From 121ffc6fe574cc0df5c77f9010acff970aa89899 Mon Sep 17 00:00:00 2001
From: sigbjorn <sigbjorn.helset@gmail.com>
Date: Thu, 19 Sep 2024 16:50:02 +0200
Subject: [PATCH] Adding proposed fix, with tests, resolves #792

---
 .../spirit/home/karma/numeric/real_policies.hpp  | 16 ++++++++++++++--
 test/karma/real3.cpp                             | 10 ++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/boost/spirit/home/karma/numeric/real_policies.hpp b/include/boost/spirit/home/karma/numeric/real_policies.hpp
index a5c7099fba..974e60fb28 100644
--- a/include/boost/spirit/home/karma/numeric/real_policies.hpp
+++ b/include/boost/spirit/home/karma/numeric/real_policies.hpp
@@ -257,8 +257,20 @@ namespace boost { namespace spirit { namespace karma
             //    generate(sink, right_align(precision, '0')[ulong], n);
             // but it's spelled out to avoid inter-modular dependencies.
 
-            typename remove_const<T>::type digits = 
-                (traits::test_zero(n) ? 1 : ceil(log10(n + T(1.))));
+	    unsigned int digits=1; //should be number of digits n(truncating any fraction)
+	    if(!boost::spirit::traits::test_zero(n)) {
+	      static constexpr uint64_t limit = UINT64_MAX / 10;
+	      const T num = floor(n);
+	      for (uint64_t x = 10u, i = 1u;; x *= 10, i++) {
+		if (num < x) {
+		  digits=i;break;
+		}
+		if (x > limit) {
+		  digits= i + 1;break;
+		}
+	      }
+	    }
+	    
             bool r = true;
             for (/**/; r && digits < precision_; digits = digits + 1)
                 r = char_inserter<>::call(sink, '0');
diff --git a/test/karma/real3.cpp b/test/karma/real3.cpp
index 2b6a014503..ab264388a5 100644
--- a/test/karma/real3.cpp
+++ b/test/karma/real3.cpp
@@ -198,6 +198,16 @@ int main()
             ));
     }
 
+    // test for #792: off by a magnitude due to fraction_part
+    // notice that it applies to all forms of fractions
+    // that end with a sequence of 0.009999
+    {
+        BOOST_TEST(test("1.09e-06",
+			double_,
+            1.09e-06
+            ));
+    }
+
     return boost::report_errors();
 }