Advent of Code 2022 Day 2

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.
Afor 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.
Xfor 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()
}