day5 part2

src/day5.rs

@@ -54,3 +54,31 @@ pub fn solve_d5_p1(input: &[u8]) -> SeatId {
         .max()
         .unwrap()
 }
+
+#[aoc(day5, part2)]
+pub fn solve_d5_p2(input: &[u8]) -> SeatId {
+    #[derive(Debug)]
+    struct MinMaxSum {
+        min: u16,
+        max: u16,
+        sum: u32,
+    }
+    let MinMaxSum { min, max, sum } = input
+        .split(|&b| b == b'\n')
+        .map(|i| SeatId::parse(i).unwrap())
+        .fold(
+            MinMaxSum {
+                min: u16::MAX,
+                max: 0,
+                sum: 0,
+            },
+            |MinMaxSum { min, max, sum }, seatid| MinMaxSum {
+                min: std::cmp::min(min, seatid.0),
+                max: std::cmp::max(max, seatid.0),
+                sum: sum + seatid.0 as u32,
+            },
+        );
+    let num_seats = max as u32 - min as u32 + 1;
+    let expected_sum = (max + min) as u32 * num_seats / 2;
+    SeatId((expected_sum - sum) as u16)
+}