From c568871f4d8e63087a7fa4a3c28fb15e0f30615f Mon Sep 17 00:00:00 2001
From: timeshifter <timeshifter@sumatrae>
Date: Thu, 2 Jul 2026 13:06:20 +0200
Subject: [PATCH] add day 15

---
 day15/Cargo.lock  |  7 ++++
 day15/Cargo.toml  |  6 +++
 day15/example.txt |  2 +
 day15/input.txt   |  6 +++
 day15/src/main.rs | 99 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 120 insertions(+)
 create mode 100644 day15/Cargo.lock
 create mode 100644 day15/Cargo.toml
 create mode 100644 day15/example.txt
 create mode 100644 day15/input.txt
 create mode 100644 day15/src/main.rs

diff --git a/day15/Cargo.lock b/day15/Cargo.lock
new file mode 100644
index 0000000..a6ac8d2
--- /dev/null
+++ b/day15/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day15"
+version = "0.1.0"
diff --git a/day15/Cargo.toml b/day15/Cargo.toml
new file mode 100644
index 0000000..c787d54
--- /dev/null
+++ b/day15/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day15"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/day15/example.txt b/day15/example.txt
new file mode 100644
index 0000000..8a94234
--- /dev/null
+++ b/day15/example.txt
@@ -0,0 +1,2 @@
+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.
diff --git a/day15/input.txt b/day15/input.txt
new file mode 100644
index 0000000..1f25755
--- /dev/null
+++ b/day15/input.txt
@@ -0,0 +1,6 @@
+Disc #1 has 13 positions; at time=0, it is at position 1.
+Disc #2 has 19 positions; at time=0, it is at position 10.
+Disc #3 has 3 positions; at time=0, it is at position 2.
+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 3.
+Disc #6 has 17 positions; at time=0, it is at position 5.
diff --git a/day15/src/main.rs b/day15/src/main.rs
new file mode 100644
index 0000000..739c65f
--- /dev/null
+++ b/day15/src/main.rs
@@ -0,0 +1,99 @@
+#[cfg(debug_assertions)]
+const FILENAME: &str = "example.txt";
+#[cfg(not(debug_assertions))]
+const FILENAME: &str = "input.txt";
+
+fn main() {
+    let mut discs: Vec<_> = std::fs::read_to_string(FILENAME)
+        .unwrap()
+        .lines()
+        .map(Disc::parse)
+        .collect();
+
+    part_1(&discs);
+
+    discs.push(Disc {
+        positions: 11,
+        current_position: 0,
+    });
+
+    part_1(&discs);
+}
+
+fn part_1(discs: &[Disc]) {
+    let mut initial_state = Simulation::new(discs);
+
+    let mut ball_drop_time = 0;
+
+    loop {
+        let mut simulation = initial_state.clone();
+
+        let success = simulation.run();
+        if success {
+            break;
+        }
+        ball_drop_time += 1;
+        initial_state.advance_discs();
+    }
+    println!("{ball_drop_time}")
+}
+
+#[derive(Clone, Debug)]
+struct Simulation {
+    discs: Vec<Disc>,
+}
+
+impl Simulation {
+    fn new(discs: &[Disc]) -> Self {
+        Self {
+            discs: discs.to_vec(),
+        }
+    }
+
+    fn run(&mut self) -> bool {
+        let mut ball = 0;
+        loop {
+            ball += 1;
+            self.advance_discs();
+
+            if let Some(disc) = self.discs.get(ball - 1) {
+                if disc.current_position != 0 {
+                    return false;
+                }
+            } else {
+                return true;
+            }
+        }
+    }
+
+    fn advance_discs(&mut self) {
+        self.discs.iter_mut().for_each(|disc| disc.advance());
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Disc {
+    positions: usize,
+    current_position: usize,
+}
+
+impl Disc {
+    fn parse(s: &str) -> Self {
+        let mut iter = s.split_whitespace();
+        let positions = iter.nth(3).unwrap().parse().unwrap();
+
+        let mut last_iter = iter.next_back().unwrap().chars();
+        last_iter.next_back();
+        let current_position = last_iter.collect::<String>().parse().unwrap();
+
+        Self {
+            positions,
+            current_position,
+        }
+    }
+
+    fn advance(&mut self) {
+        self.current_position += 1;
+        self.current_position %= self.positions;
+    }
+}