2018 day 9

Author: ictrobot

crates/year2018/src/day09.rs

@@ -0,0 +1,81 @@
+use std::collections::VecDeque;
+use utils::prelude::*;
+
+/// Simulating a marble game.
+#[derive(Clone, Debug)]
+pub struct Day09 {
+    players: u32,
+    marbles: u32,
+}
+
+impl Day09 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let (players, marbles) = parser::number_range(1..=999)
+            .with_suffix(" players; last marble is worth ")
+            .then(parser::number_range(1..=99_999))
+            .with_suffix(" points")
+            .parse_complete(input)?;
+
+        Ok(Self { players, marbles })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        Self::max_score(self.players, self.marbles)
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        Self::max_score(self.players, self.marbles * 100)
+    }
+
+    fn max_score(players: u32, marbles: u32) -> u64 {
+        let mut circle = VecDeque::with_capacity(marbles as usize);
+        circle.push_front(0u32);
+        let mut scores = vec![0u64; players as usize];
+
+        let batches = marbles / 23;
+        for base in (0..23 * batches).step_by(23) {
+            // Equivalent to the following operations, which naively add 23 marbles while keeping
+            // the current marble at the front of dequeue:
+            //  22x [push_back(pop_front), push_back(pop_front), push_front(i)]
+            //   7x [push_front(pop_back)]
+            //      [pop_front]
+            // By eliminating redundant pushes and pops the total number of operations per batch is
+            // decreased from 125 to 67.
+            let front = circle.pop_front().unwrap();
+            circle.push_back(front);
+
+            for i in 1..=18 {
+                let front = circle.pop_front().unwrap();
+                circle.push_back(front);
+                circle.push_back(base + i);
+            }
+
+            let f1 = circle.pop_front().unwrap();
+            let f2 = circle.pop_front().unwrap();
+            let f3 = circle.pop_front().unwrap();
+            let f4 = circle.pop_front().unwrap();
+
+            circle.push_front(base + 22);
+            circle.push_front(f4);
+            circle.push_front(base + 21);
+            circle.push_front(f3);
+            circle.push_front(base + 20);
+            circle.push_front(f2);
+            circle.push_front(base + 19);
+
+            scores[((base + 23) % players) as usize] += (base as u64 + 23) + (f1 as u64);
+        }
+
+        scores.iter().copied().max().unwrap()
+    }
+}
+
+examples!(Day09 -> (u64, u64) [
+    {input: "10 players; last marble is worth 1618 points", part1: 8317},
+    {input: "13 players; last marble is worth 7999 points", part1: 146373},
+    {input: "17 players; last marble is worth 1104 points", part1: 2764},
+    {input: "21 players; last marble is worth 6111 points", part1: 54718},
+    {input: "30 players; last marble is worth 5807 points", part1: 37305},
+]);

crates/year2018/src/lib.rs

@@ -10,4 +10,5 @@ utils::year!(2018 => year2018, ${
     6 => day06::Day06,
     7 => day07::Day07,
     8 => day08::Day08,
+    9 => day09::Day09,
 });