day23 part 2

input/2020/day23.txt

@@ -0,0 +1 @@
+459672813

src/day23.rs

@@ -0,0 +1,86 @@
+use aoc_runner_derive::aoc;
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+struct Cup(u32);
+impl Cup {
+    fn as_idx(self) -> usize {
+        self.0 as usize - 1
+    }
+
+    fn from_idx(idx: usize) -> Self {
+        Cup(idx as u32 + 1)
+    }
+}
+
+#[derive(Debug)]
+struct Cups {
+    current: Cup,
+    cups: Box<[Cup]>,
+}
+
+impl Cups {
+    fn parse(input: &[u8], total_len: usize) -> Option<Self> {
+        let mut cups = vec![Cup(0); total_len];
+        let input_iter = input.iter().map(|x| (x - b'0') as u32).chain(input.len() as u32 + 1 ..= total_len as u32);
+        for (curr, next) in input_iter.clone().zip(input_iter.cycle().skip(1)) {
+            cups[curr as usize - 1] = Cup(next as u32);
+        }
+
+        Some(Cups {
+            current: Cup((input[0] - b'0') as u32),
+            cups: cups.into_boxed_slice(),
+        })
+    }
+
+    fn do_move(&mut self) {
+        let taken_1 = self.next_cup(self.current);
+        let taken_2 = self.next_cup(taken_1);
+        let taken_3 = self.next_cup(taken_2);
+
+        let mut dest_cup =
+            Cup::from_idx((self.current.as_idx() + self.cups.len() - 1) % self.cups.len());
+        Cup(((self.current.0 as usize - 1 + self.cups.len() - 1) % self.cups.len()) as u32 + 1);
+        while dest_cup == taken_1 || dest_cup == taken_2 || dest_cup == taken_3 {
+            dest_cup = Cup::from_idx((dest_cup.as_idx() + self.cups.len() - 1) % self.cups.len());
+        }
+        self.cups[self.current.as_idx()] = self.next_cup(taken_3);
+        self.cups[taken_3.as_idx()] = self.next_cup(dest_cup);
+        self.cups[dest_cup.as_idx()] = taken_1;
+        self.current = self.next_cup(self.current);
+    }
+
+    fn next_cup(&self, cup: Cup) -> Cup {
+        self.cups[cup.as_idx()]
+    }
+
+    fn iter(&self, starting_cup: Cup) -> impl Iterator<Item = Cup> + '_ {
+        std::iter::successors(Some(starting_cup), move |&prev_cup| {
+            Some(self.next_cup(prev_cup))
+        })
+    }
+}
+
+#[aoc(day23, part1)]
+fn solve_d23_p1(input: &[u8]) -> usize {
+    let mut cups = Cups::parse(input, input.len()).unwrap();
+    for _ in 0..100 {
+        cups.do_move();
+    }
+    cups.iter(Cup(1))
+        .skip(1)
+        .take_while(|&cup| cup != Cup(1))
+        .fold(0, |accum, cup| accum * 10 + cup.0 as usize)
+}
+
+#[aoc(day23, part2)]
+fn solve_d23_p2(input: &[u8]) -> usize {
+    let mut cups = Cups::parse(input, 1_000_000).unwrap();
+    for _ in 0..10_000_000 {
+        cups.do_move();
+    }
+    cups.iter(Cup(1))
+        .skip(1)
+        .take(2)
+        .map(|x| x.0 as usize)
+        .product()
+}

src/lib.rs

@@ -13,6 +13,7 @@ pub mod day2;
 pub mod day20;
 pub mod day21;
 pub mod day22;
+pub mod day23;
 pub mod day3;
 pub mod day4;
 pub mod day5;