Implementation day 13

2020/src/common.rs

@@ -40,6 +40,21 @@ pub fn read_char_grid(input: &mut dyn Read) -> Vec<Vec<u8>> {
         .collect()
 }
 
+pub fn numbers_and_stuff<T, C>(input: &mut dyn Read) -> C
+where
+    T: FromStr,
+    C: FromIterator<T>,
+{
+    let mut buffer = String::new();
+
+    input.read_to_string(&mut buffer).unwrap();
+
+    buffer
+        .split(|c: char| !c.is_ascii_digit())
+        .filter_map(|t| t.parse().ok())
+        .collect()
+}
+
 /// An interface to count elements in particular categories.
 pub trait GroupingCount {
     /// The type of the categories under inspection

2020/src/day13.rs

@@ -1,12 +1,69 @@
 use std::io::Read;
 
+use num_integer::Integer;
+
+use crate::common::numbers_and_stuff;
+use crate::common::Lines;
 use crate::Solution;
 
 #[derive(Default)]
 pub struct Day13;
 
 impl Solution for Day13 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let numbers: Vec<u32> = numbers_and_stuff(input);
+
+        let departure = numbers[0];
+
+        let (wait, id) = numbers[1..]
+            .iter()
+            .map(|&interval| (interval - (departure % interval), interval))
+            .min()
+            .unwrap();
+
+        (wait * id).to_string()
+    }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let line = Lines::new(input).nth(1).unwrap();
+
+        let mut period = 1;
+        let mut wait = 0;
+
+        for (offset, freq) in line
+            .split(',')
+            .enumerate()
+            .filter_map(|(i, s)| s.parse().ok().map(|n| (i as u64, n)))
+        {
+            let offset = (freq - (offset % freq)) % freq;
+
+            // Modular integer division does this faster but this works.
+            while wait % freq != offset {
+                wait += period;
+            }
+
+            period = period.lcm(&freq);
+        }
+
+        wait.to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/13.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day13, 1, SAMPLE, 295);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation!(Day13, 2, SAMPLE, 1068781);
     }
 }