py/modmath: Add option to work around -inf bug in a port's tgamma.

This is needed for a workaround on esp32 port (in child commit),
which produces incorrect results otherwise.

This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/modmath.c

@@ -196,7 +196,17 @@ MATH_FUN_1(erf, erf)
 // erfc(x): return the complementary error function of x
 MATH_FUN_1(erfc, erfc)
 // gamma(x): return the gamma function of x
+#if MICROPY_PY_MATH_GAMMA_FIX_NEGINF
+static mp_float_t MICROPY_FLOAT_C_FUN(tgamma_func)(mp_float_t x) {
+    if (isinf(x) && x < 0) {
+        math_error();
+    }
+    return MICROPY_FLOAT_C_FUN(tgamma)(x);
+}
+MATH_FUN_1(gamma, tgamma_func)
+#else
 MATH_FUN_1(gamma, tgamma)
+#endif
 // lgamma(x): return the natural logarithm of the gamma function of x
 MATH_FUN_1(lgamma, lgamma)
 #endif

py/mpconfig.h

@@ -1393,6 +1393,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_MATH_POW_FIX_NAN (0)
 #endif
 
+// Whether to provide fix for gamma(-inf) to raise ValueError
+#ifndef MICROPY_PY_MATH_GAMMA_FIX_NEGINF
+#define MICROPY_PY_MATH_GAMMA_FIX_NEGINF (0)
+#endif
+
 // Whether to provide "cmath" module
 #ifndef MICROPY_PY_CMATH
 #define MICROPY_PY_CMATH (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)