3 files changed, 83 insertions, 0 deletions
diff --git a/d15/Cargo.toml b/d15/Cargo.toml
new file mode 100644
index 0000000..01030d9
--- /dev/null
+++ b/d15/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "d15"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/d15/input.txt b/d15/input.txt
new file mode 100644
index 0000000..396cee2
--- /dev/null
+++ b/d15/input.txt
@@ -0,0 +1,6 @@
+Disc #1 has 13 positions; at time=0, it is at position 10.
+Disc #2 has 17 positions; at time=0, it is at position 15.
+Disc #3 has 19 positions; at time=0, it is at position 17.
+Disc #4 has 7 positions; at time=0, it is at position 1.
+Disc #5 has 5 positions; at time=0, it is at position 0.
+Disc #6 has 3 positions; at time=0, it is at position 1.
diff --git a/d15/src/main.rs b/d15/src/main.rs
new file mode 100644
index 0000000..be1ce8f
--- /dev/null
+++ b/d15/src/main.rs
@@ -0,0 +1,71 @@
+use std::fs;
+
+#[derive(Clone, Debug)]
+struct Disc {
+    pos: usize,
+    period: usize,
+}
+
+fn parse(lines: Vec<&str>) -> Vec<Disc> {
+    let mut result = Vec::new();
+    // Add a dummy 0th disc for slightly cleaner code later
+    result.push(Disc { pos: 0, period: 1 });
+
+    for line in lines {
+        let words: Vec<&str> = line.split_ascii_whitespace().collect();
+        result.push(Disc {
+            pos: words[11].trim_end_matches('.').parse().unwrap(),
+            period: words[3].parse().unwrap(),
+        });
+    }
+
+    result
+}
+
+fn solve_puzzle(discs: &Vec<Disc>) -> usize {
+    let mut t_start = 0;
+
+    'outer: loop {
+        let mut dt = 1;
+        while dt < discs.len() {
+            // Calculate the disc's current position. If 0, the capsule
+            // can continue, otherwise this `t_start' must be rejected.
+            if (discs[dt].pos + t_start + dt) % discs[dt].period != 0 {
+                t_start += 1;
+                continue 'outer;
+            }
+            dt += 1;
+        }
+
+        return t_start;
+    }
+}
+
+fn main() {
+    // Read disc stats from input text file
+    let input = fs::read_to_string("input.txt").unwrap();
+    let lines = input.lines().collect();
+    let mut discs = parse(lines);
+
+    // Part 1 gives 203660 for me
+    println!("Part 1 solution: {}", solve_puzzle(&discs));
+
+    discs.push(Disc { pos: 0, period: 11 });
+    // Part 2 gives 2408135 for me
+    println!("Part 2 solution: {}", solve_puzzle(&discs));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let lines = vec![
+            "Disc #1 has 5 positions; at time=0, it is at position 4.",
+            "Disc #2 has 2 positions; at time=0, it is at position 1.",
+        ];
+        let discs = parse(lines);
+        assert_eq!(solve_puzzle(&discs), 5);
+    }
+}