defining LDBL_DECIMAL_DIG for MSVC

Author: SwayamInSync

src/csrc/scalar.c

@@ -2,7 +2,6 @@
 #include <sleef.h>
 #include <sleefquad.h>
 #include <stdlib.h>
-#include <float.h>
 
 #define PY_ARRAY_UNIQUE_SYMBOL QuadPrecType_ARRAY_API
 #define NPY_NO_DEPRECATED_API NPY_2_0_API_VERSION
@@ -332,13 +331,15 @@ QuadPrecision_str_dragon4(QuadPrecisionObject *self)
 
 static PyObject *
 QuadPrecision_str(QuadPrecisionObject *self)
+// This is just define here for debugging, we actually use QuadPrecision_str_dragon4 for __str__ method.
 {
     char buffer[128];
     if (self->backend == BACKEND_SLEEF) {
         Sleef_snprintf(buffer, sizeof(buffer), "%.*Qe", SLEEF_QUAD_DIG, self->value.sleef_value);
     }
     else {
-        snprintf(buffer, sizeof(buffer), "%.35Le", self->value.longdouble_value);
+        snprintf(buffer, sizeof(buffer), "%.*Le", LDBL_DECIMAL_DIG - 1,
+                 self->value.longdouble_value);
     }
     return PyUnicode_FromString(buffer);
 }

src/include/constants.hpp

@@ -9,6 +9,15 @@ extern "C" {
 #include <sleefquad.h>
 #include <stdint.h>
 #include <string.h>
+#include <float.h>
+
+/* LDBL_DECIMAL_DIG: minimum decimal digits needed for a lossless
+ * long-double → string → long-double round-trip. Standard C11 constant in
+ * <float.h>, but MSVC omits it. On MSVC long double is the same width as
+ * double, so DBL_DECIMAL_DIG (17) is the exact correct fallback. */
+#ifndef LDBL_DECIMAL_DIG
+#  define LDBL_DECIMAL_DIG DBL_DECIMAL_DIG
+#endif
 
 // Quad precision constants using sleef_q macro
 #define QUAD_PRECISION_ZERO sleef_q(+0x0000000000000LL, 0x0000000000000000ULL, -16383)