day(20): add solution

Signed-off-by: Matej Focko <me@mfocko.xyz>

samples/day20.txt

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+1
+2
+-3
+3
+-2
+0
+4

src/bin/day20.rs

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+use aoc_2022::*;
+
+type Input = Vec<Coordinate>;
+type Output = i64;
+
+#[derive(Debug, Clone, PartialEq)]
+struct Coordinate {
+    key: i64,
+    index: usize,
+    swapped: bool,
+}
+
+impl Coordinate {
+    fn new(key: i64, index: usize) -> Self {
+        Self {
+            key,
+            index,
+            swapped: key == 0,
+        }
+    }
+
+    fn reset(&self) -> Self {
+        Self {
+            swapped: self.key == 0,
+            ..*self
+        }
+    }
+}
+
+fn index_of<T: PartialEq>(vector: &[T], key: &T) -> Option<usize> {
+    vector
+        .iter()
+        .enumerate()
+        .find(|&(_, val)| key == val)
+        .map(|found| found.0)
+}
+
+fn mix(order: &[Coordinate], mut coordinates: Input) -> Input {
+    let length: i64 = coordinates.len().try_into().unwrap();
+
+    for coordinate in order {
+        if coordinate.swapped {
+            continue;
+        }
+
+        let old_idx = index_of(&coordinates, coordinate).unwrap();
+
+        let mut new_idx = (old_idx as i64 + coordinate.key) % (length - 1);
+        if new_idx < 0 {
+            new_idx += length - 1;
+        }
+
+        let mut c = coordinates.remove(old_idx);
+        c.swapped = true;
+        coordinates.insert(new_idx as usize, c);
+    }
+
+    coordinates.iter().map(Coordinate::reset).collect_vec()
+}
+
+fn find_sum(mixed: &[Coordinate]) -> Output {
+    let mixed = mixed.iter().map(|c| c.key).collect_vec();
+    let base_index = index_of(&mixed, &0).unwrap();
+
+    vec![1000, 2000, 3000]
+        .iter()
+        .map(|off| mixed[(base_index + off) % mixed.len()])
+        .sum()
+}
+
+struct Day20;
+impl Solution<Input, Output> for Day20 {
+    fn parse_input<P: AsRef<Path>>(pathname: P) -> Input {
+        let coordinates: Vec<i64> = file_to_structs(pathname);
+
+        coordinates
+            .iter()
+            .enumerate()
+            .map(|(i, key)| Coordinate::new(*key, i))
+            .collect_vec()
+    }
+
+    fn part_1(input: &Input) -> Output {
+        let order = input.clone();
+        let mixed = mix(&order, input.clone());
+
+        find_sum(&mixed)
+    }
+
+    fn part_2(input: &Input) -> Output {
+        let decryption_key = 811589153;
+        let with_decryption = input
+            .iter()
+            .cloned()
+            .map(|c| Coordinate::new(c.key * decryption_key, c.index))
+            .collect_vec();
+
+        let order = with_decryption.clone();
+        let mixed = (0..10).fold(with_decryption, |acc, _| mix(&order, acc));
+
+        find_sum(&mixed)
+    }
+}
+
+fn main() -> Result<()> {
+    // Day20::run("sample")
+    Day20::main()
+}
+
+test_sample!(day_20, Day20, 3, 1623178306);