1 files changed, 61 insertions, 0 deletions
diff --git a/reference/C/CONTRIB/SNIP/cubic.c b/reference/C/CONTRIB/SNIP/cubic.c
new file mode 100755
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+++ b/reference/C/CONTRIB/SNIP/cubic.c
@@ -0,0 +1,61 @@
+/*
+**  CUBIC.C - Solve a cubic polynomial
+**  public domain by Ross Cottrell
+*/
+
+#include <math.h>
+#include <stdlib.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*M_PI)/3.0) - a1/3.0;
+            x[2] = -2.0*sqrt(Q)*cos((theta+4.0*M_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 */