Use numeric_limits instead of hand-coded equivalents

As a comment noted, the reason for QLocaleData rolling its own values
describing the ranges of digits and exponents in a double were all
about std::numeric_limits's constants not being constexpr - which they
have now been since C++11, so we can do away with our own.

One of the constants was used in two places in the same way; so
abstract that use out into an inline function in qlocale_tools, to
save duplication and give somewhere to document it.

Change-Id: I7e3740ece9b499c0ec434de18d70abe69e1fe079
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale.cpp

@@ -3354,10 +3354,9 @@ QString QLocaleData::doubleToString(const QString &zero, const QString &plus, co
     int decpt;
     int bufSize = 1;
     if (precision == QLocale::FloatingPointShortest)
-        bufSize += DoubleMaxSignificant;
-    else if (form == DFDecimal) // optimize for numbers between -512k and 512k
-        bufSize += ((d > (1 << 19) || d < -(1 << 19)) ? DoubleMaxDigitsBeforeDecimal : 6) +
-                precision;
+        bufSize += std::numeric_limits<double>::max_digits10;
+    else if (form == DFDecimal)
+        bufSize += wholePartSpace(qAbs(d)) + precision;
     else // Add extra digit due to different interpretations of precision. Also, "nan" has to fit.
         bufSize += qMax(2, precision) + 1;
 

src/corelib/text/qlocale_p.h

@@ -182,17 +182,6 @@ public:
                                  QLocale::Country country);
     static const QLocaleData *c();
 
-    // Maximum number of significant digits needed to represent a double.
-    // We cannot use std::numeric_limits here without constexpr.
-    static const int DoubleMantissaBits = 53;
-    static const int Log10_2_100000 = 30103;    // log10(2) * 100000
-    // same as C++11 std::numeric_limits<T>::max_digits10
-    static const int DoubleMaxSignificant = (DoubleMantissaBits * Log10_2_100000) / 100000 + 2;
-
-    // Maximum number of digits before decimal point to represent a double
-    // Same as std::numeric_limits<double>::max_exponent10 + 1
-    static const int DoubleMaxDigitsBeforeDecimal = 309;
-
     enum DoubleForm {
         DFExponent = 0,
         DFDecimal,

src/corelib/text/qlocale_tools.cpp

@@ -139,7 +139,7 @@ void qt_doubleToAscii(double d, QLocaleData::DoubleForm form, int precision, cha
     if (precision > 999)
         precision = 999;
     else if (precision == QLocale::FloatingPointShortest)
-        precision = QLocaleData::DoubleMaxSignificant; // "shortest" mode not supported by snprintf
+        precision = std::numeric_limits<double>::max_digits10; // snprintf lacks "shortest" mode
 
     if (isZero(d)) {
         // Negative zero is expected as simple "0", not "-0". We cannot do d < 0, though.
@@ -167,8 +167,8 @@ void qt_doubleToAscii(double d, QLocaleData::DoubleForm form, int precision, cha
     switch (form) {
     case QLocaleData::DFDecimal:
         format[formatLength - 2] = 'f';
-        // <anything> '.' <precision> '\0' - optimize for numbers smaller than 512k
-        extraChars = (d > (1 << 19) ? QLocaleData::DoubleMaxDigitsBeforeDecimal : 6) + 2;
+        // <anything> '.' <precision> '\0'
+        extraChars = wholePartSpace(d) + 2;
         break;
     case QLocaleData::DFExponent:
         format[formatLength - 2] = 'e';
@@ -581,9 +581,10 @@ QString qdtoa(qreal d, int *decpt, int *sign)
     int length = 0;
 
     // Some versions of libdouble-conversion like an extra digit, probably for '\0'
-    char result[QLocaleData::DoubleMaxSignificant + 1];
-    qt_doubleToAscii(d, QLocaleData::DFSignificantDigits, QLocale::FloatingPointShortest, result,
-                     QLocaleData::DoubleMaxSignificant + 1, nonNullSign, length, nonNullDecpt);
+    constexpr int digits = std::numeric_limits<double>::max_digits10 + 1;
+    char result[digits];
+    qt_doubleToAscii(d, QLocaleData::DFSignificantDigits, QLocale::FloatingPointShortest,
+                     result, digits, nonNullSign, length, nonNullDecpt);
 
     if (sign)
         *sign = nonNullSign ? 1 : 0;

src/corelib/text/qlocale_tools_p.h

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -98,6 +98,15 @@ inline bool isZero(double d)
     }
 }
 
+// Enough space for the digits before the decimal separator:
+inline int wholePartSpace(double d)
+{
+    Q_ASSERT(d >= 0); // caller should call qAbs() if needed
+    // Optimize for numbers between -512k and 512k - otherwise, use the
+    // maximum number of digits in the whole number part of a double:
+    return d > (1 << 19) ? std::numeric_limits<double>::max_exponent10 + 1 : 6;
+}
+
 Q_CORE_EXPORT double qstrtod(const char *s00, char const **se, bool *ok);
 Q_CORE_EXPORT double qstrntod(const char *s00, int len, char const **se, bool *ok);
 qlonglong qstrtoll(const char *nptr, const char **endptr, int base, bool *ok);