Implement day 9 part 1.

2018/inputs/09.txt

@@ -0,0 +1 @@
+419 players; last marble is worth 72164 points

2018/src/day09.rs

@@ -0,0 +1,80 @@
+use common::Solution;
+use std::io::Read;
+
+fn winning_marbles(elves: usize, marbles: usize) -> usize {
+    let mut scores = vec![0usize; elves];
+    let mut state = Vec::new();
+    state.push(0);
+    let mut current = 0;
+
+    for marble in 1..=marbles {
+        if marble % 23 == 0 {
+            let player = marble % elves;
+            for _ in 0..7 {}
+            let to_remove = (current + state.len() - 7) % state.len();
+
+            scores[player] += marble + state[to_remove];
+
+            state.remove(to_remove);
+            current = to_remove;
+        } else {
+            let after = (current + 1) % state.len();
+            if after == state.len() - 1 {
+                state.push(marble);
+            } else {
+                state.insert(after + 1, marble);
+            }
+            current = after + 1;
+        }
+    }
+
+    *scores.iter().max().unwrap()
+}
+
+#[derive(Default)]
+pub struct Day09 {}
+
+impl Day09 {
+    pub fn new() -> Self {
+        Default::default()
+    }
+
+    fn read_input(input: &mut Read) -> (usize, usize) {
+        let mut data = String::new();
+        input.read_to_string(&mut data).unwrap();
+        let mut parts = data.split(" ");
+        let elves = parts.next().unwrap().parse().unwrap();
+        let marbles = parts.skip(5).next().unwrap().parse().unwrap();
+
+        (elves, marbles)
+    }
+}
+
+impl Solution for Day09 {
+    fn part1(&mut self, input: &mut Read) -> String {
+        let (elves, marbles) = Day09::read_input(input);
+
+        format!("{}", winning_marbles(elves, marbles))
+    }
+
+    fn part2(&mut self, input: &mut Read) -> String {
+        let (elves, marbles) = Day09::read_input(input);
+
+        format!("{}", winning_marbles(elves, marbles * 100))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_winning_marbles() {
+        assert_eq!(32, winning_marbles(9, 25));
+        assert_eq!(8317, winning_marbles(10, 1618));
+        assert_eq!(146373, winning_marbles(13, 7999));
+        assert_eq!(2764, winning_marbles(17, 1104));
+        assert_eq!(54718, winning_marbles(21, 6111));
+        assert_eq!(37305, winning_marbles(30, 5807));
+    }
+}

2018/src/lib.rs

@@ -11,3 +11,4 @@ pub mod day05;
 pub mod day06;
 pub mod day07;
 pub mod day08;
+pub mod day09;

2018/src/main.rs

@@ -20,6 +20,7 @@ fn get_impl(day: u32) -> Box<common::Solution> {
         6 => Box::new(day06::Day06::new()),
         7 => Box::new(day07::Day07::new()),
         8 => Box::new(day08::Day08::new()),
+        9 => Box::new(day09::Day09::new()),
         val => panic!("Unimplemented day {}", val),
     }
 }