78 lines
3.8 KiB
Markdown
78 lines
3.8 KiB
Markdown
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# 2018 Day 11: Chronal Charge
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Copyright (c) Eric Wastl
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#### [Direct Link](https://adventofcode.com/2018/day/11)
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## Part 1
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You watch the Elves and their sleigh fade into the distance as they head toward the North Pole.
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Actually, you're the one fading. The falling sensation returns.
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The low fuel warning light is illuminated on your wrist-mounted device. Tapping it once causes it to project a hologram of the situation: a **300x300** grid of fuel cells and their current power levels, some negative. You're not sure what negative power means in the context of time travel, but it can't be good.
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Each fuel cell has a coordinate ranging from **1 to 300** in both the X (horizontal) and Y (vertical) direction. In `X,Y` notation, the top-left cell is `1,1`, and the top-right cell is `300,1`.
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The interface lets you select **any 3x3 square** of fuel cells. To increase your chances of getting to your destination, you decide to choose the 3x3 square with the **largest total power**.
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The power level in a given fuel cell can be found through the following process:
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- Find the fuel cell's **rack ID**, which is its **X coordinate plus 10**.
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- Begin with a power level of the **rack ID** times the **Y coordinate**.
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- Increase the power level by the value of the **grid serial number** (your puzzle input).
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- Set the power level to itself multiplied by the **rack ID**.
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- Keep only the **hundreds digit** of the power level (so `12345` becomes 3; numbers with no hundreds digit become `0`).
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- **Subtract 5** from the power level.
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For example, to find the power level of the fuel cell at 3,5 in a grid with serial number `8`:
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- The rack ID is `3 + 10 = 13`.
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- The power level starts at `13 * 5 = 65`.
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- Adding the serial number produces `65 + 8 = 73`.
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- Multiplying by the rack ID produces `73 * 13 = 949`.
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- The hundreds digit of `949` is `9`.
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- Subtracting 5 produces `9 - 5 = 4`.
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So, the power level of this fuel cell is **`4`**.
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Here are some more example power levels:
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- Fuel cell at `122, 79`, grid serial number `57`: power level `-5`.
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- Fuel cell at `217,196`, grid serial number `39`: power level `0`.
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- Fuel cell at `101,153`, grid serial number `71`: power level `4`.
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Your goal is to find the 3x3 square which has the largest total power. The square must be entirely within the 300x300 grid. Identify this square using the `X,Y` coordinate of its **top-left fuel cell**. For example:
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For grid serial number `18`, the largest total 3x3 square has a top-left corner of **`33,45`** (with a total power of `29`); these fuel cells appear in the middle of this 5x5 region:
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```
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-2 -4 4 4 4
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-4 4 4 4 -5
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4 3 3 4 -4
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1 1 2 4 -3
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-1 0 2 -5 -2
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```
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For grid serial number `42`, the largest 3x3 square's top-left is **`21,61`** (with a total power of `30`); they are in the middle of this region:
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```
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-3 4 2 2 2
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-4 4 3 3 4
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-5 3 3 4 -4
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4 3 3 4 -3
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3 3 3 -5 -1
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```
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**What is the `X,Y` coordinate of the top-left fuel cell of the 3x3 square with the largest total power?**
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## Part 2
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You discover a dial on the side of the device; it seems to let you select a square of **any size**, not just 3x3. Sizes from 1x1 to 300x300 are supported.
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Realizing this, you now must find the **square of any size with the largest total power**. Identify this square by including its size as a third parameter after the top-left coordinate: a 9x9 square with a top-left corner of `3,5` is identified as `3,5,9`.
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For example:
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- For grid serial number `18`, the largest total square (with a total power of `113`) is 16x16 and has a top-left corner of `90,269`, so its identifier is **`90,269,16`**.
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- For grid serial number `42`, the largest total square (with a total power of `119`) is 12x12 and has a top-left corner of `232,251`, so its identifier is **`232,251,12`**.
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**What is the X,Y,size identifier of the square with the largest total power?**
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