2025b_fasttrack_source/Core/Src/benchmarks/basicmath/cubic.c

/* +++Date last modified: 05-Jul-1997 */

/*
**  CUBIC.C - Solve a cubic polynomial
**  public domain by Ross Cottrell
*/

#include <math.h>
#include <stdlib.h>
#include "benchmarks/basicmath/snipmath.h"

void SolveCubic(double a,
                double b,
                double c,
                double d,
                int *solutions,
                double *x)
{
      long double a1 = b / a, a2 = c / a, a3 = d / a;
      long double Q = (a1 * a1 - 3.0 * a2) / 9.0;
      long double R = (2.0 * a1 * a1 * a1 - 9.0 * a1 * a2 + 27.0 * a3) / 54.0;
      double R2_Q3 = R * R - Q * Q * Q;

      double theta;

      if (R2_Q3 <= 0)
      {
            *solutions = 3;
            theta = acos(R / sqrt(Q * Q * Q));
            x[0] = -2.0 * sqrt(Q) * cos(theta / 3.0) - a1 / 3.0;
            x[1] = -2.0 * sqrt(Q) * cos((theta + 2.0 * PI) / 3.0) - a1 / 3.0;
            x[2] = -2.0 * sqrt(Q) * cos((theta + 4.0 * PI) / 3.0) - a1 / 3.0;
      }
      else
      {
            *solutions = 1;
            x[0] = pow(sqrt(R2_Q3) + fabs(R), 1 / 3.0);
            x[0] += Q / x[0];
            x[0] *= (R < 0.0) ? 1 : -1;
            x[0] -= a1 / 3.0;
      }
}

#ifdef TEST

int main(void)
{
      double a1 = 1.0, b1 = -10.5, c1 = 32.0, d1 = -30.0;
      double a2 = 1.0, b2 = -4.5, c2 = 17.0, d2 = -30.0;
      double x[3];
      int solutions;

      SolveCubic(a1, b1, c1, d1, &solutions, x);

      /* should get 3 solutions: 2, 6 & 2.5   */

      SolveCubic(a2, b2, c2, d2, &solutions, x);

      /* should get 1 solution: 2.5           */

      return 0;
}

#endif /* TEST */