My Advent of Code solutions in Python.
kevinyap.ca/2019/12/going-fast-in-advent-of-code/
advent-of-code
python
Add solution for 2019/22
+92
+92
2019/day22.py
+92
2019/day22.py
···
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"""
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Credit to /u/mcpower_ for explaining the number theory involved in Part 2:
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https://www.reddit.com/r/adventofcode/comments/ee0rqi/2019_day_22_solutions/fbnkaju/
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"""
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import fileinput
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def solve_part_1(actions):
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CARDS = 10007
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POS = 2019
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for action, n in actions:
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if action == 'deal':
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if n is None:
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POS = (CARDS - 1) - POS
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else:
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POS = (n * POS) % CARDS
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else:
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if POS >= n:
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POS -= n
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else:
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POS = (CARDS - (n - POS))
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return POS
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def solve_part_2(actions):
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CARDS = 119315717514047
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SHUFFLES = 101741582076661
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def modinv(n, p):
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"""Returns the inverse of n mod p, assuming p is prime."""
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return pow(n, p - 2, p)
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def get_card(offset, increment, i):
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"""Returns the ith card in the sequence given its offset and increment."""
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return (offset + i * increment) % CARDS
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def get_sequence(iterations, increment_mul, offset_diff):
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"""Returns the final increment and offset after the given number of iterations."""
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increment = pow(increment_mul, iterations, CARDS)
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offset = (offset_diff * (1 - increment) * modinv(1 - increment_mul, CARDS)) % CARDS
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return increment, offset
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# `increment` is the difference between two adajcent numbers
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increment = 1
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# `offset` is the first number in the sequence
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offset = 0
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for action, n in actions:
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if action == 'deal':
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# deal into new stack
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if n is None:
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# The sequence gets reversed...
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increment = (increment * -1) % CARDS
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# ...and shifted one to the left
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offset = (offset + increment) % CARDS
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# deal with increment n
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else:
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increment = (increment * modinv(n, CARDS)) % CARDS
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# cut n
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else:
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offset = (offset + (n * increment)) % CARDS
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inc, off = get_sequence(SHUFFLES, increment, offset)
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return get_card(off, inc, 2020)
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actions = []
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for line in fileinput.input():
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line = line.strip()
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inst = line.split()
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verb = inst[0]
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if verb == 'cut':
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actions.append(('cut', int(inst[1])))
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else:
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if inst[1] == 'into':
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actions.append(('deal', None))
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else:
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actions.append(('deal', int(inst[-1])))
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print "Position of card 2019:", solve_part_1(actions)
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print "Card ending up in position 2020:", solve_part_2(actions)