Average Error: 13.3 → 0.4
Time: 24.0s
Precision: 64
\[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
\[\begin{array}{l} \mathbf{if}\;wj \le 6.715218772498828 \cdot 10^{-07}:\\ \;\;\;\;\mathsf{fma}\left(wj \cdot wj, wj \cdot wj, \mathsf{fma}\left(wj, wj, -\left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \mathbf{else}:\\ \;\;\;\;\frac{x}{e^{wj} \cdot wj + e^{wj}} + \left(wj - \frac{wj}{1 + wj}\right)\\ \end{array}\]
wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}
\begin{array}{l}
\mathbf{if}\;wj \le 6.715218772498828 \cdot 10^{-07}:\\
\;\;\;\;\mathsf{fma}\left(wj \cdot wj, wj \cdot wj, \mathsf{fma}\left(wj, wj, -\left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\

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

\end{array}
double f(double wj, double x) {
        double r7625989 = wj;
        double r7625990 = exp(r7625989);
        double r7625991 = r7625989 * r7625990;
        double r7625992 = x;
        double r7625993 = r7625991 - r7625992;
        double r7625994 = r7625990 + r7625991;
        double r7625995 = r7625993 / r7625994;
        double r7625996 = r7625989 - r7625995;
        return r7625996;
}

double f(double wj, double x) {
        double r7625997 = wj;
        double r7625998 = 6.715218772498828e-07;
        bool r7625999 = r7625997 <= r7625998;
        double r7626000 = r7625997 * r7625997;
        double r7626001 = r7626000 * r7625997;
        double r7626002 = -r7626001;
        double r7626003 = fma(r7625997, r7625997, r7626002);
        double r7626004 = fma(r7626000, r7626000, r7626003);
        double r7626005 = x;
        double r7626006 = exp(r7625997);
        double r7626007 = r7626006 * r7625997;
        double r7626008 = r7626007 + r7626006;
        double r7626009 = r7626005 / r7626008;
        double r7626010 = r7626004 + r7626009;
        double r7626011 = 1.0;
        double r7626012 = r7626011 + r7625997;
        double r7626013 = r7625997 / r7626012;
        double r7626014 = r7625997 - r7626013;
        double r7626015 = r7626009 + r7626014;
        double r7626016 = r7625999 ? r7626010 : r7626015;
        return r7626016;
}

Error

Bits error versus wj

Bits error versus x

Target

Original13.3
Target12.7
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 < 6.715218772498828e-07

    1. Initial program 13.0

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Using strategy rm
    3. Applied div-sub13.0

      \[\leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
    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 0.3

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

      \[\leadsto \color{blue}{\mathsf{fma}\left(wj \cdot wj, wj \cdot wj, wj \cdot wj - wj \cdot \left(wj \cdot wj\right)\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    7. Using strategy rm
    8. Applied fma-neg0.3

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

    if 6.715218772498828e-07 < wj

    1. Initial program 26.6

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Using strategy rm
    3. Applied div-sub26.6

      \[\leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
    4. Applied associate--r-26.6

      \[\leadsto \color{blue}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}}\]
    5. Using strategy rm
    6. Applied *-un-lft-identity26.6

      \[\leadsto \left(wj - \frac{wj \cdot e^{wj}}{\color{blue}{1 \cdot e^{wj}} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    7. Applied distribute-rgt-out26.7

      \[\leadsto \left(wj - \frac{wj \cdot e^{wj}}{\color{blue}{e^{wj} \cdot \left(1 + wj\right)}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    8. Applied associate-/r*26.4

      \[\leadsto \left(wj - \color{blue}{\frac{\frac{wj \cdot e^{wj}}{e^{wj}}}{1 + wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    9. Simplified2.0

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

    \[\leadsto \begin{array}{l} \mathbf{if}\;wj \le 6.715218772498828 \cdot 10^{-07}:\\ \;\;\;\;\mathsf{fma}\left(wj \cdot wj, wj \cdot wj, \mathsf{fma}\left(wj, wj, -\left(wj \cdot wj\right) \cdot wj\right)\right) + \frac{x}{e^{wj} \cdot wj + e^{wj}}\\ \mathbf{else}:\\ \;\;\;\;\frac{x}{e^{wj} \cdot wj + e^{wj}} + \left(wj - \frac{wj}{1 + wj}\right)\\ \end{array}\]

Reproduce

herbie shell --seed 2019158 +o rules:numerics
(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))))))