RootSolver

package com.hypixel.hytale.server.npc.util;

public class RootSolver {
   public static final double M_PI = 3.141592653589793;
   public static final double EQN_EPS = 1.0E-15;

   public RootSolver() {
   }

   protected static boolean isZero(double x) {
      return x > -1.0E-15 && x < 1.0E-15;
   }

   protected static double cubicRoot(double x) {
      return x > 0.0 ? Math.pow(x, 0.3333333333333333) : (x < 0.0 ? -Math.pow(-x, 0.3333333333333333) : 0.0);
   }

   public static int solveQuadric(double c2, double c1, double c0, double[] results, int resultIndex) {
      double p = c1 / (2.0 * c2);
      double q = c0 / c2;
      double D = p * p - q;
      if (isZero(D)) {
         results[resultIndex] = -p;
         return 1;
      } else if (D < 0.0) {
         return 0;
      } else {
         double sqrt_D = Math.sqrt(D);
         results[resultIndex] = sqrt_D - p;
         results[resultIndex + 1] = -sqrt_D - p;
         return 2;
      }
   }

   public static int solveCubic(double c3, double c2, double c1, double c0, double[] results) {
      double A = c2 / c3;
      double B = c1 / c3;
      double C = c0 / c3;
      double sq_A = A * A;
      double p = 0.3333333333333333 * (-0.3333333333333333 * sq_A + B);
      double q = 0.5 * (0.07407407407407407 * A * sq_A - 0.3333333333333333 * A * B + C);
      double cb_p = p * p * p;
      double D = q * q + cb_p;
      int num;
      if (isZero(D)) {
         if (isZero(q)) {
            results[0] = 0.0;
            num = 1;
         } else {
            double u = cubicRoot(-q);
            results[0] = 2.0 * u;
            results[1] = -u;
            num = 2;
         }
      } else if (D < 0.0) {
         double phi = 0.3333333333333333 * Math.acos(-q / Math.sqrt(-cb_p));
         double t = 2.0 * Math.sqrt(-p);
         results[0] = t * Math.cos(phi);
         results[1] = -t * Math.cos(phi + 1.0471975511965976);
         results[2] = -t * Math.cos(phi - 1.0471975511965976);
         num = 3;
      } else {
         double sqrt_D = Math.sqrt(D);
         double u = cubicRoot(sqrt_D - q);
         double v = -cubicRoot(sqrt_D + q);
         results[0] = u + v;
         num = 1;
      }

      double sub = 0.3333333333333333 * A;

      for (int i = 0; i < num; i++) {
         results[i] -= sub;
      }

      return num;
   }

   public static int solveQuartic(double c4, double c3, double c2, double c1, double c0, double[] results) {
      double A = c3 / c4;
      double B = c2 / c4;
      double C = c1 / c4;
      double D = c0 / c4;
      double sq_A = A * A;
      double p = -0.375 * sq_A + B;
      double q = 0.125 * sq_A * A - 0.5 * A * B + C;
      double r = -0.01171875 * sq_A * sq_A + 0.0625 * sq_A * B - 0.25 * A * C + D;
      int num;
      if (isZero(r)) {
         double coeff2 = 0.0;
         double coeff3 = 1.0;
         num = solveCubic(coeff3, coeff2, p, q, results);
         results[num++] = 0.0;
      } else {
         double coeff0 = 0.5 * r * p - 0.125 * q * q;
         double coeff1 = -r;
         double coeff2 = -0.5 * p;
         double coeff3 = 1.0;
         solveCubic(coeff3, coeff2, coeff1, coeff0, results);
         double z = results[0];
         double u = z * z - r;
         double v = 2.0 * z - p;
         if (isZero(u)) {
            u = 0.0;
         } else {
            if (!(u > 0.0)) {
               return 0;
            }

            u = Math.sqrt(u);
         }

         if (isZero(v)) {
            v = 0.0;
         } else {
            if (!(v > 0.0)) {
               return 0;
            }

            v = Math.sqrt(v);
         }

         coeff0 = z - u;
         coeff1 = q < 0.0 ? -v : v;
         coeff2 = 1.0;
         num = solveQuadric(coeff2, coeff1, coeff0, results, 0);
         coeff0 = z + u;
         coeff1 = q < 0.0 ? v : -v;
         coeff2 = 1.0;
         num += solveQuadric(coeff2, coeff1, coeff0, results, num);
      }

      double sub = 0.25 * A;

      for (int i = 0; i < num; i++) {
         results[i] -= sub;
      }

      return num;
   }
}