57 lines
3.8 KiB
Markdown
57 lines
3.8 KiB
Markdown
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# 2020 Day 15: Rambunctious Recitation
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#### [Direct Link](https://adventofcode.com/2020/day/15)
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## Part 1
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You catch the airport shuttle and try to book a new flight to your vacation island. Due to the storm, all direct flights have been cancelled, but a route is available to get around the storm. You take it.
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While you wait for your flight, you decide to check in with the Elves back at the North Pole. They're playing a **memory game** and are ever so excited to explain the rules!
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In this game, the players take turns saying **numbers**. They begin by taking turns reading from a list of **starting numbers** (your puzzle input). Then, each turn consists of considering the **most recently spoken number**:
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- If that was the **first** time the number has been spoken, the current player says **`0`**.
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- Otherwise, the number had been spoken before; the current player announces **how many turns apart** the number is from when it was previously spoken.
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So, after the starting numbers, each turn results in that player speaking aloud either **`0`** (if the last number is new) or an **age** (if the last number is a repeat).
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For example, suppose the starting numbers are `0,3,6`:
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- **Turn 1**: The `1`st number spoken is a starting number, **`0`**.
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- **Turn 2**: The `2`nd number spoken is a starting number, **`3`**.
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- **Turn 3**: The `3`rd number spoken is a starting number, **`6`**.
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- **Turn 4**: Now, consider the last number spoken, `6`. Since that was the first time the number had been spoken, the `4`th number spoken is **`0`**.
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- **Turn 5**: Next, again consider the last number spoken, `0`. Since it had been spoken before, the next number to speak is the difference between the turn number when it was last spoken (the previous turn, `4`) and the turn number of the time it was most recently spoken before then (turn `1`). Thus, the 5th number spoken is `4 - 1`, **`3`**.
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- **Turn 6**: The last number spoken, `3` had also been spoken before, most recently on turns `5` and `2`. So, the `6`th number spoken is `5 - 2`, **`3`**.
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- **Turn 7**: Since `3` was just spoken twice in a row, and the last two turns are `1` turn apart, the `7`th number spoken is **`1`**.
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- **Turn 8**: Since `1` is new, the `8`th number spoken is **`0`**.
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- **Turn 9**: `0` was last spoken on turns `8` and `4`, so the `9`th number spoken is the difference between them, **`4`**.
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- **Turn 10**: `4` is new, so the `10`th number spoken is **`0`**.
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(The game ends when the Elves get sick of playing or dinner is ready, whichever comes first.)
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Their question for you is: what will be the **`2020`th** number spoken? In the example above, the `2020`th number spoken will be `436`.
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Here are a few more examples:
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- Given the starting numbers `1,3,2`, the `2020`th number spoken is `1`.
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- Given the starting numbers `2,1,3`, the `2020`th number spoken is `10`.
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- Given the starting numbers `1,2,3`, the `2020`th number spoken is `27`.
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- Given the starting numbers `2,3,1`, the `2020`th number spoken is `78`.
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- Given the starting numbers `3,2,1`, the `2020`th number spoken is `438`.
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- Given the starting numbers `3,1,2`, the `2020`th number spoken is `1836`.
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Given your starting numbers, **what will be the 2020th number spoken?**
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## Part 2
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Impressed, the Elves issue you a challenge: determine the 30000000th number spoken. For example, given the same starting numbers as above:
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- Given `0,3,6`, the `30000000th` number spoken is `175594`.
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- Given `1,3,2`, the `30000000th` number spoken is `2578`.
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- Given `2,1,3`, the `30000000th` number spoken is `3544142`.
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- Given `1,2,3`, the `30000000th` number spoken is `261214`.
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- Given `2,3,1`, the `30000000th` number spoken is `6895259`.
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- Given `3,2,1`, the `30000000th` number spoken is `18`.
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- Given `3,1,2`, the `30000000th` number spoken is `362`.
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Given your starting numbers, **what will be the 30000000th number spoken?**
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