70 lines
4.7 KiB
Markdown
70 lines
4.7 KiB
Markdown
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# 2020 Day 12: Rain Risk
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#### [Direct Link](https://adventofcode.com/2020/day/12)
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## Part 1
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Your ferry made decent progress toward the island, but the storm came in faster than anyone expected. The ferry needs to take **evasive actions**!
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Unfortunately, the ship's navigation computer seems to be malfunctioning; rather than giving a route directly to safety, it produced extremely circuitous instructions. When the captain uses the [PA system](https://en.wikipedia.org/wiki/Public_address_system) to ask if anyone can help, you quickly volunteer.
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The navigation instructions (your puzzle input) consists of a sequence of single-character **actions** paired with integer input **values**. After staring at them for a few minutes, you work out what they probably mean:
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- Action **`N`** means to move **north** by the given value.
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- Action **`S`** means to move **south** by the given value.
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- Action **`E`** means to move **east** by the given value.
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- Action **`W`** means to move **west** by the given value.
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- Action **`L`** means to turn **left** the given number of degrees.
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- Action **`R`** means to turn **right** the given number of degrees.
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- Action **`F`** means to move **forward** by the given value in the direction the ship is currently facing.
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The ship starts by facing **east**. Only the `L` and `R` actions change the direction the ship is facing. (That is, if the ship is facing east and the next instruction is `N10`, the ship would move north 10 units, but would still move east if the following action were `F`.)
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For example:
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```
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F10
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N3
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F7
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R90
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F11
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```
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These instructions would be handled as follows:
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- `F10` would move the ship 10 units east (because the ship starts by facing east) to **east 10, north 0**.
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- `N3` would move the ship 3 units north to **east 10, north 3**.
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- `F7` would move the ship another 7 units east (because the ship is still facing east) to **east 17, north 3**.
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- `R90` would cause the ship to turn right by 90 degrees and face south; it remains at **east 17, north 3**.
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- `F11` would move the ship 11 units south to **east 17, south 8**.
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At the end of these instructions, the ship's [Manhattan distance](https://en.wikipedia.org/wiki/Manhattan_distance) (sum of the absolute values of its east/west position and its north/south position) from its starting position is `17 + 8` = **`25`**.
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Figure out where the navigation instructions lead. **What is the Manhattan distance between that location and the ship's starting position?**
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## Part 2
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Before you can give the destination to the captain, you realize that the actual action meanings were printed on the back of the instructions the whole time.
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Almost all of the actions indicate how to move a **waypoint** which is relative to the ship's position:
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- Action **`N`** means to move the waypoint **north** by the given value.
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- Action **`S`** means to move the waypoint **south** by the given value.
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- Action **`E`** means to move the waypoint **east** by the given value.
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- Action **`W`** means to move the waypoint **west** by the given value.
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- Action **`L`** means to rotate the waypoint around the ship **left** (**counter-clockwise**) the given number of degrees.
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- Action **`R`** means to rotate the waypoint around the ship **right** (**clockwise**) the given number of degrees.
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- Action **`F`** means to move **forward** to the waypoint a number of times equal to the given value.
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The waypoint starts **10 units east and 1 unit north** relative to the ship. The waypoint is relative to the ship; that is, if the ship moves, the waypoint moves with it.
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For example, using the same instructions as above:
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- `F10` moves the ship to the waypoint 10 times (a total of **100 units east and 10 units north**), leaving the ship at **east 100, north 10**. The waypoint stays 10 units east and 1 unit north of the ship.
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- `N3` moves the waypoint 3 units north to **10 units east and 4 units north of the ship**. The ship remains at **east 100, north 10**.
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- `F7` moves the ship to the waypoint 7 times (a total of **70 units east and 28 units north**), leaving the ship at **east 170, north 38**. The waypoint stays 10 units east and 4 units north of the ship.
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- `R90` rotates the waypoint around the ship clockwise 90 degrees, moving it to **4 units east and 10 units south of the ship**. The ship remains at **east 170, north 38**.
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- `F11` moves the ship to the waypoint 11 times (a total of **44 units east and 110 units south**), leaving the ship at **east 214, south 72**. The waypoint stays 4 units east and 10 units south of the ship.
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After these operations, the ship's Manhattan distance from its starting position is `214 + 72` = **`286`**.
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Figure out where the navigation instructions actually lead. **What is the Manhattan distance between that location and the ship's starting position?**
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