    return total
+
+
+def part_1(seats: list[str]) -> None:
+    R = len(seats)
+    C = len(seats[0])
+    while True:
+        has_changed = False
+        next_iter = []
+        for r in range(R):
+            new_row = ""
+            for c in range(C):
+                seat = seats[r][c]
+                if seat != ".":
+                    occupants = seats_around(seats, r, c)
+                    if seat == "L" and occupants == 0:
+                        seat = "#"
+                        has_changed = True
+                    elif seat == "#" and occupants >= 4:
+                        seat = "L"
+                        has_changed = True
+                new_row += seat
+            next_iter.append(new_row)
+        if not has_changed:
+            break
+        seats = next_iter
+
+    print(f"There are {free_seats(seats)} seats occupied")
+
+
+if __name__ == "__main__":
+    part_1(seats)