This blog post is second in the Advent of Code 2021 series and shows a JavaScript-based solution to the problem described in Day 2: Dive!.

All solutions are in this GitHub repository.

## Solving Part One

The goal is to find the horizontal position and depth of a moving submarine based on a set of given instructions.

According to the problem the initial depth and horizontal positions are `0`

.

```
let depth = 0
let horizontalPosition = 0
```

We can read the set of instructions using the `readInput`

utility created on Day 1. Then we can split the data line by line.

```
const commands = data.split('\n')
```

Each line contains a command type and a number of steps. To use these, we can split the line by the space character, then convert the steps to a number.

```
const [type, stepsAsString] = command.split(' ')
const steps = Number(stepsAsString)
```

Finally, we can calculate the depth and horizontal positions based on the command type and steps to move according to the rules described in the challenge.

```
switch (type) {
case 'forward':
horizontalPosition += steps
break
case 'down':
depth += steps
break
case 'up':
depth -= steps
break
default:
break
}
```

The final solution:

```
import { URL, fileURLToPath } from 'url'
import { readInput } from '../utils/readInput.js'
const solve = (data) => {
const commands = data.split('\n')
let depth = 0
let horizontalPosition = 0
commands.forEach((command) => {
const [type, stepsAsString] = command.split(' ')
const steps = Number(stepsAsString)
switch (type) {
case 'forward':
horizontalPosition += steps
break
case 'down':
depth += steps
break
case 'up':
depth -= steps
break
default:
break
}
})
return depth * horizontalPosition
}
const inputPath = fileURLToPath(new URL('./input.txt', import.meta.url))
const data = await readInput(inputPath)
console.log(solve(data))
```

## Solving Part Two

The second part of the problem requires calculating depth and horizontal position in a slightly different way using new rules and an aim value.

```
let aim = 0
```

The new rules can be written in JavaScript as follows:

```
switch (type) {
case 'forward':
horizontalPosition += steps
depth += aim * steps
break
case 'down':
aim += steps
break
case 'up':
aim -= steps
break
default:
break
}
```

The final solution for part two is on my GitHub repository.

## Conclusion

Day 2 was about calculating values such as horizontal position and depth of a moving submarine based on a set of given commands. I had fun solving the challenge and the utility functions added on Day 1 helped solve the second challenge somewhat faster.