Implement day 15 of 2016.

2016/src/day15.rs

@@ -0,0 +1,69 @@
+use std::io;
+use std::io::prelude::*;
+use regex;
+use common;
+
+#[derive(Default)]
+pub struct Day15 {
+    disks: Vec<(i32, i32)>,
+}
+
+impl Day15 {
+    pub fn new() -> Day15 {
+        Default::default()
+    }
+
+    fn read_disks(&mut self, input: &mut io::Read) {
+        // Note: this implementation assumes the input is sorted.
+        let re = regex::Regex::new(r"Disc #(\d+) has (\d+) positions; at time=0, it is at position (\d+).")
+            .unwrap();
+        let reader = io::BufReader::new(input);
+        for line in reader.lines() {
+            let contents = line.unwrap();
+            let groups = re.captures(&contents).unwrap();
+            let disk_size: i32 = groups.get(2).unwrap().as_str().parse().unwrap();
+            let start_pos: i32 = groups.get(3).unwrap().as_str().parse().unwrap();
+            self.disks.push((disk_size, start_pos));
+        }
+    }
+
+    fn first_pass(&self) -> i32 {
+        let mut to_wait = 0;
+        let mut multiplier = 1;
+        for (i, (size, start)) in self.disks.iter().enumerate() {
+            while (i as i32 + start + to_wait + 1) % size != 0 {
+                to_wait += multiplier;
+            }
+            multiplier = common::lcm(*size, multiplier);
+        }
+        to_wait
+    }
+}
+
+impl common::Solution for Day15 {
+    fn part1(&mut self, input: &mut io::Read) -> String {
+        self.read_disks(input);
+        format!("{}", self.first_pass())
+    }
+
+    fn part2(&mut self, input: &mut io::Read) -> String {
+        self.read_disks(input);
+        self.disks.push((11, 0));
+        format!("{}", self.first_pass())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use common::Solution;
+
+    const SAMPLE: &str = "Disc #1 has 5 positions; at time=0, it is at position 4.\n\
+                          Disc #2 has 2 positions; at time=0, it is at position 1.";
+
+    #[test]
+    fn sample_part1() {
+        let mut instance = Day15::new();
+        assert_eq!("5", instance.part1(&mut SAMPLE.as_bytes()));
+    }
+}