Day 6 solutions

2023/data/day6.txt

@@ -0,0 +1,2 @@
+Time:        44     82     69     81
+Distance:   202   1076   1138   1458

2023/src/day6.rs

@@ -0,0 +1,44 @@
+const RACES: [(u64, u64); 4] = [(44, 202), (82, 1076), (69, 1138), (81, 1458)];
+
+pub fn day6a() -> String {
+    let score = RACES
+        .iter()
+        .map(|(time, winning_distance)| optimal_winners(*time, *winning_distance).len())
+        .fold(1, |acc, val| acc * val);
+    format!("{}", score)
+}
+
+pub fn day6b() -> String {
+    format!("{}", optimal_winners(44826981, 202107611381458).len())
+}
+
+fn optimal_winners(total_time: u64, winning_distance: u64) -> Vec<u64> {
+    race_distances(total_time)
+        .filter(|(_, distance)| *distance > winning_distance)
+        .map(|(time, _)| time)
+        .collect::<Vec<u64>>()
+}
+
+fn race_distances(total_time: u64) -> impl Iterator<Item = (u64, u64)> {
+    (0..total_time)
+        .map(move |charge_time| (charge_time, distance(charge_time, total_time - charge_time)))
+}
+
+fn distance(charge_time: u64, remaining_time: u64) -> u64 {
+    charge_time * remaining_time
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_winners() {
+        assert_eq!(optimal_winners(7, 9), vec![2, 3, 4, 5]);
+        assert_eq!(optimal_winners(15, 40), vec![4, 5, 6, 7, 8, 9, 10, 11]);
+        assert_eq!(
+            optimal_winners(30, 200),
+            vec![11, 12, 13, 14, 15, 16, 17, 18, 19]
+        );
+    }
+}

2023/src/main.rs

@@ -3,6 +3,7 @@ mod day2;
 mod day3;
 mod day4;
 mod day5;
+mod day6;
 
 fn main() {
     let day = std::env::args().skip(1).next();
@@ -18,6 +19,8 @@ fn main() {
         Some("4b") => day4::day4b(),
         Some("5a") => day5::day5a(),
         Some("5b") => day5::day5b(),
+        Some("6a") => day6::day6a(),
+        Some("6b") => day6::day6b(),
         _ => panic!("unrecognized day"),
     };