From b27ede6524ea8794dd8a95213d1861cd9ff2d745 Mon Sep 17 00:00:00 2001
From: Kjetil Orbekk <kjetil.orbekk@gmail.com>
Date: Sun, 15 Dec 2019 08:53:28 -0500
Subject: Moons

---
 advent/Cargo.toml |  11 ++++++
 advent/src/12.rs  | 113 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+)
 create mode 100644 advent/Cargo.toml
 create mode 100644 advent/src/12.rs

diff --git a/advent/Cargo.toml b/advent/Cargo.toml
new file mode 100644
index 0000000..46c4fb0
--- /dev/null
+++ b/advent/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "advent"
+version = "0.1.0"
+authors = ["Kjetil Orbekk <kjetil.orbekk@gmail.com>"]
+edition = "2018"
+
+[[bin]]
+name = "12"
+path = "src/12.rs"
+
+[dependencies]
diff --git a/advent/src/12.rs b/advent/src/12.rs
new file mode 100644
index 0000000..4eefad0
--- /dev/null
+++ b/advent/src/12.rs
@@ -0,0 +1,113 @@
+use std::fmt;
+
+#[derive(Debug)]
+struct Moon {
+    position: (i64, i64, i64),
+    velocity: (i64, i64, i64),
+}
+
+fn sign(x: i64) -> i64 {
+    if x < 0 {
+        -1
+    } else if x > 0 {
+        1
+    } else {
+        0
+    }
+}
+
+impl Moon {
+    fn step(&self, moons: &Vec<Moon>) -> Moon {
+        let (x, y, z) = self.position;
+        let (mut gx, mut gy, mut gz) = self.velocity;
+        for moon in moons {
+            let (ox, oy, oz) = moon.position;
+            gx += sign(ox - x);
+            gy += sign(oy - y);
+            gz += sign(oz - z);
+        }
+        let position = (x + gx, y + gy, z + gz);
+        Moon { position: position, velocity: (gx, gy, gz) }
+    }
+
+    fn energy(&self) -> i64 {
+        let (x, y, z) = self.position;
+        let (p, q, r) = self.velocity;
+        (x.abs() + y.abs() + z.abs()) * (p.abs() + q.abs() + r.abs())
+    }
+}
+
+fn make_moon(x: i64, y: i64, z: i64) -> Moon {
+    Moon { position: (x, y, z), velocity: (0, 0, 0) }
+}
+
+impl fmt::Display for Moon {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let (x, y, z) = self.position;
+        let (p, q, r) = self.velocity;
+        write!(f, "pos=<x={:3}, y={:3}, z={:3}>, vel=<x={:3}, y={:3}, z={:3}>",
+                x, y, z, p, q, r)
+    }
+}
+
+#[derive(Debug)]
+struct Moons(Vec<Moon>);
+
+impl Moons {
+    fn step(&self) -> Moons {
+        Moons(self.0.iter().map(|m| m.step(&self.0)).collect())
+    }
+
+    fn energy(&self) -> i64 {
+        self.0.iter().map(|m| m.energy()).sum()
+    }
+}
+
+impl fmt::Display for Moons {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        for moon in &self.0 {
+            moon.fmt(f)?;
+            write!(f, "\n")?;
+        }
+        Ok(())
+    }
+}
+
+fn run_steps(mut m: Moons, steps: i64, debug: bool) {
+    for i in 0..steps {
+        if debug {
+            println!("After {} steps:", i);
+            println!("{}", m);
+        }
+        m = m.step()
+    }
+    println!("After {} steps:", steps);
+    println!("{}", m);
+    println!("energy={:3}", m.energy())
+}
+
+fn main() {
+    let _ex1 = Moons(vec!(
+        make_moon(-1, 0, 2),
+        make_moon(2, -10, -7),
+        make_moon(4, -8, 8),
+        make_moon(3, 5, -1),
+    ));
+    let _ex2 = Moons(vec!(
+        make_moon(-8, -10, 0),
+        make_moon(5, 5, 10),
+        make_moon(2, -7, 3),
+        make_moon(9, -8, -3),
+        ));
+    run_steps(_ex1, 10, true);
+    println!();
+    run_steps(_ex2, 1000, false);
+}
+
+// After 1000 steps:
+// pos=<x=-26, y= 45, z=-31>, vel=<x=  2, y= 17, z=  6>
+//     pos=<x= 87, y= 23, z=-12>, vel=<x=-14, y=-13, z=  9>
+//     pos=<x=-33, y= -2, z=-24>, vel=<x= -2, y=-15, z= -5>
+//     pos=<x=-20, y=-86, z= 77>, vel=<x= 14, y= 11, z=-10>
+// 
+//     energy=14645
-- 
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