210 lines
7.0 KiB
Plaintext
210 lines
7.0 KiB
Plaintext
Advent of Code
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--- Day 22: Slam Shuffle ---
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There isn't much to do while you wait for the droids to repair your ship. At least you're drifting in the right direction.
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You decide to practice a new card shuffle you've been working on.
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Digging through the ship's storage, you find a deck of space cards! Just like any deck of space cards, there are 10007 cards
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in the deck numbered 0 through 10006. The deck must be new - they're still in factory order, with 0 on the top, then 1, then
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2, and so on, all the way through to 10006 on the bottom.
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You've been practicing three different techniques that you use while shuffling. Suppose you have a deck of only 10 cards
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(numbered 0 through 9):
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To deal into new stack, create a new stack of cards by dealing the top card of the deck onto the top of the new stack
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repeatedly until you run out of cards:
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Top Bottom
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0 1 2 3 4 5 6 7 8 9 Your deck
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New stack
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1 2 3 4 5 6 7 8 9 Your deck
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0 New stack
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2 3 4 5 6 7 8 9 Your deck
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1 0 New stack
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3 4 5 6 7 8 9 Your deck
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2 1 0 New stack
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Several steps later...
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9 Your deck
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8 7 6 5 4 3 2 1 0 New stack
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Your deck
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9 8 7 6 5 4 3 2 1 0 New stack
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Finally, pick up the new stack you've just created and use it as the deck for the next technique.
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To cut N cards, take the top N cards off the top of the deck and move them as a single unit to the bottom of the deck,
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retaining their order. For example, to cut 3:
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Top Bottom
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0 1 2 3 4 5 6 7 8 9 Your deck
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3 4 5 6 7 8 9 Your deck
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0 1 2 Cut cards
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3 4 5 6 7 8 9 Your deck
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0 1 2 Cut cards
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3 4 5 6 7 8 9 0 1 2 Your deck
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You've also been getting pretty good at a version of this technique where N is negative! In that case, cut (the absolute
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value of) N cards from the bottom of the deck onto the top. For example, to cut -4:
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Top Bottom
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0 1 2 3 4 5 6 7 8 9 Your deck
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0 1 2 3 4 5 Your deck
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6 7 8 9 Cut cards
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0 1 2 3 4 5 Your deck
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6 7 8 9 Cut cards
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6 7 8 9 0 1 2 3 4 5 Your deck
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To deal with increment N, start by clearing enough space on your table to lay out all of the cards individually in a long
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line. Deal the top card into the leftmost position. Then, move N positions to the right and deal the next card there. If you
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would move into a position past the end of the space on your table, wrap around and keep counting from the leftmost card
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again. Continue this process until you run out of cards.
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For example, to deal with increment 3:
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0 1 2 3 4 5 6 7 8 9 Your deck
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. . . . . . . . . . Space on table
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^ Current position
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Deal the top card to the current position:
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1 2 3 4 5 6 7 8 9 Your deck
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0 . . . . . . . . . Space on table
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^ Current position
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Move the current position right 3:
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1 2 3 4 5 6 7 8 9 Your deck
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0 . . . . . . . . . Space on table
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^ Current position
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Deal the top card:
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2 3 4 5 6 7 8 9 Your deck
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0 . . 1 . . . . . . Space on table
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^ Current position
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Move right 3 and deal:
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3 4 5 6 7 8 9 Your deck
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0 . . 1 . . 2 . . . Space on table
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^ Current position
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Move right 3 and deal:
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4 5 6 7 8 9 Your deck
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0 . . 1 . . 2 . . 3 Space on table
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^ Current position
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Move right 3, wrapping around, and deal:
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5 6 7 8 9 Your deck
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0 . 4 1 . . 2 . . 3 Space on table
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^ Current position
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And so on:
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0 7 4 1 8 5 2 9 6 3 Space on table
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Positions on the table which already contain cards are still counted; they're not skipped. Of course, this technique is
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carefully designed so it will never put two cards in the same position or leave a position empty.
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Finally, collect the cards on the table so that the leftmost card ends up at the top of your deck, the card to its right ends
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up just below the top card, and so on, until the rightmost card ends up at the bottom of the deck.
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The complete shuffle process (your puzzle input) consists of applying many of these techniques. Here are some examples that
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combine techniques; they all start with a factory order deck of 10 cards:
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deal with increment 7
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deal into new stack
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deal into new stack
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Result: 0 3 6 9 2 5 8 1 4 7
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cut 6
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deal with increment 7
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deal into new stack
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Result: 3 0 7 4 1 8 5 2 9 6
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deal with increment 7
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deal with increment 9
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cut -2
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Result: 6 3 0 7 4 1 8 5 2 9
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deal into new stack
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cut -2
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deal with increment 7
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cut 8
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cut -4
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deal with increment 7
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cut 3
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deal with increment 9
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deal with increment 3
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cut -1
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Result: 9 2 5 8 1 4 7 0 3 6
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Positions within the deck count from 0 at the top, then 1 for the card immediately below the top card, and so on to the
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bottom. (That is, cards start in the position matching their number.)
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After shuffling your factory order deck of 10007 cards, what is the position of card 2019?
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Your puzzle answer was 6129.
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--- Part Two ---
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After a while, you realize your shuffling skill won't improve much more with merely a single deck of cards. You ask every 3D
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printer on the ship to make you some more cards while you check on the ship repairs. While reviewing the work the droids have
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finished so far, you think you see Halley's Comet fly past!
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When you get back, you discover that the 3D printers have combined their power to create for you a single, giant, brand new,
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factory order deck of 119315717514047 space cards.
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Finally, a deck of cards worthy of shuffling!
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You decide to apply your complete shuffle process (your puzzle input) to the deck 101741582076661 times in a row.
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You'll need to be careful, though - one wrong move with this many cards and you might overflow your entire ship!
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After shuffling your new, giant, factory order deck that many times, what number is on the card that ends up in position
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2020?
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Your puzzle answer was 71345377301237.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, all that is left is for you to admire your Advent calendar.
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If you still want to see it, you can get your puzzle input.
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References
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Visible links
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. https://adventofcode.com/
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. https://adventofcode.com/2019/about
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. https://adventofcode.com/2019/events
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. https://adventofcode.com/2019/settings
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. https://adventofcode.com/2019/auth/logout
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. Advent of Code Supporter
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https://adventofcode.com/2019/support
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019/support
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. https://adventofcode.com/2019/sponsors
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. https://adventofcode.com/2019/leaderboard
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. https://adventofcode.com/2019/stats
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. https://adventofcode.com/2019/sponsors
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. https://en.wikipedia.org/wiki/Shuffling
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. https://en.wikipedia.org/wiki/Halley%27s_Comet
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019/day/22/input
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