Add 2019 solutions

2019/AoC.playground/Pages/Day 12.xcplaygroundpage/Contents.swift

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+struct Moon: Hashable {
+    var x: Int
+    var y: Int
+    var z: Int
+
+    var vx = 0
+    var vy = 0
+    var vz = 0
+
+    init(x: Int, y: Int, z: Int) {
+        self.x = x
+        self.y = y
+        self.z = z
+    }
+
+    mutating func move() {
+        x += vx
+        y += vy
+        z += vz
+    }
+
+    mutating func applyGravity(from other: Moon) {
+        vx += grav(x, other.x)
+        vy += grav(y, other.y)
+        vz += grav(z, other.z)
+    }
+
+    var energy: Int {
+        let pot = abs(x) + abs(y) + abs(z)
+        let kin = abs(vx) + abs(vy) + abs(vz)
+        return pot * kin
+    }
+}
+
+func grav(_ a: Int, _ b: Int) -> Int
+{
+    if a < b {
+        return 1
+    } else if a == b {
+        return 0
+    } else {
+        return -1
+    }
+}
+
+
+var moons = [
+    Moon(x: -13, y: 14, z: -7),
+    Moon(x: -18, y: 9, z: 0),
+    Moon(x: 0, y: -3, z: -3),
+    Moon(x: -15, y: 3, z: -13)
+]
+
+@discardableResult
+func step() -> Int {
+    for a in 0..<moons.count {
+        for b in 0..<moons.count where a != b {
+            moons[a].applyGravity(from: moons[b])
+        }
+    }
+
+    var energy = 0
+    for i in 0..<moons.count {
+        moons[i].move()
+        energy += moons[i].energy
+    }
+    return energy
+}
+
+var seen: Set<[Moon]> = []
+
+var steps = 0
+
+repeat {
+    seen.insert(moons)
+    step()
+    steps += 1
+} while !seen.contains(moons)
+