1 files changed, 67 insertions, 0 deletions
diff --git a/day20.cc b/day20.cc
new file mode 100644
index 0000000..33c1fb3
--- /dev/null
+++ b/day20.cc
@@ -0,0 +1,67 @@
+#include <algorithm>
+#include <iostream>
+#include <numeric>
+#include <sstream>
+#include <tuple>
+#include <vector>
+
+using namespace std;
+
+struct particle {
+	struct v3 { int x, y, z; };
+	v3 p, v, a;
+	bool operator<(const particle &o) const {
+		return tie(p.x, p.y, p.z) < tie(o.p.x, o.p.y, o.p.z);
+	}
+	int dist() const {
+		return abs(p.x) + abs(p.y) + abs(p.z);
+	}
+	void move() {
+		v.x += a.x; v.y += a.y; v.z += a.z;
+		p.x += v.x; p.y += v.y; p.z += v.z;
+	}
+};
+
+int
+main() {
+	
+	vector<particle> particles;
+
+	for (string line; getline(cin, line); ) {
+		replace_if(begin(line), end(line),
+		    [](char c1) { return !(isdigit(c1) || c1 == '-'); }, ' ');
+
+		particle t;
+		istringstream{line} >> t.p.x >> t.p.y >> t.p.z
+				    >> t.v.x >> t.v.y >> t.v.z
+				    >> t.a.x >> t.a.y >> t.a.z;
+		particles.push_back(t);
+	}
+
+	vector<particle> particles1 = particles;
+	for (auto &p : particles1)
+		for (int i = 0; i < 1000; i++)
+			p.move();
+	auto x = min_element(begin(particles1), end(particles1), 
+	    [](auto& p1, auto& p2) { return p1.dist() < p2.dist(); });
+	cout << distance(begin(particles1), x) << endl;
+
+	vector<particle> new_particles;
+	for (int i = 0; i < 1000; i++) {
+		new_particles.clear();
+	
+		sort(begin(particles), end(particles));
+
+		for (auto i = begin(particles); i != end(particles); ) {
+			auto j = upper_bound(i, end(particles), *i);
+			if (distance(i, j) == 1) {
+				i->move();
+				new_particles.push_back(*i);
+			}
+			i = j;
+		}
+
+		particles.swap(new_particles);
+	}
+	cout << size(particles) << endl;
+}