Average Error: 13.5 → 0.4
Time: 22.1s
Precision: 64
\[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
\[\begin{array}{l} \mathbf{if}\;wj \le 2.7403440668061758 \cdot 10^{-12}:\\ \;\;\;\;\left(wj \cdot wj + \left(\left(wj \cdot wj\right) \cdot \left(wj \cdot wj\right) - \left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \mathbf{else}:\\ \;\;\;\;\frac{\left(wj \cdot wj\right) \cdot wj - \left(\frac{wj}{1 + wj} \cdot \frac{wj}{1 + wj}\right) \cdot \frac{wj}{1 + wj}}{wj \cdot wj + \frac{wj}{1 + wj} \cdot \left(\frac{wj}{1 + wj} + wj\right)} + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \end{array}\]
wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}
\begin{array}{l}
\mathbf{if}\;wj \le 2.7403440668061758 \cdot 10^{-12}:\\
\;\;\;\;\left(wj \cdot wj + \left(\left(wj \cdot wj\right) \cdot \left(wj \cdot wj\right) - \left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\

\mathbf{else}:\\
\;\;\;\;\frac{\left(wj \cdot wj\right) \cdot wj - \left(\frac{wj}{1 + wj} \cdot \frac{wj}{1 + wj}\right) \cdot \frac{wj}{1 + wj}}{wj \cdot wj + \frac{wj}{1 + wj} \cdot \left(\frac{wj}{1 + wj} + wj\right)} + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\

\end{array}
double f(double wj, double x) {
        double r8863416 = wj;
        double r8863417 = exp(r8863416);
        double r8863418 = r8863416 * r8863417;
        double r8863419 = x;
        double r8863420 = r8863418 - r8863419;
        double r8863421 = r8863417 + r8863418;
        double r8863422 = r8863420 / r8863421;
        double r8863423 = r8863416 - r8863422;
        return r8863423;
}

double f(double wj, double x) {
        double r8863424 = wj;
        double r8863425 = 2.7403440668061758e-12;
        bool r8863426 = r8863424 <= r8863425;
        double r8863427 = r8863424 * r8863424;
        double r8863428 = r8863427 * r8863427;
        double r8863429 = r8863427 * r8863424;
        double r8863430 = r8863428 - r8863429;
        double r8863431 = r8863427 + r8863430;
        double r8863432 = x;
        double r8863433 = exp(r8863424);
        double r8863434 = r8863433 * r8863424;
        double r8863435 = r8863434 + r8863433;
        double r8863436 = r8863432 / r8863435;
        double r8863437 = r8863431 + r8863436;
        double r8863438 = 1.0;
        double r8863439 = r8863438 + r8863424;
        double r8863440 = r8863424 / r8863439;
        double r8863441 = r8863440 * r8863440;
        double r8863442 = r8863441 * r8863440;
        double r8863443 = r8863429 - r8863442;
        double r8863444 = r8863440 + r8863424;
        double r8863445 = r8863440 * r8863444;
        double r8863446 = r8863427 + r8863445;
        double r8863447 = r8863443 / r8863446;
        double r8863448 = r8863447 + r8863436;
        double r8863449 = r8863426 ? r8863437 : r8863448;
        return r8863449;
}

Error

Bits error versus wj

Bits error versus x

Try it out

Your Program's Arguments

Results

Enter valid numbers for all inputs

Target

Original13.5
Target12.8
Herbie0.4
\[wj - \left(\frac{wj}{wj + 1} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)\]

Derivation

  1. Split input into 2 regimes
  2. if wj < 2.7403440668061758e-12

    1. Initial program 13.1

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Using strategy rm
    3. Applied div-sub13.1

      \[\leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
    4. Applied associate--r-6.9

      \[\leadsto \color{blue}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}}\]
    5. Taylor expanded around 0 0.2

      \[\leadsto \color{blue}{\left(\left({wj}^{2} + {wj}^{4}\right) - {wj}^{3}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    6. Simplified0.2

      \[\leadsto \color{blue}{\left(\left(\left(wj \cdot wj\right) \cdot \left(wj \cdot wj\right) - \left(wj \cdot wj\right) \cdot wj\right) + wj \cdot wj\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]

    if 2.7403440668061758e-12 < wj

    1. Initial program 23.9

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Using strategy rm
    3. Applied div-sub23.9

      \[\leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
    4. Applied associate--r-23.9

      \[\leadsto \color{blue}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}}\]
    5. Using strategy rm
    6. Applied flip3--23.9

      \[\leadsto \color{blue}{\frac{{wj}^{3} - {\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right)}^{3}}{wj \cdot wj + \left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} \cdot \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} + wj \cdot \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right)}} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    7. Simplified23.8

      \[\leadsto \frac{\color{blue}{\left(wj \cdot wj\right) \cdot wj - \frac{wj}{\frac{wj + 1}{1}} \cdot \left(\frac{wj}{\frac{wj + 1}{1}} \cdot \frac{wj}{\frac{wj + 1}{1}}\right)}}{wj \cdot wj + \left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} \cdot \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} + wj \cdot \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    8. Simplified5.5

      \[\leadsto \frac{\left(wj \cdot wj\right) \cdot wj - \frac{wj}{\frac{wj + 1}{1}} \cdot \left(\frac{wj}{\frac{wj + 1}{1}} \cdot \frac{wj}{\frac{wj + 1}{1}}\right)}{\color{blue}{\frac{wj}{\frac{wj + 1}{1}} \cdot \left(wj + \frac{wj}{\frac{wj + 1}{1}}\right) + wj \cdot wj}} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
  3. Recombined 2 regimes into one program.
  4. Final simplification0.4

    \[\leadsto \begin{array}{l} \mathbf{if}\;wj \le 2.7403440668061758 \cdot 10^{-12}:\\ \;\;\;\;\left(wj \cdot wj + \left(\left(wj \cdot wj\right) \cdot \left(wj \cdot wj\right) - \left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \mathbf{else}:\\ \;\;\;\;\frac{\left(wj \cdot wj\right) \cdot wj - \left(\frac{wj}{1 + wj} \cdot \frac{wj}{1 + wj}\right) \cdot \frac{wj}{1 + wj}}{wj \cdot wj + \frac{wj}{1 + wj} \cdot \left(\frac{wj}{1 + wj} + wj\right)} + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \end{array}\]

Reproduce

herbie shell --seed 2019163 
(FPCore (wj x)
  :name "Jmat.Real.lambertw, newton loop step"

  :herbie-target
  (- wj (- (/ wj (+ wj 1)) (/ x (+ (exp wj) (* wj (exp wj))))))

  (- wj (/ (- (* wj (exp wj)) x) (+ (exp wj) (* wj (exp wj))))))