Move snprintf_float to format.h

include/fmt/format.h

@@ -1325,10 +1325,116 @@ FMT_HEADER_ONLY_CONSTEXPR20 auto format_float(T value, int precision,
                                               float_specs specs,
                                               buffer<char>& buf) -> int;
 
+#ifndef _MSC_VER
+#  define FMT_SNPRINTF snprintf
+#else
+FMT_API auto fmt_snprintf(char* buf, size_t size, const char* fmt, ...) -> int;
+#  define FMT_SNPRINTF fmt_snprintf
+#endif  // _MSC_VER
+
 // Formats a floating-point number with snprintf.
 template <typename T>
 auto snprintf_float(T value, int precision, float_specs specs,
-                    buffer<char>& buf) -> int;
+                    buffer<char>& buf) -> int {
+  // Buffer capacity must be non-zero, otherwise MSVC's vsnprintf_s will fail.
+  FMT_ASSERT(buf.capacity() > buf.size(), "empty buffer");
+  static_assert(!std::is_same<T, float>::value, "");
+
+  // Subtract 1 to account for the difference in precision since we use %e for
+  // both general and exponent format.
+  if (specs.format == float_format::general ||
+      specs.format == float_format::exp) {
+    precision = (precision >= 0 ? precision : 6) - 1;
+  }
+
+  // Build the format string.
+  char format[7];  // The longest format is "%#.*Le".
+  char* format_ptr = format;
+  *format_ptr++ = '%';
+  if (specs.showpoint && specs.format == float_format::hex) *format_ptr++ = '#';
+  if (precision >= 0) {
+    *format_ptr++ = '.';
+    *format_ptr++ = '*';
+  }
+  if (std::is_same<T, long double>()) *format_ptr++ = 'L';
+  *format_ptr++ = specs.format != float_format::hex
+                      ? (specs.format == float_format::fixed ? 'f' : 'e')
+                      : (specs.upper ? 'A' : 'a');
+  *format_ptr = '\0';
+
+  // Format using snprintf.
+  auto offset = buf.size();
+  for (;;) {
+    auto begin = buf.data() + offset;
+    auto capacity = buf.capacity() - offset;
+#ifdef FMT_FUZZ
+    if (precision > 100000)
+      throw std::runtime_error("fuzz mode: avoid large allocation in snprintf");
+#endif
+    // Suppress the warning about a nonliteral format string.
+    // Cannot use auto because of a bug in MinGW (#1532).
+    int (*snprintf_ptr)(char*, size_t, const char*, ...) = FMT_SNPRINTF;
+    int result = precision >= 0
+                     ? snprintf_ptr(begin, capacity, format, precision, value)
+                     : snprintf_ptr(begin, capacity, format, value);
+    if (result < 0) {
+      // The buffer will grow exponentially.
+      buf.try_reserve(buf.capacity() + 1);
+      continue;
+    }
+    auto size = to_unsigned(result);
+    // Size equal to capacity means that the last character was truncated.
+    if (size >= capacity) {
+      buf.try_reserve(size + offset + 1);  // Add 1 for the terminating '\0'.
+      continue;
+    }
+    auto is_digit = [](char c) { return c >= '0' && c <= '9'; };
+    if (specs.format == float_format::fixed) {
+      if (precision == 0) {
+        buf.try_resize(size);
+        return 0;
+      }
+      // Find and remove the decimal point.
+      auto end = begin + size, p = end;
+      do {
+        --p;
+      } while (is_digit(*p));
+      int fraction_size = static_cast<int>(end - p - 1);
+      std::memmove(p, p + 1, to_unsigned(fraction_size));
+      buf.try_resize(size - 1);
+      return -fraction_size;
+    }
+    if (specs.format == float_format::hex) {
+      buf.try_resize(size + offset);
+      return 0;
+    }
+    // Find and parse the exponent.
+    auto end = begin + size, exp_pos = end;
+    do {
+      --exp_pos;
+    } while (*exp_pos != 'e');
+    char sign = exp_pos[1];
+    FMT_ASSERT(sign == '+' || sign == '-', "");
+    int exp = 0;
+    auto p = exp_pos + 2;  // Skip 'e' and sign.
+    do {
+      FMT_ASSERT(is_digit(*p), "");
+      exp = exp * 10 + (*p++ - '0');
+    } while (p != end);
+    if (sign == '-') exp = -exp;
+    int fraction_size = 0;
+    if (exp_pos != begin + 1) {
+      // Remove trailing zeros.
+      auto fraction_end = exp_pos - 1;
+      while (*fraction_end == '0') --fraction_end;
+      // Move the fractional part left to get rid of the decimal point.
+      fraction_size = static_cast<int>(fraction_end - begin - 1);
+      std::memmove(begin + 1, begin + 2, to_unsigned(fraction_size));
+    }
+    buf.try_resize(to_unsigned(fraction_size) + offset + 1);
+    return exp - fraction_size;
+  }
+}
 
 template <typename T>
 using convert_float_result =
@@ -3203,14 +3309,6 @@ extern template auto format_float<double>(double value, int precision,
 extern template auto format_float<long double>(long double value, int precision,
                                                float_specs specs,
                                                buffer<char>& buf) -> int;
-void snprintf_float(float, int, float_specs, buffer<char>&) = delete;
-extern template auto snprintf_float<double>(double value, int precision,
-                                            float_specs specs,
-                                            buffer<char>& buf) -> int;
-extern template auto snprintf_float<long double>(long double value,
-                                                 int precision,
-                                                 float_specs specs,
-                                                 buffer<char>& buf) -> int;
 #endif  // FMT_HEADER_ONLY
 
 FMT_END_DETAIL_NAMESPACE