Merge pull request #269 from boostorg/267

Fix all 9s rounding on the right side of the decimal point with fixed precision

Author: mborland

include/boost/charconv/detail/dragonbox/floff.hpp

@@ -4018,9 +4018,10 @@ BOOST_CHARCONV_SAFEBUFFERS to_chars_result floff(const double x, int precision,
                 ++decimal_dot_pos;
             }
         }
-        else if (decimal_exponent_normalized == 0)
+        else if (decimal_exponent_normalized == 0 || remaining_digits == 1)
         {
             // For the case 0.99...9 -> 1.00...0, the rounded digit is one before the first digit written.
+            // This same case applies for 0.099 -> 0.10 in the precision = 2 instance
             // Note: decimal_exponent_normalized was negative before the increment (++decimal_exponent_normalized),
             //       so we already have printed "00" onto the buffer.
             //       Hence, --digit_starting_pos doesn't go more than the starting position of the buffer.

test/Jamfile

@@ -70,3 +70,4 @@ run github_issue_166.cpp ;
 run github_issue_166_float128.cpp ;
 run github_issue_186.cpp ;
 run github_issue_212.cpp ;
+run github_issue_267.cpp ;

test/github_issue_267.cpp

@@ -0,0 +1,35 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// See: https://github.com/boostorg/charconv/issues/267
+
+#include <boost/charconv.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename T>
+void test(T value, int precision, const char* correct_result)
+{
+    char buffer[64] {};
+    const auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), value, boost::charconv::chars_format::fixed, precision);
+    BOOST_TEST_CSTR_EQ(buffer, correct_result);
+    BOOST_TEST(r);
+}
+
+int main()
+{
+    test(0.09, 2, "0.09");
+    test(0.099, 2, "0.10");
+    test(0.0999, 2, "0.10");
+    test(0.09999, 2, "0.10");
+    test(0.099999, 2, "0.10");
+    test(0.0999999, 2, "0.10");
+    test(0.09999999, 2, "0.10");
+    test(0.099999999, 2, "0.10");
+    test(0.0999999999, 2, "0.10");
+    test(0.09999999999, 2, "0.10");
+    test(0.099999999999, 2, "0.10");
+    test(0.0999999999999, 2, "0.10");
+
+    return boost::report_errors();
+}