From 9f7c2c04c84982403db4b75998e346a5d2afdbca Mon Sep 17 00:00:00 2001
From: abolz <lt.morris.schaffer@googlemail.com>
Date: Mon, 15 Jan 2018 20:58:02 +0100
Subject: [PATCH] Use the Grisu2 algorithm for formatting 'float's and
 'double's

---
 develop/detail/serializer.hpp | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/develop/detail/serializer.hpp b/develop/detail/serializer.hpp
index b5f2b06a..59b9a606 100644
--- a/develop/detail/serializer.hpp
+++ b/develop/detail/serializer.hpp
@@ -14,6 +14,7 @@
 #include <string> // string
 #include <type_traits> // is_same
 
+#include "detail/conversions/to_chars.hpp"
 #include "detail/macro_scope.hpp"
 #include "detail/meta.hpp"
 #include "detail/parsing/output_adapters.hpp"
@@ -636,12 +637,34 @@ class serializer
     void dump_float(number_float_t x)
     {
         // NaN / inf
-        if (not std::isfinite(x) or std::isnan(x))
+        if (not std::isfinite(x))
         {
             o->write_characters("null", 4);
             return;
         }
 
+        // If number_float_t is an IEEE-754 single or double precision number,
+        // use the Grisu2 algorithm to produce short numbers which are guaranteed
+        // to round-trip, using strtof and strtod, resp.
+        //
+        // NB: The test below works if <long double> == <double>.
+        static constexpr bool is_ieee_single_or_double
+            = (std::numeric_limits<number_float_t>::is_iec559 and std::numeric_limits<number_float_t>::digits == 24 and std::numeric_limits<number_float_t>::max_exponent == 128) or
+              (std::numeric_limits<number_float_t>::is_iec559 and std::numeric_limits<number_float_t>::digits == 53 and std::numeric_limits<number_float_t>::max_exponent == 1024);
+
+        dump_float(x, std::integral_constant<bool, is_ieee_single_or_double>());
+    }
+
+    void dump_float(number_float_t x, std::true_type /*is_ieee_single_or_double*/)
+    {
+        char* begin = number_buffer.data();
+        char* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x);
+
+        o->write_characters(begin, static_cast<size_t>(end - begin));
+    }
+
+    void dump_float(number_float_t x, std::false_type /*is_ieee_single_or_double*/)
+    {
         // get number of digits for a text -> float -> text round-trip
         static constexpr auto d = std::numeric_limits<number_float_t>::digits10;