Make exponent threshold depend on representation (#3649)

Author: vitaut

include/fmt/format.h

@@ -2332,7 +2332,7 @@ template <typename Char, typename OutputIt, typename DecimalFP,
           typename Grouping = digit_grouping<Char>>
 FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
                                     const format_specs& specs, sign s,
-                                    locale_ref loc) -> OutputIt {
+                                    int exp_upper, locale_ref loc) -> OutputIt {
   auto significand = f.significand;
   int significand_size = get_significand_size(f);
   const Char zero = static_cast<Char>('0');
@@ -2348,7 +2348,7 @@ FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
     if (specs.type() == presentation_type::fixed) return false;
     // Use the fixed notation if the exponent is in [exp_lower, exp_upper),
     // e.g. 0.0001 instead of 1e-04. Otherwise use the exponent notation.
-    const int exp_lower = -4, exp_upper = 16;
+    const int exp_lower = -4;
     return output_exp < exp_lower ||
            output_exp >= (specs.precision > 0 ? specs.precision : exp_upper);
   };
@@ -2451,12 +2451,13 @@ template <typename Char> class fallback_digit_grouping {
 template <typename Char, typename OutputIt, typename DecimalFP>
 FMT_CONSTEXPR20 auto write_float(OutputIt out, const DecimalFP& f,
                                  const format_specs& specs, sign s,
-                                 locale_ref loc) -> OutputIt {
+                                 int exp_upper, locale_ref loc) -> OutputIt {
   if (is_constant_evaluated()) {
     return do_write_float<Char, OutputIt, DecimalFP,
-                          fallback_digit_grouping<Char>>(out, f, specs, s, loc);
+                          fallback_digit_grouping<Char>>(out, f, specs, s,
+                                                         exp_upper, loc);
   } else {
-    return do_write_float<Char>(out, f, specs, s, loc);
+    return do_write_float<Char>(out, f, specs, s, exp_upper, loc);
   }
 }
 
@@ -3288,6 +3289,12 @@ FMT_CONSTEXPR20 auto format_float(Float value, int precision,
   return exp;
 }
 
+// Numbers with exponents greater or equal to the returned value will use
+// the exponential notation.
+template <typename T> constexpr auto exp_upper() -> int {
+  return min_of(15, std::numeric_limits<T>::digits10) + 1;
+}
+
 template <typename Char, typename OutputIt, typename T>
 FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
                                  locale_ref loc) -> OutputIt {
@@ -3303,6 +3310,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
     if (specs.width != 0) --specs.width;
   }
 
+  constexpr int exp_upper = detail::exp_upper<T>();
   int precision = specs.precision;
   if (precision < 0) {
     if (specs.type() != presentation_type::none) {
@@ -3311,7 +3319,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
       // Use Dragonbox for the shortest format.
       using floaty = conditional_t<sizeof(T) >= sizeof(double), double, float>;
       auto dec = dragonbox::to_decimal(static_cast<floaty>(value));
-      return write_float<Char>(out, dec, specs, s, loc);
+      return write_float<Char>(out, dec, specs, s, exp_upper, loc);
     }
   }
 
@@ -3339,7 +3347,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
 
   specs.precision = precision;
   auto f = big_decimal_fp{buffer.data(), static_cast<int>(buffer.size()), exp};
-  return write_float<Char>(out, f, specs, s, loc);
+  return write_float<Char>(out, f, specs, s, exp_upper, loc);
 }
 
 template <typename Char, typename OutputIt, typename T,
@@ -3366,7 +3374,7 @@ FMT_CONSTEXPR20 auto write(OutputIt out, T value) -> OutputIt {
     return write_nonfinite<Char>(out, std::isnan(value), specs, s);
 
   auto dec = dragonbox::to_decimal(static_cast<floaty>(value));
-  return write_float<Char>(out, dec, specs, s, {});
+  return write_float<Char>(out, dec, specs, s, exp_upper<T>(), {});
 }
 
 template <typename Char, typename OutputIt, typename T,

test/format-impl-test.cc

@@ -312,6 +312,7 @@ template <> struct numeric_limits<double_double> {
   // is_iec559 is true for double-double in libstdc++.
   static constexpr bool is_iec559 = true;
   static constexpr int digits = 106;
+  static constexpr int digits10 = 33;
 };
 
 template <> struct is_floating_point<slow_float> : std::true_type {};
@@ -341,7 +342,7 @@ TEST(format_impl_test, write_dragon_even) {
   auto s = std::string();
   fmt::detail::write<char>(std::back_inserter(s), slow_float(33554450.0f), {});
   // Specializing is_floating_point is broken in MSVC.
-  if (!FMT_MSC_VERSION) EXPECT_EQ(s, "33554450");
+  if (!FMT_MSC_VERSION) EXPECT_EQ(s, "3.355445e+07");
 }
 
 #if defined(_WIN32) && !defined(FMT_USE_WRITE_CONSOLE)

test/format-test.cc

@@ -1068,7 +1068,8 @@ TEST(format_test, precision) {
   EXPECT_EQ(fmt::format("{:#.0f}", 123.0), "123.");
   EXPECT_EQ(fmt::format("{:.02f}", 1.234), "1.23");
   EXPECT_EQ(fmt::format("{:.1g}", 0.001), "0.001");
-  EXPECT_EQ(fmt::format("{}", 1019666432.0f), "1019666400");
+  EXPECT_EQ(fmt::format("{}", 123456789.0f), "1.2345679e+08");
+  EXPECT_EQ(fmt::format("{}", 1019666432.0f), "1.0196664e+09");
   EXPECT_EQ(fmt::format("{:.0e}", 9.5), "1e+01");
   EXPECT_EQ(fmt::format("{:.1e}", 1e-34), "1.0e-34");