Merge pull request #320 from toughengineer/int_multiplication_by_power_of_10

Implemented multiplication of integer by a power of 10

Author: lemire

README.md

@@ -377,6 +377,33 @@ int main() {
 }
 ```
 
+## Multiplication of an integer by a power of 10
+An integer `W` can be multiplied by a power of ten `10^Q` and
+converted to `double` with correctly rounded value
+(in "round to nearest, tie to even" fashion) using
+`fast_float::integer_times_pow10()`, e.g.:
+```C++
+const uint64_t W = 12345678901234567;
+const int Q = 23;
+const double result = fast_float::integer_times_pow10(W, Q);
+std::cout.precision(17);
+std::cout << W << " * 10^" << Q << " = " << result << " ("
+  << (result == 12345678901234567e23 ? "==" : "!=") << "expected)\n";
+```
+outputs
+```
+12345678901234567 * 10^23 = 1.2345678901234567e+39 (==expected)
+```
+`fast_float::integer_times_pow10()` gives the same result as
+using `fast_float::from_chars()` when parsing the string `"WeQ"`
+(in this example `"12345678901234567e23"`),
+except `fast_float::integer_times_pow10()` does not report out-of-range errors, and
+underflows to zero or overflows to infinity when the resulting value is
+out of range.
+
+Overloads of `fast_float::integer_times_pow10()` are provided for
+signed and unsigned integer types: `int64_t`, `uint64_t`, etc.
+
 
 ## Users and Related Work
 

include/fast_float/fast_float.h

@@ -45,6 +45,26 @@ FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
 from_chars_advanced(UC const *first, UC const *last, T &value,
                     parse_options_t<UC> options) noexcept;
 
+/**
+ * This function multiplies an integer number by a power of 10 and returns
+ * the result as a double precision floating-point value that is correctly
+ * rounded. The resulting floating-point value is the closest floating-point
+ * value, using the "round to nearest, tie to even" convention for values that
+ * would otherwise fall right in-between two values. That is, we provide exact
+ * conversion according to the IEEE standard.
+ *
+ * On overflow infinity is returned, on underflow 0 is returned.
+ *
+ * The implementation does not throw and does not allocate memory (e.g., with
+ * `new` or `malloc`).
+ */
+FASTFLOAT_CONSTEXPR20 inline double
+integer_times_pow10(uint64_t mantissa, int decimal_exponent) noexcept;
+FASTFLOAT_CONSTEXPR20 inline
+    typename std::enable_if<is_supported_float_type<double>::value,
+                            double>::type
+    integer_times_pow10(int64_t mantissa, int decimal_exponent) noexcept;
+
 /**
  * from_chars for integer types.
  */

include/fast_float/parse_number.h

@@ -188,32 +188,17 @@ from_chars(UC const *first, UC const *last, T &value,
                                     parse_options_t<UC>(fmt));
 }
 
-/**
- * This function overload takes parsed_number_string_t structure that is created
- * and populated either by from_chars_advanced function taking chars range and
- * parsing options or other parsing custom function implemented by user.
- */
-template <typename T, typename UC>
-FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
-from_chars_advanced(parsed_number_string_t<UC> &pns, T &value) noexcept {
-
-  static_assert(is_supported_float_type<T>::value,
-                "only some floating-point types are supported");
-  static_assert(is_supported_char_type<UC>::value,
-                "only char, wchar_t, char16_t and char32_t are supported");
-
-  from_chars_result_t<UC> answer;
-
-  answer.ec = std::errc(); // be optimistic
-  answer.ptr = pns.lastmatch;
+template <typename T>
+fastfloat_really_inline FASTFLOAT_CONSTEXPR20 bool
+clinger_fast_path_impl(uint64_t mantissa, int64_t exponent, bool is_negative,
+                       T &value) noexcept {
   // The implementation of the Clinger's fast path is convoluted because
   // we want round-to-nearest in all cases, irrespective of the rounding mode
   // selected on the thread.
   // We proceed optimistically, assuming that detail::rounds_to_nearest()
   // returns true.
-  if (binary_format<T>::min_exponent_fast_path() <= pns.exponent &&
-      pns.exponent <= binary_format<T>::max_exponent_fast_path() &&
-      !pns.too_many_digits) {
+  if (binary_format<T>::min_exponent_fast_path() <= exponent &&
+      exponent <= binary_format<T>::max_exponent_fast_path()) {
     // Unfortunately, the conventional Clinger's fast path is only possible
     // when the system rounds to the nearest float.
     //
@@ -224,41 +209,64 @@ from_chars_advanced(parsed_number_string_t<UC> &pns, T &value) noexcept {
     if (!cpp20_and_in_constexpr() && detail::rounds_to_nearest()) {
       // We have that fegetround() == FE_TONEAREST.
       // Next is Clinger's fast path.
-      if (pns.mantissa <= binary_format<T>::max_mantissa_fast_path()) {
-        value = T(pns.mantissa);
-        if (pns.exponent < 0) {
-          value = value / binary_format<T>::exact_power_of_ten(-pns.exponent);
+      if (mantissa <= binary_format<T>::max_mantissa_fast_path()) {
+        value = T(mantissa);
+        if (exponent < 0) {
+          value = value / binary_format<T>::exact_power_of_ten(-exponent);
         } else {
-          value = value * binary_format<T>::exact_power_of_ten(pns.exponent);
+          value = value * binary_format<T>::exact_power_of_ten(exponent);
         }
-        if (pns.negative) {
+        if (is_negative) {
           value = -value;
         }
-        return answer;
+        return true;
       }
     } else {
       // We do not have that fegetround() == FE_TONEAREST.
       // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's
       // proposal
-      if (pns.exponent >= 0 &&
-          pns.mantissa <=
-              binary_format<T>::max_mantissa_fast_path(pns.exponent)) {
+      if (exponent >= 0 &&
+          mantissa <= binary_format<T>::max_mantissa_fast_path(exponent)) {
 #if defined(__clang__) || defined(FASTFLOAT_32BIT)
         // Clang may map 0 to -0.0 when fegetround() == FE_DOWNWARD
-        if (pns.mantissa == 0) {
-          value = pns.negative ? T(-0.) : T(0.);
-          return answer;
+        if (mantissa == 0) {
+          value = is_negative ? T(-0.) : T(0.);
+          return true;
         }
 #endif
-        value = T(pns.mantissa) *
-                binary_format<T>::exact_power_of_ten(pns.exponent);
-        if (pns.negative) {
+        value = T(mantissa) * binary_format<T>::exact_power_of_ten(exponent);
+        if (is_negative) {
           value = -value;
         }
-        return answer;
+        return true;
       }
     }
   }
+  return false;
+}
+
+/**
+ * This function overload takes parsed_number_string_t structure that is created
+ * and populated either by from_chars_advanced function taking chars range and
+ * parsing options or other parsing custom function implemented by user.
+ */
+template <typename T, typename UC>
+FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
+from_chars_advanced(parsed_number_string_t<UC> &pns, T &value) noexcept {
+  static_assert(is_supported_float_type<T>::value,
+                "only some floating-point types are supported");
+  static_assert(is_supported_char_type<UC>::value,
+                "only char, wchar_t, char16_t and char32_t are supported");
+
+  from_chars_result_t<UC> answer;
+
+  answer.ec = std::errc(); // be optimistic
+  answer.ptr = pns.lastmatch;
+
+  if (!pns.too_many_digits &&
+      clinger_fast_path_impl(pns.mantissa, pns.exponent, pns.negative, value))
+    return answer;
+
   adjusted_mantissa am =
       compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
   if (pns.too_many_digits && am.power2 >= 0) {
@@ -336,6 +344,49 @@ from_chars(UC const *first, UC const *last, T &value, int base) noexcept {
   return from_chars_advanced(first, last, value, options);
 }
 
+FASTFLOAT_CONSTEXPR20 inline double
+integer_times_pow10(uint64_t mantissa, int decimal_exponent) noexcept {
+  double value;
+  if (clinger_fast_path_impl(mantissa, decimal_exponent, false, value))
+    return value;
+
+  adjusted_mantissa am =
+      compute_float<binary_format<double>>(decimal_exponent, mantissa);
+  to_float(false, am, value);
+  return value;
+}
+
+FASTFLOAT_CONSTEXPR20 inline double
+integer_times_pow10(int64_t mantissa, int decimal_exponent) noexcept {
+  const bool is_negative = mantissa < 0;
+  const uint64_t m = static_cast<uint64_t>(is_negative ? -mantissa : mantissa);
+
+  double value;
+  if (clinger_fast_path_impl(m, decimal_exponent, is_negative, value))
+    return value;
+
+  adjusted_mantissa am =
+      compute_float<binary_format<double>>(decimal_exponent, m);
+  to_float(is_negative, am, value);
+  return value;
+}
+
+// the following overloads are here to avoid surprising ambiguity for int,
+// unsigned, etc.
+template <typename Int>
+FASTFLOAT_CONSTEXPR20 inline typename std::enable_if<
+    std::is_integral<Int>::value && !std::is_signed<Int>::value, double>::type
+integer_times_pow10(Int mantissa, int decimal_exponent) noexcept {
+  return integer_times_pow10(static_cast<uint64_t>(mantissa), decimal_exponent);
+}
+
+template <typename Int>
+FASTFLOAT_CONSTEXPR20 inline typename std::enable_if<
+    std::is_integral<Int>::value && std::is_signed<Int>::value, double>::type
+integer_times_pow10(Int mantissa, int decimal_exponent) noexcept {
+  return integer_times_pow10(static_cast<int64_t>(mantissa), decimal_exponent);
+}
+
 template <typename T, typename UC>
 FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
 from_chars_int_advanced(UC const *first, UC const *last, T &value,

tests/CMakeLists.txt

@@ -61,6 +61,7 @@ fast_float_add_cpp_test(wide_char_test)
 fast_float_add_cpp_test(supported_chars_test)
 fast_float_add_cpp_test(example_test)
 fast_float_add_cpp_test(example_comma_test)
+fast_float_add_cpp_test(example_integer_times_pow10)
 fast_float_add_cpp_test(basictest)
 option(FASTFLOAT_CONSTEXPR_TESTS "Require constexpr tests (build will fail if the compiler won't support it)" OFF)
 if (FASTFLOAT_CONSTEXPR_TESTS)