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04.py
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# --- Day 4: Giant Squid ---
#
# You're already almost 1.5km (almost a mile) below the surface of the
# ocean, already so deep that you can't see any sunlight. What you
# can see, however, is a giant squid that has attached itself to the
# outside of your submarine.
#
# Maybe it wants to play bingo?
#
# Bingo is played on a set of boards each consisting of a 5x5 grid of
# numbers. Numbers are chosen at random, and the chosen number is
# marked on all boards on which it appears. (Numbers may not appear
# on all boards.) If all numbers in any row or any column of a board
# are marked, that board wins. (Diagonals don't count.)
#
# The submarine has a bingo subsystem to help passengers (currently,
# you and the giant squid) pass the time. It automatically generates
# a random order in which to draw numbers and a random set of boards
# (your puzzle input). For example:
#
# 7,4,9,5,11,17,23,2,0,14,21,24,10,16,13,6,15,25,12,22,18,20,8,19,3,26,1
#
# 22 13 17 11 0
# 8 2 23 4 24
# 21 9 14 16 7
# 6 10 3 18 5
# 1 12 20 15 19
#
# 3 15 0 2 22
# 9 18 13 17 5
# 19 8 7 25 23
# 20 11 10 24 4
# 14 21 16 12 6
#
# 14 21 17 24 4
# 10 16 15 9 19
# 18 8 23 26 20
# 22 11 13 6 5
# 2 0 12 3 7
#
# After the first five numbers are drawn (7, 4, 9, 5, and 11), there
# are no winners, but the boards are marked as follows (shown here
# adjacent to each other to save space):
#
# 22 13 17 (11) 0 | 3 15 0 2 22 | 14 21 17 24 ( 4)
# 8 2 23 ( 4) 24 | ( 9) 18 13 17 ( 5) | 10 16 15 ( 9) 19
# 21 ( 9) 14 16 ( 7) | 19 8 ( 7) 25 23 | 18 8 23 26 20
# 6 10 3 18 ( 5) | 20 (11) 10 24 ( 4) | 22 (11) 13 6 ( 5)
# 1 12 20 15 19 | 14 21 16 12 6 | 2 0 12 3 ( 7)
#
# After the next six numbers are drawn (17, 23, 2, 0, 14, and 21),
# there are still no winners:
#
# 22 13 (17)(11)( 0) | 3 15 ( 0)( 2) 22 | (14)(21)(17) 24 ( 4)
# 8 ( 2)(23)( 4) 24 | ( 9) 18 13 (17)( 5) | 10 16 15 ( 9) 19
# (21)( 9)(14) 16 ( 7) | 19 8 ( 7) 25 (23) | 18 8 (23) 26 20
# 6 10 3 18 ( 5) | 20 (11) 10 24 ( 4) | 22 (11) 13 6 ( 5)
# 1 12 20 15 19 | (14)(21) 16 12 6 | ( 2)( 0) 12 3 ( 7)
#
# Finally, 24 is drawn:
#
# 22 13 (17)(11)( 0) | 3 15 ( 0)( 2) 22 | (14)(21)(17)(24)( 4)
# 8 ( 2)(23)( 4)(24) | ( 9) 18 13 (17)( 5) | 10 16 15 ( 9) 19
# (21)( 9)(14) 16 ( 7) | 19 8 ( 7) 25 (23) | 18 8 (23) 26 20
# 6 10 3 18 ( 5) | 20 (11) 10 (24)( 4) | 22 (11) 13 6 ( 5)
# 1 12 20 15 19 | (14)(21) 16 12 6 | ( 2)( 0) 12 3 ( 7)
#
# At this point, the third board wins because it has at least one
# complete row or column of marked numbers (in this case, the entire
# top row is marked: 14 21 17 24 4).
#
# The score of the winning board can now be calculated. Start by
# finding the sum of all unmarked numbers on that board; in this case,
# the sum is 188. Then, multiply that sum by the number that was just
# called when the board won, 24, to get the final score,
# 188 * 24 = 4512.
#
# To guarantee victory against the giant squid, figure out which board
# will win first. What will your final score be if you choose that
# board?
BS = 5 # board size
class Board:
def __init__(self, lines):
self.nums = {}
for r, l in enumerate(lines):
for c, n in enumerate(map(int, l.split())):
# We assume a number appears at most once on a board.
assert n not in self.nums
self.nums[n] = (r, c)
self.clear()
self.won = False
def clear(self):
self.marked = [[False]*BS for _ in range(BS)]
def mark(self, num):
if not self.won and num in self.nums:
row, col = self.nums[num]
self.marked[row][col] = True
if (
all(self.marked[r][col] for r in range(BS))
or all(self.marked[row][c] for c in range(BS))
):
self.won = True
return sum(
n for n, (r, c) in self.nums.items()
if not self.marked[r][c]
)
return None
lines = open("04.in").readlines()
nums = [int(n) for n in lines[0].split(",")]
boards = [Board(lines[i+1:i+BS+1]) for i in range(1, len(lines), BS+1)]
score = None
for n in nums:
for b in boards:
result = b.mark(n)
if result != None:
score = n*result
break
if score != None:
break
print(score)
# --- Part Two ---
#
# On the other hand, it might be wise to try a different strategy: let
# the giant squid win.
#
# You aren't sure how many bingo boards a giant squid could play at
# once, so rather than waste time counting its arms, the safe thing to
# do is to figure out which board will win last and choose that one.
# That way, no matter which boards it picks, it will win for sure.
#
# In the above example, the second board is the last to win, which
# happens after 13 is eventually called and its middle column is
# completely marked. If you were to keep playing until this point,
# the second board would have a sum of unmarked numbers equal to 148
# for a final score of 148 * 13 = 1924.
#
# Figure out which board will win last. Once it wins, what would its
# final score be?
from common import each_do
each_do(Board.clear, boards)
score = None
for n in nums:
for b in boards:
result = b.mark(n)
if result != None:
score = n*result
print(score)