Rust

Part 2 is crazy slow, but it works, so thats cool :D

~~Edit: Gonna fix this, because pt2 is stupid.~~

Much better, 2.4s. Still slow, but not 6 minutes slow.

#[cfg(test)]
mod tests {
    use std::collections::HashMap;
    use std::iter::zip;

    fn step(start: usize) -> usize {
        let mut next = start;
        next = ((next * 64) ^ next) % 16777216;
        next = ((next / 32) ^ next) % 16777216;
        next = ((next * 2048) ^ next) % 16777216;
        next
    }

    fn simulate(initial: usize) -> usize {
        let mut next = initial;
        for _ in 0..2000 {
            next = step(next);
        }
        next
    }
    #[test]
    fn test_step() {
        assert_eq!(15887950, step(123));
    }
    #[test]
    fn test_simulate() {
        assert_eq!(8685429, simulate(1));
    }

    #[test]
    fn day22_part1_test() {
        let input = std::fs::read_to_string("src/input/day_22.txt").unwrap();
        let initial_values = input
            .split("\n")
            .map(|s| s.parse::<usize>().unwrap())
            .collect::<Vec<usize>>();

        let mut total = 0;

        for value in initial_values {
            total += simulate(value);
        }

        println!("{}", total);
    }

    #[test]
    fn day22_part2_test() {
        let input = std::fs::read_to_string("src/input/day_22.txt").unwrap();
        let initial_values = input
            .split("\n")
            .map(|s| s.parse::<usize>().unwrap())
            .collect::<Vec<usize>>();

        let mut all_deltas = vec![];
        let mut all_values = vec![];

        for value in initial_values {
            let mut deltas = String::with_capacity(2000);
            let mut values = vec![];
            let mut prev = value;
            for _ in 0..2000 {
                let next = step(prev);
                values.push(next % 10);
                deltas.push((10u8 + b'A' + ((prev % 10) as u8) - ((next % 10) as u8)) as char);
                prev = next;
            }

            all_deltas.push(deltas);
            all_values.push(values);
        }

        let mut totals = HashMap::with_capacity(100000);

        for (delta, value) in zip(&all_deltas, &all_values) {
            let mut cache = HashMap::with_capacity(2000);
            for j in 0..delta.len() - 4 {
                let seq = &delta[j..j + 4];
                let bananas = value[j + 3];
                cache.entry(seq).or_insert(bananas);
            }
            for (key, value) in cache {
                *totals.entry(key).or_insert(0) += value;
            }
        }

        let max_bananas = totals.values().max().unwrap();

        println!("{}", max_bananas);
    }
}