From 756f22d58bb198b8f34589c112e1003614ccdcd6 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sun, 2 Nov 2025 21:41:58 +0100
Subject: aoc 2017, days 1-20

---
 2017/src/day20.cpp | 162 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 162 insertions(+)
 create mode 100644 2017/src/day20.cpp

(limited to '2017/src/day20.cpp')

diff --git a/2017/src/day20.cpp b/2017/src/day20.cpp
new file mode 100644
index 0000000..fd09535
--- /dev/null
+++ b/2017/src/day20.cpp
@@ -0,0 +1,162 @@
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+using point_type    = tuple<long, long, long>;
+using particle_type = tuple<point_type, point_type, point_type>;
+
+#if 0
+ostream&
+operator<<(ostream& strm, const point_type& point)
+{
+	const auto [x, y, z] = point;
+	strm << '<' << x << ',' << y << ',' << z << '>';
+	return strm;
+}
+
+ostream&
+operator<<(ostream& strm, const particle_type& particle)
+{
+	const auto [pos, velo, accel] = particle;
+	strm << "p=" << pos << ", v=" << velo << ", a=" << accel;
+	return strm;
+}
+#endif
+
+vector<long>
+split(const string& line, const string& delimiters)
+{
+	vector<long> result;
+
+	size_t start = 0;
+	size_t end   = 0;
+
+	while ( (end = line.find_first_of(delimiters, start)) != string::npos ) {
+		if ( end != start ) {
+			result.emplace_back(stol(line.substr(start, end - start)));
+		}
+
+		start = end + 1;
+	}
+
+	if ( start != line.size() ) {
+		result.emplace_back(stol(line.substr(start)));
+	}
+
+	return result;
+}
+
+vector<particle_type>
+read_file(const filesystem::path& filename)
+{
+	ifstream              file{ filename };
+	vector<particle_type> data;
+
+	for ( string line; getline(file, line); ) {
+		auto parts = split(line, "p=<>,va ");
+
+		point_type position{ parts.at(0), parts.at(1), parts.at(2) };
+		point_type velocity{ parts.at(3), parts.at(4), parts.at(5) };
+		point_type acceleration{ parts.at(6), parts.at(7), parts.at(8) };
+
+		data.emplace_back(position, velocity, acceleration);
+	}
+
+	return data;
+}
+
+point_type
+add(const point_type& lhs, const point_type& rhs)
+{
+	return { get<0>(lhs) + get<0>(rhs), get<1>(lhs) + get<1>(rhs), get<2>(lhs) + get<2>(rhs) };
+}
+
+void
+update(particle_type& particle)
+{
+	auto [pos, vel, acc] = particle;
+
+	vel = add(vel, acc);
+	pos = add(pos, vel);
+
+	particle = { pos, vel, acc };
+}
+
+long
+distance(const point_type& point)
+{
+	return abs(get<0>(point)) + abs(get<1>(point)) + abs(get<2>(point));
+}
+
+void
+part1(vector<particle_type> particles)
+{
+	size_t current_min_index = particles.size();
+
+	for ( int round = 0; round != 10000; ++round ) {
+		// Alle updaten
+		ranges::for_each(particles, update);
+
+		long   step_min_dist  = numeric_limits<long>::max();
+		size_t step_min_index = current_min_index;
+
+		// kleinste Distanz suchen
+		for ( size_t i = 0; i != particles.size(); ++i ) {
+			auto min_dist = distance(get<0>(particles.at(i)));
+
+			if ( min_dist < step_min_dist ) {
+				step_min_dist  = min_dist;
+				step_min_index = i;
+			}
+		}
+
+		// Prüfen, ob stabil
+		if ( step_min_index != current_min_index ) {
+			current_min_index = step_min_index;
+			round             = 0;
+		}
+	}
+
+	cout << "Part1: " << current_min_index << '\n';
+}
+
+void
+part2(vector<particle_type> particles)
+{
+	for ( int round = 0; round != 10000; ++round ) {
+		// Alle updaten
+		ranges::for_each(particles, update);
+
+		map<point_type, size_t> collision_counter;
+
+		for ( const auto& [pos, vel, accl]: particles ) {
+			collision_counter[pos]++;
+		}
+
+		for ( const auto& [pos, count]: collision_counter ) {
+			if ( count > 1 ) {
+				erase_if(particles, [&pos](const auto& particle) { return get<0>(particle) == pos; });
+				round = 0;
+			}
+		}
+	}
+
+	cout << "Part2: " << particles.size() << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto data = read_file("data/day20.txt");
+	part1(data);
+	part2(data);
+}
-- 
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