2020: day13: part 1

aoc2020/src/day13.rs

@@ -0,0 +1,64 @@
+use std::fmt::Write;
+
+use aoc::err;
+
+const INPUT: &str = include_str!("../input/day13.txt");
+
+pub fn run() -> aoc::Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> aoc::Result<u64> {
+    let mut lines = input.lines();
+
+    let earliest_timestamp = lines
+        .next()
+        .ok_or_else(|| err!("input was empty"))?
+        .parse::<u64>()
+        .map_err(|e| err!("couldn't parse first line: {}", e))?;
+
+    let bus_ids = lines
+        .next()
+        .ok_or_else(|| err!("no second line"))?
+        .split(',')
+        .filter_map(|num| {
+            if num == "x" {
+                None
+            } else {
+                Some(num.parse::<u64>().map_err(|e| err!("{}", e)))
+            }
+        })
+        .collect::<aoc::Result<Vec<_>>>()?;
+
+    let (bus_id, earliest_departure) = bus_ids
+        .iter()
+        .map(|id| {
+            let next_departure = ((earliest_timestamp / id) * id) + id;
+            (id, next_departure)
+        })
+        .min_by_key(|(_, next_departure)| *next_departure)
+        .unwrap();
+
+    Ok(bus_id * (earliest_departure - earliest_timestamp))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day13_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 295);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 3269);
+    }
+}

aoc2020/src/lib.rs

@@ -10,3 +10,4 @@ pub mod day09;
 pub mod day10;
 pub mod day11;
 pub mod day12;
+pub mod day13;

aoc2020/src/main.rs

@@ -13,6 +13,7 @@ use aoc2020::day09;
 use aoc2020::day10;
 use aoc2020::day11;
 use aoc2020::day12;
+use aoc2020::day13;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -28,6 +29,7 @@ fn main() -> Result<()> {
         day10::run,
         day11::run,
         day12::run,
+        day13::run,
     ];
 
     aoc::run(days)