day15.py

import io
import re

# --- Day 15: Timing is Everything ---
# 
# The halls open into an interior plaza containing a large kinetic
# sculpture. The sculpture is in a sealed enclosure and seems to involve
# a set of identical spherical capsules that are carried to the top and
# allowed to bounce through the maze of spinning pieces.
# 
# Part of the sculpture is even interactive! When a button is pressed, a
# capsule is dropped and tries to fall through slots in a set of
# rotating discs to finally go through a little hole at the bottom and
# come out of the sculpture. If any of the slots aren't aligned with the
# capsule as it passes, the capsule bounces off the disc and soars
# away. You feel compelled to get one of those capsules.
# 
# The discs pause their motion each second and come in different sizes;
# they seem to each have a fixed number of positions at which they
# stop. You decide to call the position with the slot 0, and count up
# for each position it reaches next.
# 
# Furthermore, the discs are spaced out so that after you push the
# button, one second elapses before the first disc is reached, and one
# second elapses as the capsule passes from one disk to the one below
# it. So, if you push the button at time=100, then the capsule reaches
# the top disc at time=101, the second disc at time=102, the third disc
# at time=103, and so on.
# 
# The button will only drop a capsule at an integer time - no fractional
# seconds allowed.
# 
# For example, at time=0, suppose you see the following arrangement:
# 
# Disc #1 has 5 positions; at time=0, it is at position 4.
# Disc #2 has 2 positions; at time=0, it is at position 1.
# 
# If you press the button exactly at time=0, the capsule would start to
# fall; it would reach the first disc at time=1. Since the first disc
# was at position 4 at time=0, by time=1 it has ticked one position
# forward. As a five-position disc, the next position is 0, and the
# capsule falls through the slot.
# 
# Then, at time=2, the capsule reaches the second disc. The second disc
# has ticked forward two positions at this point: it started at position
# 1, then continued to position 0, and finally ended up at position 1
# again. Because there's only a slot at position 0, the capsule bounces
# away.
# 
# If, however, you wait until time=5 to push the button, then when the
# capsule reaches each disc, the first disc will have ticked forward 5+1
# = 6 times (to position 0), and the second disc will have ticked
# forward 5+2 = 7 times (also to position 0). In this case, the capsule
# would fall through the discs and come out of the machine.
# 
# However, your situation has more than two discs; you've noted their
# positions in your puzzle input. What is the first time you can press
# the button to get a capsule?
# 

pattern_disc = re.compile('Disc #([0-9]+) has ([0-9]+) positions; at time=0, it is at position ([0-9]+).')

def line2disc(line):
    """
    Turn a string description of a disc into a pair description (nb position, current position)
    """
    match = pattern_disc.match(line)
    nb_positions = int(match.group(2))
    current_position = int(match.group(3))
    return (nb_positions, current_position)

def resolves(discs):
    start_time = 0

    i = 0
    while i < len(discs):
        (nb_positions, current_position) = discs[i]

        # Moving forward the disc
        shift = start_time + 1 + i
        current_position = (current_position + shift) % nb_positions

        if current_position == 0:
            # Continue investigating this time
            i += 1
        else:
            # A disc is not aligned, pass to the next time
            i = 0
            start_time += 1

    return start_time

with io.open('inputs/day15.txt') as f:
    discs = [line2disc(line.strip()) for line in f]

    print('The first time you can press the button to get a capsule with {} discs: {}'.format(len(discs), resolves(discs)))

# --- Part Two ---
# 
# After getting the first capsule (it contained a star! what great
# fortune!), the machine detects your success and begins to rearrange
# itself.
# 
# When it's done, the discs are back in their original configuration as
# if it were time=0 again, but a new disc with 11 positions and starting
# at position 0 has appeared exactly one second below the
# previously-bottom disc.
# 
# With this new disc, and counting again starting from time=0 with the
# configuration in your puzzle input, what is the first time you can
# press the button to get another capsule?

with io.open('inputs/day15.txt') as f:
    discs = [line2disc(line.strip()) for line in f]
    # Adding the special capsule
    discs.append((11, 0))

    print('The first time you can press the button to get another capsule with {} discs: {}'.format(len(discs), resolves(discs)))