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09.rb
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# --- Day 9: Marble Mania ---
#
# You talk to the Elves while you wait for your navigation system to
# initialize. To pass the time, they introduce you to their favorite
# marble game.
#
# The Elves play this game by taking turns arranging the marbles in a
# circle according to very particular rules. The marbles are numbered
# starting with 0 and increasing by 1 until every marble has a number.
#
# First, the marble numbered 0 is placed in the circle. At this
# point, while it contains only a single marble, it is still a circle:
# the marble is both clockwise from itself and counter-clockwise from
# itself. This marble is designated the current marble.
#
# Then, each Elf takes a turn placing the lowest-numbered remaining
# marble into the circle between the marbles that are 1 and 2 marbles
# clockwise of the current marble. (When the circle is large enough,
# this means that there is one marble between the marble that was just
# placed and the current marble.) The marble that was just placed
# then becomes the current marble.
#
# However, if the marble that is about to be placed has a number which
# is a multiple of 23, something entirely different happens. First,
# the current player keeps the marble they would have placed, adding
# it to their score. In addition, the marble 7 marbles
# counter-clockwise from the current marble is removed from the circle
# and also added to the current player's score. The marble located
# immediately clockwise of the marble that was removed becomes the new
# current marble.
#
# For example, suppose there are 9 players. After the marble with
# value 0 is placed in the middle, each player (shown in square
# brackets) takes a turn. The result of each of those turns would
# produce circles of marbles like this, where clockwise is to the
# right and the resulting current marble is in parentheses:
#
# [-] (0)
# [1] 0 (1)
# [2] 0 (2) 1
# [3] 0 2 1 (3)
# [4] 0 (4) 2 1 3
# [5] 0 4 2 (5) 1 3
# [6] 0 4 2 5 1 (6) 3
# [7] 0 4 2 5 1 6 3 (7)
# [8] 0 (8) 4 2 5 1 6 3 7
# [9] 0 8 4 (9) 2 5 1 6 3 7
# [1] 0 8 4 9 2(10) 5 1 6 3 7
# [2] 0 8 4 9 2 10 5(11) 1 6 3 7
# [3] 0 8 4 9 2 10 5 11 1(12) 6 3 7
# [4] 0 8 4 9 2 10 5 11 1 12 6(13) 3 7
# [5] 0 8 4 9 2 10 5 11 1 12 6 13 3(14) 7
# [6] 0 8 4 9 2 10 5 11 1 12 6 13 3 14 7(15)
# [7] 0(16) 8 4 9 2 10 5 11 1 12 6 13 3 14 7 15
# [8] 0 16 8(17) 4 9 2 10 5 11 1 12 6 13 3 14 7 15
# [9] 0 16 8 17 4(18) 9 2 10 5 11 1 12 6 13 3 14 7 15
# [1] 0 16 8 17 4 18 9(19) 2 10 5 11 1 12 6 13 3 14 7 15
# [2] 0 16 8 17 4 18 9 19 2(20)10 5 11 1 12 6 13 3 14 7 15
# [3] 0 16 8 17 4 18 9 19 2 20 10(21) 5 11 1 12 6 13 3 14 7 15
# [4] 0 16 8 17 4 18 9 19 2 20 10 21 5(22)11 1 12 6 13 3 14 7 15
# [5] 0 16 8 17 4 18(19) 2 20 10 21 5 22 11 1 12 6 13 3 14 7 15
# [6] 0 16 8 17 4 18 19 2(24)20 10 21 5 22 11 1 12 6 13 3 14 7 15
# [7] 0 16 8 17 4 18 19 2 24 20(25)10 21 5 22 11 1 12 6 13 3 14 7 15
#
# The goal is to be the player with the highest score after the last
# marble is used up. Assuming the example above ends after the marble
# numbered 25, the winning score is 23+9=32 (because player 5 kept
# marble 23 and removed marble 9, while no other player got any points
# in this very short example game).
#
# Here are a few more examples:
#
# - 10 players; last marble is worth 1618 points: high score is 8317
# - 13 players; last marble is worth 7999 points: high score is 146373
# - 17 players; last marble is worth 1104 points: high score is 2764
# - 21 players; last marble is worth 6111 points: high score is 54718
# - 30 players; last marble is worth 5807 points: high score is 37305
#
# What is the winning Elf's score?
Input = {num_players: 452, last_value: 71250}
class Marble
attr_reader :value
attr_accessor :prev, :next
def initialize(value)
@value = value
@prev = @next = self
end
def insert_before(other)
@prev = other.prev
other.prev = self
@next = other
@prev.next = self
self
end
def remove
@next.prev = @prev
@prev.next = @next
self
end
end
def solve(last_value)
m = Marble.new(0)
p = [0]*Input[:num_players]
i = 0
v = 1
while true
if v%23 == 0
7.times do
m = m.prev
end
p[i] += v + m.value
m = m.remove.next
else
m = Marble.new(v).insert_before(m.next.next)
end
break if v == last_value
i = (i+1)%Input[:num_players]
v += 1
end
p.max
end
puts solve(Input[:last_value])
# --- Part Two ---
#
# Amused by the speed of your answer, the Elves are curious:
#
# What would the new winning Elf's score be if the number of the last
# marble were 100 times larger?
puts solve(Input[:last_value]*100)