content: import java.util.UUID;
  
  public final class NBodyProblem {
      static final class Body {
          public double x, y, z, vx, vy, vz, mass;
  
          Body(double x, double y, double z,
               double vx, double vy, double vz, double mass) {
  
              this.x = x;
              this.y = y;
              this.z = z;
              this.vx = vx;
              this.vy = vy;
              this.vz = vz;
              this.mass = mass;
          }
      }
  
      static boolean roughlyEquals(double a, double b, double precision) {
          return Math.abs(a - b) < precision;
      }
  
      static final double PI = 3.141592653589793;
      static final double SOLAR_MASS = 4 * PI * PI;
      static final double DAYS_PER_YEAR = 365.24;
  
      static void offsetMomentum(Body[] bodies) {
          double px = 0.0;
          double py = 0.0;
          double pz = 0.0;
          Body b;
          for (int i=0; i < bodies.length; i++) {
              b = bodies[i];
              px += b.vx * b.mass;
              py += b.vy * b.mass;
              pz += b.vz * b.mass;
          }
          b = bodies[0];
          b.vx = -px / SOLAR_MASS;
          b.vy = -py / SOLAR_MASS;
          b.vz = -pz / SOLAR_MASS;
      }
  
      static double energy(Body[] bodies) {
          double e = 0.0;
          int numBodies = bodies.length;
          for (int i=0; i < numBodies; i++) {
              Body b = bodies[i];
              double sq = b.vx * b.vx + b.vy * b.vy + b.vz * b.vz;
              e += 0.5 * bodies[i].mass * sq;
              for (int j=i+1; j < numBodies; ++j) {
                  double dx = b.x - bodies[j].x;
                  double dy = b.y - bodies[j].y;
                  double dz = b.z - bodies[j].z;
                  double dsq = dx * dx + dy * dy + dz * dz;
                  e -= (b.mass * bodies[j].mass) / Math.sqrt(dsq);
              }
          }
          return e;
      }
  
      static void advance(Body[] bodies, double dt) {
          int numBodies = bodies.length;
              for (int i=0; i < numBodies; i++) {
                  for (int j=i+1; j < numBodies; ++j) {
                  double dx = bodies[i].x - bodies[j].x;
                  double dy = bodies[i].y - bodies[j].y;
                  double dz = bodies[i].z - bodies[j].z;
                  double dsq = dx*dx + dy*dy + dz*dz;
                  double mag = dt / (dsq * Math.sqrt(dsq));
  
                  double mj = bodies[j].mass * mag;
                  bodies[i].vx -= dx * mj;
                  bodies[i].vy -= dy * mj;
                  bodies[i].vz -= dz * mj;
  
                  double mi = bodies[i].mass * mag;
                  bodies[j].vx += dx * mi;
                  bodies[j].vy += dy * mi;
                  bodies[j].vz += dz * mi;
              }
          }
  
          for (int i=0; i < numBodies; i++) {
              bodies[i].x += bodies[i].vx * dt;
              bodies[i].y += bodies[i].vy * dt;
              bodies[i].z += bodies[i].vz * dt;
          }
      }
  
      public static void main(String[] args) {
          // int n = Integer.parseInt(args[0]);
          // int n = 50_000_000; // num simulation steps in the original program
          int n = 1_000_000;
  
          long time = System.currentTimeMillis();
  
          var bodies = new Body[] {
              // sun
              new Body(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, SOLAR_MASS),
              // jupiter
              new Body(
                  4.84143144246472090e+00,
                  -1.16032004402742839e+00,
                  -1.03622044471123109e-01,
                  1.66007664274403694e-03 * DAYS_PER_YEAR,
                  7.69901118419740425e-03 * DAYS_PER_YEAR,
                  -6.90460016972063023e-05 * DAYS_PER_YEAR,
                  9.54791938424326609e-04 * SOLAR_MASS
              ),
              new Body(
                  // saturn
                  8.34336671824457987e+00,
                  4.12479856412430479e+00,
                  -4.03523417114321381e-01,
                  -2.76742510726862411e-03 * DAYS_PER_YEAR,
                  4.99852801234917238e-03 * DAYS_PER_YEAR,
                  2.30417297573763929e-05 * DAYS_PER_YEAR,
                  2.85885980666130812e-04 * SOLAR_MASS
              ),
              new Body(
                  // uranus
                  1.28943695621391310e+01,
                  -1.51111514016986312e+01,
                  -2.23307578892655734e-01,
                  2.96460137564761618e-03 * DAYS_PER_YEAR,
                  2.37847173959480950e-03 * DAYS_PER_YEAR,
                  -2.96589568540237556e-05 * DAYS_PER_YEAR,
                  4.36624404335156298e-05 * SOLAR_MASS
              ),
              new Body(
                  // neptune
                  1.53796971148509165e+01,
                  -2.59193146099879641e+01,
                  1.79258772950371181e-01,
                  2.68067772490389322e-03 * DAYS_PER_YEAR,
                  1.62824170038242295e-03 * DAYS_PER_YEAR,
                  -9.51592254519715870e-05 * DAYS_PER_YEAR,
                  5.15138902046611451e-05 * SOLAR_MASS
              )
          };
  
          offsetMomentum(bodies);
          System.out.println("Running simulation...");
          double energyBefore = energy(bodies);
          System.out.println("Initial energy: " + energyBefore);
          System.out.println("Roughly equals: " + roughlyEquals(energyBefore, -0.169075164, 1e-3));
  
          for (int i=0; i<n; i++) {
              advance(bodies, 0.01);
          }
  
          double energyAfter = energy(bodies);
          System.out.println("Roughly equals: " + roughlyEquals(energyAfter, -0.169059907, 1e-3));
          System.out.println("Final energy: " + energyAfter);
          System.out.println("Energy change minimal: " + roughlyEquals(energyBefore, energyAfter, 1e-3));
          System.out.println("Sun final position: " + bodies[0].x + " " + bodies[0].y + " " + bodies[0].z);
  
          System.out.println("Simulation took: " + (System.currentTimeMillis() - time) + "ms");
      }
  
      public static String getString() {
          return "Hello, World!";
      }
  
      public static void printString(String s) {
          System.out.println(s);
      }
  
      public static String randomUUID() {
          return UUID.randomUUID().toString();
      }
  
      public static String[] exampleArray() {
          return new String[] { "Hello", "World", "Lol", null };
      }
  }
  