Use header-only fast_float library instead of std::from_chars due…

… to incomplete standard library implementations

LICENSE

@@ -251,3 +251,33 @@ ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+-------------------------------------------------------------------------------
+
+All files in symengine/utilities/fast_float were taken from the fastfloat library
+and are licensed under the MIT License:
+
+Copyright (c) 2021 The fast_float authors
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

symengine/CMakeLists.txt

@@ -248,6 +248,7 @@ set(HEADERS
     refine.h
     simplify.h
     utilities/stream_fmt.h
+    utilities/fast_float/fast_float.h
     tuple.h
     matrix_expressions.h
     matrices/matrix_expr.h

symengine/parser/parser.cpp

@@ -4,29 +4,11 @@
 #include <symengine/real_mpfr.h>
 #include <symengine/ntheory_funcs.h>
 #include <symengine/parser/tokenizer.h>
-#if ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
-#include <charconv>
-#endif
+#include <symengine/utilities/fast_float/fast_float.h>
 
 namespace SymEngine
 {
 
-static double string_to_double(const char *str, char **endptr)
-{
-#if ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
-    // use std::from_chars: locale-independent, non-allocating, non-throwing
-    double result;
-    auto [ptr, ec] = std::from_chars(str, nullptr, result);
-    if (endptr != nullptr) {
-        *endptr = const_cast<char *>(ptr);
-    }
-    return result;
-#else
-    // otherwise fall-back to using strtod
-    return std::strtod(str, endptr);
-#endif
-}
-
 RCP<const Basic>
 parse(const std::string &s, bool convert_xor,
       const std::map<const std::string, const RCP<const Basic>> &constants)
@@ -329,8 +311,8 @@ RCP<const Basic> Parser::parse_numeric(const std::string &expr)
             }
         }
         if (digits <= 15) {
-            char *endptr = 0;
-            double d = string_to_double(startptr, &endptr);
+            double d;
+            fast_float::from_chars(startptr, startptr + expr.size(), d);
             return real_double(d);
         } else {
             // mpmath.libmp.libmpf.dps_to_prec
@@ -339,8 +321,8 @@ RCP<const Basic> Parser::parse_numeric(const std::string &expr)
             return real_mpfr(mpfr_class(expr, prec));
         }
 #else
-        char *endptr = 0;
-        double d = string_to_double(startptr, &endptr);
+        double d;
+        fast_float::from_chars(startptr, startptr + expr.size(), d);
         return real_double(d);
 #endif
     }
@@ -350,8 +332,9 @@ std::tuple<RCP<const Basic>, RCP<const Basic>>
 Parser::parse_implicit_mul(const std::string &expr)
 {
     const char *startptr = expr.c_str();
-    char *endptr = 0;
-    string_to_double(startptr, &endptr);
+    double result;
+    const char *endptr
+        = fast_float::from_chars(startptr, startptr + expr.size(), result).ptr;
 
     RCP<const Basic> num = one, sym;