Cannot sample enough valid points. (more)

\[-\frac{1}{\frac{\pi}{4}} \cdot \log \left(\frac{e^{\frac{\pi}{4} \cdot f} + e^{-\frac{\pi}{4} \cdot f}}{e^{\frac{\pi}{4} \cdot f} - e^{-\frac{\pi}{4} \cdot f}}\right)\]
-\frac{1}{\frac{\pi}{4}} \cdot \log \left(\frac{e^{\frac{\pi}{4} \cdot f} + e^{-\frac{\pi}{4} \cdot f}}{e^{\frac{\pi}{4} \cdot f} - e^{-\frac{\pi}{4} \cdot f}}\right)
double code(double f) {
	return ((double) -(((double) (((double) (1.0 / ((double) (((double) M_PI) / 4.0)))) * ((double) log(((double) (((double) (((double) exp(((double) (((double) (((double) M_PI) / 4.0)) * f)))) + ((double) exp(((double) -(((double) (((double) (((double) M_PI) / 4.0)) * f)))))))) / ((double) (((double) exp(((double) (((double) (((double) M_PI) / 4.0)) * f)))) - ((double) exp(((double) -(((double) (((double) (((double) M_PI) / 4.0)) * f))))))))))))))));
}

Reproduce

herbie shell --seed 2020113 +o rules:numerics
(FPCore (f)
  :name "VandenBroeck and Keller, Equation (20)"
  :precision binary64
  (- (* (/ 1 (/ PI 4)) (log (/ (+ (exp (* (/ PI 4) f)) (exp (- (* (/ PI 4) f)))) (- (exp (* (/ PI 4) f)) (exp (- (* (/ PI 4) f)))))))))