Simplified clinger.

include/fast_float/float_common.h

@@ -221,6 +221,55 @@ constexpr static double powers_of_ten_double[] = {
 constexpr static float powers_of_ten_float[] = {1e0, 1e1, 1e2, 1e3, 1e4, 1e5,
                                                 1e6, 1e7, 1e8, 1e9, 1e10};
 
+// Largest integer value v so that (5**index * v) <= 1<<53.
+// 0x10000000000000 == 1 << 53
+constexpr static uint64_t max_mantissa_double[] = {
+      0x10000000000000,
+      0x10000000000000 / 5,
+      0x10000000000000 / (5 * 5),
+      0x10000000000000 / (5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5 * 5 * 5 * 5 * 5),
+      0x10000000000000 / (uint64_t(5) * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 *
+           5 * 5 * 5 * 5 * 5 * 5)};
+  // Largest integer value v so that (5**index * v) <= 1<<24.
+  // 0x1000000 == 1<<24
+  constexpr static uint64_t max_mantissa_float[] = {
+      0x1000000,
+      0x1000000 / 5,
+      0x1000000 / (5 * 5),
+      0x1000000 / (5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5),
+      0x1000000 / (5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5)};
+
 template <typename T> struct binary_format {
   using equiv_uint = typename std::conditional<sizeof(T) == 4, uint32_t, uint64_t>::type;
 
@@ -228,11 +277,10 @@ template <typename T> struct binary_format {
   static inline constexpr int minimum_exponent();
   static inline constexpr int infinite_power();
   static inline constexpr int sign_index();
-  static inline constexpr int min_exponent_fast_path();
   static inline constexpr int max_exponent_fast_path();
   static inline constexpr int max_exponent_round_to_even();
   static inline constexpr int min_exponent_round_to_even();
-  static inline constexpr uint64_t max_mantissa_fast_path();
+  static inline constexpr uint64_t max_mantissa_fast_path(int64_t power);
   static inline constexpr int largest_power_of_ten();
   static inline constexpr int smallest_power_of_ten();
   static inline constexpr T exact_power_of_ten(int64_t power);
@@ -282,21 +330,6 @@ template <> inline constexpr int binary_format<float>::infinite_power() {
 template <> inline constexpr int binary_format<double>::sign_index() { return 63; }
 template <> inline constexpr int binary_format<float>::sign_index() { return 31; }
 
-template <> inline constexpr int binary_format<double>::min_exponent_fast_path() {
-#if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
-  return 0;
-#else
-  return -22;
-#endif
-}
-template <> inline constexpr int binary_format<float>::min_exponent_fast_path() {
-#if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
-  return 0;
-#else
-  return -10;
-#endif
-}
-
 template <> inline constexpr int binary_format<double>::max_exponent_fast_path() {
   return 22;
 }
@@ -304,11 +337,17 @@ template <> inline constexpr int binary_format<float>::max_exponent_fast_path()
   return 10;
 }
 
-template <> inline constexpr uint64_t binary_format<double>::max_mantissa_fast_path() {
+template <> inline constexpr uint64_t binary_format<double>::max_mantissa_fast_path(int64_t power) {
-  return uint64_t(2) << mantissa_explicit_bits();
+  // caller is responsible to ensure that
+  // power >= 0 && power <= 22
+  //
+  return max_mantissa_double[power];
 }
-template <> inline constexpr uint64_t binary_format<float>::max_mantissa_fast_path() {
+template <> inline constexpr uint64_t binary_format<float>::max_mantissa_fast_path(int64_t power) {
-  return uint64_t(2) << mantissa_explicit_bits();
+  // caller is responsible to ensure that
+  // power >= 0 && power <= 10
+  //
+  return max_mantissa_float[power];
 }
 
 template <>

include/fast_float/parse_number.h

@@ -87,11 +87,10 @@ from_chars_result from_chars_advanced(const char *first, const char *last,
   }
   answer.ec = std::errc(); // be optimistic
   answer.ptr = pns.lastmatch;
-  // Next is Clinger's fast path.
+  // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's proposal
-  if (binary_format<T>::min_exponent_fast_path() <= pns.exponent && pns.exponent <= binary_format<T>::max_exponent_fast_path() && pns.mantissa <=binary_format<T>::max_mantissa_fast_path() && !pns.too_many_digits) {
+  if (pns.exponent >= 0 && pns.exponent <= binary_format<T>::max_exponent_fast_path() && pns.mantissa <=binary_format<T>::max_mantissa_fast_path(pns.exponent) && !pns.too_many_digits) {
     value = T(pns.mantissa);
-    if (pns.exponent < 0) { value = value / binary_format<T>::exact_power_of_ten(-pns.exponent); }
+    value = value * binary_format<T>::exact_power_of_ten(pns.exponent);
-    else { value = value * binary_format<T>::exact_power_of_ten(pns.exponent); }
     if (pns.negative) { value = -value; }
     return answer;
   }

tests/CMakeLists.txt

@@ -56,6 +56,8 @@ endfunction(fast_float_add_cpp_test)
 fast_float_add_cpp_test(example_test)
 fast_float_add_cpp_test(example_comma_test)
 fast_float_add_cpp_test(basictest)
+target_compile_features(basictest PRIVATE cxx_std_17)
+
 fast_float_add_cpp_test(long_test)
 fast_float_add_cpp_test(powersoffive_hardround)
 fast_float_add_cpp_test(string_test)