day13 only part 1

input/2020/day13.txt

@@ -0,0 +1,2 @@
+1004345
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,379,x,x,x,x,x,x,x,23,x,x,x,x,13,x,x,x,17,x,x,x,x,x,x,x,x,x,x,x,29,x,557,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19

src/day13.rs

@@ -0,0 +1,42 @@
+use aoc_runner_derive::aoc;
+
+#[aoc(day13, part1)]
+fn solve_d13_p1(input: &str) -> usize {
+    let first_newline = input.find('\n').unwrap();
+    let (t, bus_ids) = input.split_at(first_newline);
+    let bus_ids = bus_ids.trim();
+
+    let t: usize = t.parse().unwrap();
+
+    struct WaitTime {
+        bus_id: usize,
+        wait_time: usize,
+    }
+    let min_wait_time = bus_ids
+        .split(',')
+        .filter(|&x| x != "x")
+        .map(|x| x.parse::<usize>().unwrap())
+        .fold(
+            WaitTime {
+                bus_id: usize::MAX,
+                wait_time: usize::MAX,
+            },
+            |min, bus_id| {
+                let offset = t % bus_id;
+                let wait_time = if offset == 0 {
+                    0
+                } else {
+                    bus_id - offset
+                };
+                if min.wait_time < wait_time {
+                    min
+                } else {
+                    WaitTime{
+                        bus_id,
+                        wait_time,
+                    }
+                }
+            },
+        );
+    min_wait_time.bus_id * min_wait_time.wait_time
+}

src/lib.rs

@@ -2,6 +2,7 @@ pub mod day1;
 pub mod day10;
 pub mod day11;
 pub mod day12;
+pub mod day13;
 pub mod day2;
 pub mod day3;
 pub mod day4;