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
, orC
. - The second letter is
X
,Y
, orZ
.
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