Average Error: 40.3 → 0.5
Time: 17.5s
Precision: binary64
\[\cos \left(x + \varepsilon\right) - \cos x \]
↓
\[\begin{array}{l} \mathbf{if}\;\varepsilon \leq -0.0023136737066387857 \lor \neg \left(\varepsilon \leq 0.002833034179938811\right):\\ \;\;\;\;\cos x \cdot \cos \varepsilon - \mathsf{fma}\left(\sin \varepsilon, \sin x, \cos x\right)\\ \mathbf{else}:\\ \;\;\;\;\left(0.16666666666666666 \cdot \left(\sin x \cdot {\varepsilon}^{3}\right) + 0.041666666666666664 \cdot \left(\cos x \cdot {\varepsilon}^{4}\right)\right) - \left(0.5 \cdot \left(\cos x \cdot {\varepsilon}^{2}\right) + \varepsilon \cdot \sin x\right)\\ \end{array} \]
\cos \left(x + \varepsilon\right) - \cos x
↓
\begin{array}{l}
\mathbf{if}\;\varepsilon \leq -0.0023136737066387857 \lor \neg \left(\varepsilon \leq 0.002833034179938811\right):\\
\;\;\;\;\cos x \cdot \cos \varepsilon - \mathsf{fma}\left(\sin \varepsilon, \sin x, \cos x\right)\\

\mathbf{else}:\\
\;\;\;\;\left(0.16666666666666666 \cdot \left(\sin x \cdot {\varepsilon}^{3}\right) + 0.041666666666666664 \cdot \left(\cos x \cdot {\varepsilon}^{4}\right)\right) - \left(0.5 \cdot \left(\cos x \cdot {\varepsilon}^{2}\right) + \varepsilon \cdot \sin x\right)\\


\end{array}

(FPCore (x eps) :precision binary64 (- (cos (+ x eps)) (cos x)))
↓
(FPCore (x eps)
 :precision binary64
 (if (or (<= eps -0.0023136737066387857) (not (<= eps 0.002833034179938811)))
   (- (* (cos x) (cos eps)) (fma (sin eps) (sin x) (cos x)))
   (-
    (+
     (* 0.16666666666666666 (* (sin x) (pow eps 3.0)))
     (* 0.041666666666666664 (* (cos x) (pow eps 4.0))))
    (+ (* 0.5 (* (cos x) (pow eps 2.0))) (* eps (sin x))))))
double code(double x, double eps) {
	return cos(x + eps) - cos(x);
}
↓
double code(double x, double eps) {
	double tmp;
	if ((eps <= -0.0023136737066387857) || !(eps <= 0.002833034179938811)) {
		tmp = (cos(x) * cos(eps)) - fma(sin(eps), sin(x), cos(x));
	} else {
		tmp = ((0.16666666666666666 * (sin(x) * pow(eps, 3.0))) + (0.041666666666666664 * (cos(x) * pow(eps, 4.0)))) - ((0.5 * (cos(x) * pow(eps, 2.0))) + (eps * sin(x)));
	}
	return tmp;
}

Error

Bits error versus x

Bits error versus eps

Derivation

  1. Split input into 2 regimes

  2. if eps < -0.00231367370663878571 or 0.0028330341799388111 < eps

    1. Initial program 30.8

      \[\cos \left(x + \varepsilon\right) - \cos x \]

    2. Applied cos-sum_binary640.8

      \[\leadsto \color{blue}{\left(\cos x \cdot \cos \varepsilon - \sin x \cdot \sin \varepsilon\right)} - \cos x \]

    3. Applied associate--l-_binary640.8

      \[\leadsto \color{blue}{\cos x \cdot \cos \varepsilon - \left(\sin x \cdot \sin \varepsilon + \cos x\right)} \]

    4. Simplified0.8

      \[\leadsto \cos x \cdot \cos \varepsilon - \color{blue}{\mathsf{fma}\left(\sin \varepsilon, \sin x, \cos x\right)} \]

    if -0.00231367370663878571 < eps < 0.0028330341799388111

    1. Initial program 49.6

      \[\cos \left(x + \varepsilon\right) - \cos x \]

    2. Taylor expanded in eps around 0 0.2

      \[\leadsto \color{blue}{\left(0.16666666666666666 \cdot \left({\varepsilon}^{3} \cdot \sin x\right) + 0.041666666666666664 \cdot \left({\varepsilon}^{4} \cdot \cos x\right)\right) - \left(0.5 \cdot \left({\varepsilon}^{2} \cdot \cos x\right) + \varepsilon \cdot \sin x\right)} \]
  3. Recombined 2 regimes into one program.

  4. Final simplification0.5

    \[\leadsto \begin{array}{l} \mathbf{if}\;\varepsilon \leq -0.0023136737066387857 \lor \neg \left(\varepsilon \leq 0.002833034179938811\right):\\ \;\;\;\;\cos x \cdot \cos \varepsilon - \mathsf{fma}\left(\sin \varepsilon, \sin x, \cos x\right)\\ \mathbf{else}:\\ \;\;\;\;\left(0.16666666666666666 \cdot \left(\sin x \cdot {\varepsilon}^{3}\right) + 0.041666666666666664 \cdot \left(\cos x \cdot {\varepsilon}^{4}\right)\right) - \left(0.5 \cdot \left(\cos x \cdot {\varepsilon}^{2}\right) + \varepsilon \cdot \sin x\right)\\ \end{array} \]

Reproduce

herbie shell --seed 2021224 
(FPCore (x eps)
  :name "2cos (problem 3.3.5)"
  :precision binary64
  (- (cos (+ x eps)) (cos x)))