107 lines
3.1 KiB
Plaintext
107 lines
3.1 KiB
Plaintext
Advent of Code
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--- Day 19: An Elephant Named Joseph ---
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The Elves contact you over a highly secure emergency channel. Back at the
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North Pole, the Elves are busy misunderstanding White Elephant parties.
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Each Elf brings a present. They all sit in a circle, numbered starting with
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position 1. Then, starting with the first Elf, they take turns stealing all
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the presents from the Elf to their left. An Elf with no presents is removed
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from the circle and does not take turns.
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For example, with five Elves (numbered 1 to 5):
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1
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5 2
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4 3
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• Elf 1 takes Elf 2's present.
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• Elf 2 has no presents and is skipped.
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• Elf 3 takes Elf 4's present.
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• Elf 4 has no presents and is also skipped.
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• Elf 5 takes Elf 1's two presents.
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• Neither Elf 1 nor Elf 2 have any presents, so both are skipped.
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• Elf 3 takes Elf 5's three presents.
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So, with five Elves, the Elf that sits starting in position 3 gets all the
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presents.
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With the number of Elves given in your puzzle input, which Elf gets all the
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presents?
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Your puzzle input was 3014387.
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Your puzzle answer was 1834471.
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--- Part Two ---
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Realizing the folly of their present-exchange rules, the Elves agree to
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instead steal presents from the Elf directly across the circle. If two Elves
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are across the circle, the one on the left (from the perspective of the
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stealer) is stolen from. The other rules remain unchanged: Elves with no
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presents are removed from the circle entirely, and the other elves move in
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slightly to keep the circle evenly spaced.
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For example, with five Elves (again numbered 1 to 5):
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• The Elves sit in a circle; Elf 1 goes first:
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1
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5 2
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4 3
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• Elves 3 and 4 are across the circle; Elf 3's present is stolen, being
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the one to the left. Elf 3 leaves the circle, and the rest of the Elves
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move in:
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1 1
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5 2 --> 5 2
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4 - 4
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• Elf 2 steals from the Elf directly across the circle, Elf 5:
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1 1
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- 2 --> 2
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4 4
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• Next is Elf 4 who, choosing between Elves 1 and 2, steals from Elf 1:
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- 2
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2 -->
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4 4
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• Finally, Elf 2 steals from Elf 4:
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2
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--> 2
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-
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So, with five Elves, the Elf that sits starting in position 2 gets all the
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presents.
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With the number of Elves given in your puzzle input, which Elf now gets all
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the presents?
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Your puzzle input was 3014387.
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Your puzzle answer was 1420064.
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References
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. https://en.wikipedia.org/wiki/White_elephant_gift_exchange
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