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gh-103172: Add π to the math module #103178

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gh-103172: Add π to the math module #103178

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26 changes: 25 additions & 1 deletion Modules/mathmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3948,6 +3948,30 @@ math_ulp_impl(PyObject *module, double x)
return x2 - x;
}

/*
Future-proof pi in case we ever move away from IEEE 754. Import time is
increased, but the added robustness makes this an acceptable trade-off.

TODO: Slightly better algorithms are known.
*/
static double
math_calc_pi(void)
{
double pi = 0.0;

for (int n = 0; ; ++n) {
double term = 4.0 / ((double) (2*n + 1));
if (n & 1)
term = -term;
double next_pi = pi + term;
if (next_pi == pi)
break;
pi = next_pi;
}

return pi;
}

static int
math_exec(PyObject *module)
{
Expand All @@ -3965,7 +3989,7 @@ math_exec(PyObject *module)
if (state->str___trunc__ == NULL) {
return -1;
}
if (PyModule_AddObject(module, "pi", PyFloat_FromDouble(Py_MATH_PI)) < 0) {
if (PyModule_AddObject(module, "π", PyFloat_FromDouble(math_calc_pi())) < 0) {
return -1;
}
if (PyModule_AddObject(module, "e", PyFloat_FromDouble(Py_MATH_E)) < 0) {
Expand Down