day_2.rs

///--- Day 2: Rock Paper Scissors ---
///
/// The Elves begin to set up camp on the beach. To decide whose tent gets to be
/// closest to the snack storage, a giant Rock Paper Scissors tournament is
/// already in progress.
///
/// Rock Paper Scissors is a game between two players. Each game contains many
/// rounds; in each round, the players each simultaneously choose one of Rock,
/// Paper, or Scissors using a hand shape. Then, a winner for that round is
/// selected: Rock defeats Scissors, Scissors defeats Paper, and Paper defeats
/// Rock. If both players choose the same shape, the round instead ends in a
/// draw.
///
/// Appreciative of your help yesterday, one Elf gives you an encrypted strategy
/// guide (your puzzle input) that they say will be sure to help you win. "The
/// first column is what your opponent is going to play: A for Rock, B for
/// Paper, and C for Scissors. The second column--" Suddenly, the Elf is called
/// away to help with someone's tent.
///
/// The second column, you reason, must be what you should play in response: X
/// for Rock, Y for Paper, and Z for Scissors. Winning every time would be
/// suspicious, so the responses must have been carefully chosen.
///
/// The winner of the whole tournament is the player with the highest score.
/// Your total score is the sum of your scores for each round. The score for a
/// single round is the score for the shape you selected (1 for Rock, 2 for
/// Paper, and 3 for Scissors) plus the score for the outcome of the round (0 if
/// you lost, 3 if the round was a draw, and 6 if you won).
///
/// Since you can't be sure if the Elf is trying to help you or trick you, you
/// should calculate the score you would get if you were to follow the strategy
/// guide.
///
/// For example, suppose you were given the following strategy guide:
///
/// A Y
/// B X
/// C Z
///
/// This strategy guide predicts and recommends the following:
///
/// In the first round, your opponent will choose Rock (A), and you should
/// choose Paper (Y). This ends in a win for you with a score of 8 (2 because
/// you chose Paper + 6 because you won).
///
/// In the second round, your opponent will choose Paper (B), and you should
/// choose Rock (X). This ends in a loss for you with a score of 1 (1 + 0).
///
/// The third round is a draw with both players choosing Scissors, giving you a
/// score of 3 + 3 = 6.
///
/// In this example, if you were to follow the strategy guide, you would get a
/// total score of 15 (8 + 1 + 6).
///
/// What would your total score be if everything goes exactly according to your
/// strategy guide?
///
/// --- Part Two ---
///
/// The Elf finishes helping with the tent and sneaks back over to you. "Anyway,
/// the second column says how the round needs to end: X means you need to lose,
/// Y means you need to end the round in a draw, and Z means you need to win.
/// Good luck!"
///
/// The total score is still calculated in the same way, but now you need to
/// figure out what shape to choose so the round ends as indicated. The example
/// above now goes like this:
///
/// In the first round, your opponent will choose Rock (A), and you need the
/// round to end in a draw (Y), so you also choose Rock. This gives you a score
/// of 1 + 3 = 4.
///
/// In the second round, your opponent will choose Paper (B), and you choose
/// Rock so you lose (X) with a score of 1 + 0 = 1.
///
/// In the third round, you will defeat your opponent's Scissors with Rock for a
/// score of 1 + 6 = 7.
///
/// Now that you're correctly decrypting the ultra top secret strategy guide,
/// you would get a total score of 12.
///
/// Following the Elf's instructions for the second column, what would your
/// total score be if everything goes exactly according to your strategy guide?

pub fn part_1(inputs: &str) -> u64 {
    let mut total_score = 0;
    for line in inputs.split('\n') {
        if line.is_empty() {
            continue;
        }

        let mut it = line.split(' ');
        let other = it.next().unwrap();
        let me = it.next().unwrap();

        let play_score = match me {
            "X" => 1,
            "Y" => 2,
            "Z" => 3,
            _ => unreachable!(),
        };

        total_score += play_score;

        let round_score = match (other, me) {
            ("A", "X") => 3,
            ("A", "Y") => 6,
            ("A", "Z") => 0,
            ("B", "X") => 0,
            ("B", "Y") => 3,
            ("B", "Z") => 6,
            ("C", "X") => 6,
            ("C", "Y") => 0,
            ("C", "Z") => 3,
            _ => unreachable!(),
        };

        total_score += round_score;
    }
    total_score
}

pub fn part_2(inputs: &str) -> u64 {
    let mut total_score = 0;
    for line in inputs.split('\n') {
        if line.is_empty() {
            continue;
        }

        let mut it = line.split(' ');
        let other = it.next().unwrap();
        let me = it.next().unwrap();

        let play_score = match me {
            "X" => 0,
            "Y" => 3,
            "Z" => 6,
            _ => unreachable!(),
        };

        total_score += play_score;

        let round_score = match (other, me) {
            ("A", "X") => 3,
            ("A", "Y") => 1,
            ("A", "Z") => 2,
            ("B", "X") => 1,
            ("B", "Y") => 2,
            ("B", "Z") => 3,
            ("C", "X") => 2,
            ("C", "Y") => 3,
            ("C", "Z") => 1,
            _ => unreachable!(),
        };

        total_score += round_score;
    }
    total_score
}

#[cfg(test)]
pub mod tests {
    use crate::day_2::{part_1, part_2};

    const INPUTS: &str = r#"A Y
B X
C Z"#;

    #[test]
    pub fn test_day_2_example_part1() {
        assert_eq!(part_1(INPUTS), 15);
    }

    #[test]
    pub fn test_day_2_part1() {
        assert_eq!(part_1(include_str!("input/day_2.txt")), 12855);
    }

    #[test]
    pub fn test_day_2_example_part2() {
        assert_eq!(part_2(INPUTS), 12);
    }

    #[test]
    pub fn test_day_2_part2() {
        assert_eq!(part_2(include_str!("input/day_2.txt")), 13726);
    }
}