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gh-120010: Fix invalid (nan+nanj) results in _Py_c_prod() (GH-120287)

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In some cases, previously computed as (nan+nanj), we could recover
meaningful component values in the result, see e.g. the C11, Annex
G.5.1, routine _Cmultd():

>>> z = 1e300+1j
>>> z*(nan+infj)  # was (nan+nanj)
(-inf+infj)

That also fix some complex powers for small integer exponents, computed
with optimized algorithm (by squaring):

>>> z**5  # was (nan+nanj)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    z**5
    ~^^~
OverflowError: complex exponentiation
This commit is contained in:
Sergey B Kirpichev 2024-12-06 13:28:32 +03:00 • committed by GitHub
parent e991ac8f20
commit 8b7c194c7b
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3 changed files with 75 additions and 4 deletions
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60
Objects/complexobject.c
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@ -85,11 +85,63 @@ _Py_c_neg(Py_complex a)
}
Py_complex
_Py_c_prod(Py_complex a, Py_complex b)
_Py_c_prod(Py_complex z, Py_complex w)
{
Py_complex r;
r.real = a.real*b.real - a.imag*b.imag;
r.imag = a.real*b.imag + a.imag*b.real;
double a = z.real, b = z.imag, c = w.real, d = w.imag;
double ac = a*c, bd = b*d, ad = a*d, bc = b*c;
Py_complex r = {ac - bd, ad + bc};
/* Recover infinities that computed as nan+nanj. See e.g. the C11,
Annex G.5.1, routine _Cmultd(). */
if (isnan(r.real) && isnan(r.imag)) {
int recalc = 0;
if (isinf(a) || isinf(b)) { /* z is infinite */
/* "Box" the infinity and change nans in the other factor to 0 */
a = copysign(isinf(a) ? 1.0 : 0.0, a);
b = copysign(isinf(b) ? 1.0 : 0.0, b);
if (isnan(c)) {
c = copysign(0.0, c);
}
if (isnan(d)) {
d = copysign(0.0, d);
}
recalc = 1;
}
if (isinf(c) || isinf(d)) { /* w is infinite */
/* "Box" the infinity and change nans in the other factor to 0 */
c = copysign(isinf(c) ? 1.0 : 0.0, c);
d = copysign(isinf(d) ? 1.0 : 0.0, d);
if (isnan(a)) {
a = copysign(0.0, a);
}
if (isnan(b)) {
b = copysign(0.0, b);
}
recalc = 1;
}
if (!recalc && (isinf(ac) || isinf(bd) || isinf(ad) || isinf(bc))) {
/* Recover infinities from overflow by changing nans to 0 */
if (isnan(a)) {
a = copysign(0.0, a);
}
if (isnan(b)) {
b = copysign(0.0, b);
}
if (isnan(c)) {
c = copysign(0.0, c);
}
if (isnan(d)) {
d = copysign(0.0, d);
}
recalc = 1;
}
if (recalc) {
r.real = Py_INFINITY*(a*c - b*d);
r.imag = Py_INFINITY*(a*d + b*c);
}
}
return r;
}