Average Error: 28.4 → 5.1
Time: 7.9s
Precision: binary64
\[\left(\left(1.0536712127723509 \cdot 10^{-8} < a \land a < 94906265.62425156\right) \land \left(1.0536712127723509 \cdot 10^{-8} < b \land b < 94906265.62425156\right)\right) \land \left(1.0536712127723509 \cdot 10^{-8} < c \land c < 94906265.62425156\right)\]
\[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a} \]
↓
\[\begin{array}{l} \mathbf{if}\;b \leq 0.5227477288412871:\\ \;\;\;\;\frac{\sqrt{\mathsf{fma}\left(b, b, c \cdot \left(a \cdot -3\right)\right) + \mathsf{fma}\left(-c, 3 \cdot a, c \cdot \left(3 \cdot a\right)\right)} - b}{3 \cdot a}\\ \mathbf{else}:\\ \;\;\;\;\mathsf{fma}\left(\frac{{c}^{3} \cdot \left(a \cdot a\right)}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{a \cdot \left(c \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)\\ \end{array} \]
\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a}
↓
\begin{array}{l}
\mathbf{if}\;b \leq 0.5227477288412871:\\
\;\;\;\;\frac{\sqrt{\mathsf{fma}\left(b, b, c \cdot \left(a \cdot -3\right)\right) + \mathsf{fma}\left(-c, 3 \cdot a, c \cdot \left(3 \cdot a\right)\right)} - b}{3 \cdot a}\\

\mathbf{else}:\\
\;\;\;\;\mathsf{fma}\left(\frac{{c}^{3} \cdot \left(a \cdot a\right)}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{a \cdot \left(c \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)\\


\end{array}

(FPCore (a b c)
 :precision binary64
 (/ (+ (- b) (sqrt (- (* b b) (* (* 3.0 a) c)))) (* 3.0 a)))
↓
(FPCore (a b c)
 :precision binary64
 (if (<= b 0.5227477288412871)
   (/
    (-
     (sqrt
      (+ (fma b b (* c (* a -3.0))) (fma (- c) (* 3.0 a) (* c (* 3.0 a)))))
     b)
    (* 3.0 a))
   (fma
    (/ (* (pow c 3.0) (* a a)) (pow b 5.0))
    -0.5625
    (fma
     (/ (* (pow a 3.0) (pow c 4.0)) (pow b 7.0))
     -1.0546875
     (fma (/ (* a (* c c)) (pow b 3.0)) -0.375 (* -0.5 (/ c b)))))))
double code(double a, double b, double c) {
	return (-b + sqrt((b * b) - ((3.0 * a) * c))) / (3.0 * a);
}
↓
double code(double a, double b, double c) {
	double tmp;
	if (b <= 0.5227477288412871) {
		tmp = (sqrt(fma(b, b, (c * (a * -3.0))) + fma(-c, (3.0 * a), (c * (3.0 * a)))) - b) / (3.0 * a);
	} else {
		tmp = fma(((pow(c, 3.0) * (a * a)) / pow(b, 5.0)), -0.5625, fma(((pow(a, 3.0) * pow(c, 4.0)) / pow(b, 7.0)), -1.0546875, fma(((a * (c * c)) / pow(b, 3.0)), -0.375, (-0.5 * (c / b)))));
	}
	return tmp;
}

Error

Bits error versus a

Bits error versus b

Bits error versus c

Derivation

  1. Split input into 2 regimes

  2. if b < 0.522747728841287063

    1. Initial program 11.1

      \[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a} \]

    2. Applied prod-diff_binary6411.0

      \[\leadsto \frac{\left(-b\right) + \sqrt{\color{blue}{\mathsf{fma}\left(b, b, -c \cdot \left(3 \cdot a\right)\right) + \mathsf{fma}\left(-c, 3 \cdot a, c \cdot \left(3 \cdot a\right)\right)}}}{3 \cdot a} \]

    if 0.522747728841287063 < b

    1. Initial program 31.3

      \[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a} \]

    2. Taylor expanded in b around inf 4.4

      \[\leadsto \frac{\color{blue}{-\left(1.5 \cdot \frac{c \cdot a}{b} + \left(1.125 \cdot \frac{{c}^{2} \cdot {a}^{2}}{{b}^{3}} + \left(3.1640625 \cdot \frac{{c}^{4} \cdot {a}^{4}}{{b}^{7}} + 1.6875 \cdot \frac{{c}^{3} \cdot {a}^{3}}{{b}^{5}}\right)\right)\right)}}{3 \cdot a} \]

    3. Simplified4.4

      \[\leadsto \frac{\color{blue}{-1.5 \cdot \frac{c \cdot a}{b} - \mathsf{fma}\left(1.125, \frac{\left(c \cdot a\right) \cdot \left(c \cdot a\right)}{{b}^{3}}, \mathsf{fma}\left(1.6875, \frac{{\left(c \cdot a\right)}^{3}}{{b}^{5}}, 3.1640625 \cdot \frac{{c}^{4} \cdot {a}^{4}}{{b}^{7}}\right)\right)}}{3 \cdot a} \]

    4. Taylor expanded in c around 0 4.1

      \[\leadsto \color{blue}{-\left(0.5625 \cdot \frac{{c}^{3} \cdot {a}^{2}}{{b}^{5}} + \left(1.0546875 \cdot \frac{{c}^{4} \cdot {a}^{3}}{{b}^{7}} + \left(0.375 \cdot \frac{{c}^{2} \cdot a}{{b}^{3}} + 0.5 \cdot \frac{c}{b}\right)\right)\right)} \]

    5. Simplified4.1

      \[\leadsto \color{blue}{\mathsf{fma}\left(\frac{{c}^{3} \cdot \left(a \cdot a\right)}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{a \cdot \left(c \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)} \]
  3. Recombined 2 regimes into one program.

  4. Final simplification5.1

    \[\leadsto \begin{array}{l} \mathbf{if}\;b \leq 0.5227477288412871:\\ \;\;\;\;\frac{\sqrt{\mathsf{fma}\left(b, b, c \cdot \left(a \cdot -3\right)\right) + \mathsf{fma}\left(-c, 3 \cdot a, c \cdot \left(3 \cdot a\right)\right)} - b}{3 \cdot a}\\ \mathbf{else}:\\ \;\;\;\;\mathsf{fma}\left(\frac{{c}^{3} \cdot \left(a \cdot a\right)}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{a \cdot \left(c \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)\\ \end{array} \]

Reproduce

herbie shell --seed 2022004 
(FPCore (a b c)
  :name "Cubic critical, narrow range"
  :precision binary64
  :pre (and (and (and (< 1.0536712127723509e-8 a) (< a 94906265.62425156)) (and (< 1.0536712127723509e-8 b) (< b 94906265.62425156))) (and (< 1.0536712127723509e-8 c) (< c 94906265.62425156)))
  (/ (+ (- b) (sqrt (- (* b b) (* (* 3.0 a) c)))) (* 3.0 a)))