From 81fbb2694db59f70ed459b785b52deabc20a544a Mon Sep 17 00:00:00 2001
From: "Eric N. Vander Weele" <ericvw@gmail.com>
Date: Sun, 19 May 2024 08:12:48 -0400
Subject: [PATCH] 2019: Day 12

---
 2019/input_12.txt         |   4 +
 2019/src/bin/puzzle_12.rs | 161 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 165 insertions(+)
 create mode 100644 2019/input_12.txt
 create mode 100644 2019/src/bin/puzzle_12.rs

diff --git a/2019/input_12.txt b/2019/input_12.txt
new file mode 100644
index 0000000..b7033b7
--- /dev/null
+++ b/2019/input_12.txt
@@ -0,0 +1,4 @@
+<x=17, y=-7, z=-11>
+<x=1, y=4, z=-1>
+<x=6, y=-2, z=-6>
+<x=19, y=11, z=9>
diff --git a/2019/src/bin/puzzle_12.rs b/2019/src/bin/puzzle_12.rs
new file mode 100644
index 0000000..5facca7
--- /dev/null
+++ b/2019/src/bin/puzzle_12.rs
@@ -0,0 +1,161 @@
+use std::cmp::Ordering;
+use std::io;
+
+use aoc2019::num;
+
+#[derive(Default, Clone, Copy)]
+struct Vec3 {
+    x: i32,
+    y: i32,
+    z: i32,
+}
+
+#[derive(Default, Clone, Copy)]
+struct Moon {
+    position: Vec3,
+    velocity: Vec3,
+}
+
+fn velocity_delta(pos1: i32, pos2: i32) -> i32 {
+    match pos1.cmp(&pos2) {
+        Ordering::Less => 1,
+        Ordering::Equal => 0,
+        Ordering::Greater => -1,
+    }
+}
+
+impl Moon {
+    fn apply_gravity(&mut self, other: &Moon) {
+        self.velocity.x += velocity_delta(self.position.x, other.position.x);
+        self.velocity.y += velocity_delta(self.position.y, other.position.y);
+        self.velocity.z += velocity_delta(self.position.z, other.position.z);
+    }
+
+    fn apply_velocity(&mut self) {
+        self.position.x += self.velocity.x;
+        self.position.y += self.velocity.y;
+        self.position.z += self.velocity.z;
+    }
+
+    fn potential_energy(&self) -> i32 {
+        self.position.x.abs() + self.position.y.abs() + self.position.z.abs()
+    }
+
+    fn kinectic_energy(&self) -> i32 {
+        self.velocity.x.abs() + self.velocity.y.abs() + self.velocity.z.abs()
+    }
+    fn energy(&self) -> i32 {
+        self.potential_energy() * self.kinectic_energy()
+    }
+}
+
+fn step(moons: &mut [Moon], index_pairs: &[(usize, usize)]) {
+    for &(i, j) in index_pairs {
+        let (left, right) = moons.split_at_mut(j);
+        left[i].apply_gravity(&right[0]);
+        right[0].apply_gravity(&left[i]);
+    }
+
+    for moon in moons {
+        moon.apply_velocity();
+    }
+}
+
+fn index_pairs(size: usize) -> Vec<(usize, usize)> {
+    let indices = Vec::from_iter(0..size);
+    let mut result = Vec::new();
+
+    for i in 0..indices.len() - 1 {
+        for j in i + 1..indices.len() {
+            result.push((indices[i], indices[j]));
+        }
+    }
+
+    result
+}
+
+fn simulate_motion(mut moons: Vec<Moon>, steps: usize) -> Vec<Moon> {
+    let index_pairs = index_pairs(moons.len());
+    for _ in 0..steps {
+        step(&mut moons, &index_pairs);
+    }
+    moons
+}
+
+fn total_system_energy(moons: &[Moon]) -> i32 {
+    moons.iter().map(|m| m.energy()).sum()
+}
+
+fn period_step_for_positions(positions: &[i32]) -> u64 {
+    let index_pairs = index_pairs(positions.len());
+
+    let mut positions = positions.to_vec();
+    let mut velocities = vec![0; positions.len()];
+
+    let init_positions = positions.clone();
+    let init_velocities = velocities.clone();
+
+    let mut steps = 0;
+    loop {
+        for &(i, j) in &index_pairs {
+            velocities[i] += velocity_delta(positions[i], positions[j]);
+            velocities[j] += velocity_delta(positions[j], positions[i]);
+        }
+
+        for (i, pos) in positions.iter_mut().enumerate() {
+            *pos += velocities[i];
+        }
+
+        steps += 1;
+        if velocities == init_velocities && positions == init_positions {
+            break;
+        }
+    }
+    steps
+}
+
+fn lcm(a: u64, b: u64) -> u64 {
+    a * b / num::gcd(a.try_into().unwrap(), b.try_into().unwrap())
+}
+
+fn steps_until_repeating_state(moons: &[Moon]) -> u64 {
+    let x_repeat_steps =
+        period_step_for_positions(&moons.iter().map(|m| m.position.x).collect::<Vec<_>>());
+    let y_repeat_steps =
+        period_step_for_positions(&moons.iter().map(|m| m.position.y).collect::<Vec<_>>());
+    let z_repeat_steps =
+        period_step_for_positions(&moons.iter().map(|m| m.position.z).collect::<Vec<_>>());
+    lcm(x_repeat_steps, lcm(y_repeat_steps, z_repeat_steps))
+}
+
+fn main() {
+    let unparsed_moon_positions = io::stdin()
+        .lines()
+        .collect::<Result<Vec<String>, _>>()
+        .unwrap();
+
+    let moons: Vec<_> = unparsed_moon_positions
+        .iter()
+        .map(|s| &s[1..s.len() - 1])
+        .map(|s| {
+            s.split(", ")
+                .map(|s| s[s.find('=').unwrap() + 1..].parse::<i32>().unwrap())
+                .collect::<Vec<_>>()
+        })
+        .map(|coords| Moon {
+            position: Vec3 {
+                x: coords[0],
+                y: coords[1],
+                z: coords[2],
+            },
+            ..Default::default()
+        })
+        .collect();
+
+    println!(
+        "Part 1: {}",
+        total_system_energy(&simulate_motion(moons.clone(), 1000))
+    );
+
+    println!("Part 2: {}", steps_until_repeating_state(&moons));
+}