## Day 2: Rock Paper Scissors

https://adventofcode.com/2022/day/2

TL;DR: my solution in Rust

We're playing a Rock, Paper, Scissors tournament with the elves.

The input represents an encryped strategy guide.

Each line has 2 letters seperated by a space.

- The first letter is
`A`

,`B`

, or`C`

. - The second letter is
`X`

,`Y`

, or`Z`

.

An example input looks like this:

```
A Y
B X
C Z
```

Each round is worth some points, all scores get summed up, and whoever has the highest total at the end of the tournament wins.

Your score for a single round is the sum of the score for the *shape* you played, and the score for the *outcome* of the round.

Shape scores:

- Rock: 1
- Paper: 2
- Scissors: 3

Outcome scores:

- Loss: 0
- Draw: 3
- Win: 6

The first letter in the input is what your opponent is going to play.

`A`

for Rock, `B`

for Paper, and `C`

for Scissors.

Before the elf can tell you what the second letter means, they leave.

## Part 1

Winning every round would be suspicious, so whatever that second letter is, it has to be important.

You assume the second letter is the shape you should play.

`X`

for Rock, `Y`

for Paper, and `Z`

for Scissors.

If "Rock", "Paper", and "Scissors" have positions in a list.

- To win, move 1 position to the right (and wrap around from "Scissors" to "Rock"!)
- To draw, keep the same position
- To lose, move 1 position to the left (and wrap around from "Rock" to "Scissors"!)

So I translated both `A`

, `B`

, `C`

, and `X`

, `Y`

, `Z`

to `0`

, `1`

, and `2`

respectively.

Thankfully, both ABC and XYZ are sequences where the ASCII value of a letter increases by 1 each step.

- The value for the
*shape*the opponent plays is known. - The value for the
*shape*I play is known. - To calculate the score for this round, we need to know the
*outcome*of the round.

With those two pieces of information a game of Rock, Paper, Scissors can be expressed as the following equation:

`outcome = my_shape - opponent_shape + 1 (mod 3)`

This expresses `outcome`

as a number from 0 to 2:

- 0 for loss
- 1 for draw
- 2 for win

The `mod 3`

handles the wrapping around logic.

```
pub fn part_1() -> String {
let input = std::fs::read_to_string("src/day02.txt").unwrap();
input
.lines()
// map every line to the score for that round
.map(|line| {
// transform A B C and X Y Z to 0 1 2 respectively by using their ASCII order
let bytes = line.as_bytes();
let left = (bytes[0] - b'A') as i8;
let right = (bytes[2] - b'X') as i8;
// 0 for rock, 1 for paper, 2 for scissors
// 0 for loss, 1 for draw, 2 for win
let opponent_shape = left;
let my_shape = right;
let outcome = (my_shape - opponent_shape + 1).rem_euclid(3);
let shape_score = my_shape + 1;
let outcome_score = 3 * outcome;
(shape_score + outcome_score) as u32
})
.sum::<u32>()
.to_string()
}
```

## Part 2

The elf returns and tells you that the second letter in the input is the desired *outcome*.

`X`

for loss, `Y`

for draw, `Z`

for win.

- The value for the
*shape*the opponent plays is known. - The value for the
*outcome*of the round is known. - To calculate the score for this round, we need to know the
*shape*we need to play.

We rearrange the equation from part1 to solve for `my_shape`

instead of `outcome`

:

`my_shape = opponent_shape - 1 + outcome (mod 3)`

The `mod 3`

handles the wrapping around logic.

```
pub fn part_2() -> String {
let input = std::fs::read_to_string("src/day02.txt").unwrap();
input
.lines()
// map every line to the score for that round
.map(|line| {
// transform A B C and X Y Z to 0 1 2 respectively by using their ASCII order
let bytes = line.as_bytes();
let left = (bytes[0] - b'A') as i8;
let right = (bytes[2] - b'X') as i8;
// 0 for rock, 1 for paper, 2 for scissors
// 0 for loss, 1 for draw, 2 for win
let opponent_shape = left;
let outcome = right;
let my_shape = (opponent_shape - 1 + outcome).rem_euclid(3);
let shape_score = my_shape + 1;
let outcome_score = 3 * outcome;
(shape_score + outcome_score) as u32
})
.sum::<u32>()
.to_string()
}
```

## Top comments (3)

Awesome Article!!

I also tried doing these challenges in Rust

What's your opinion on this => github.com/Liftoff-Studios/AdventO...

I'd love feedback on mine (although mine is beginner-level code)

Thank you!

Awesome, Advent of Code is great to learn new patterns and ways of doing things in a different language.

Have fun!

Had a quick look and I can only think of minor things for different ways to do the same thing in Rust.

For example in github.com/Liftoff-Studios/AdventO... you could also loop over a range instead of an array.

It doesn't matter for such a small loop, both ways are perfectly fine.

with the array:

with an inclusive range:

Ah, thank you so much 'bout that tip. I had totally forgotten about using ranges lol

I'll use it from next time :D