Fix fixed precision case where we are short zero to the right of the decimal point

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

@@ -1333,6 +1333,7 @@ BOOST_CHARCONV_SAFEBUFFERS to_chars_result floff(const double x, int precision,
 
     auto buffer_size = static_cast<std::size_t>(last - first);
     auto buffer = first;
+    bool trailing_zeros_removed = false;
 
     BOOST_CHARCONV_ASSERT(precision >= 0);
     using namespace detail;
@@ -3849,6 +3850,7 @@ BOOST_CHARCONV_SAFEBUFFERS to_chars_result floff(const double x, int precision,
         }
 
         // Remove trailing zeros.
+        trailing_zeros_removed = true;
         while (true)
         {
             auto prev = buffer - 1;
@@ -3903,7 +3905,16 @@ BOOST_CHARCONV_SAFEBUFFERS to_chars_result floff(const double x, int precision,
             print_2_digits(static_cast<std::uint32_t>(decimal_exponent_normalized), buffer);
             buffer += 2;
         }
-    }    
+    }
+    else if (!trailing_zeros_removed && buffer - (decimal_dot_pos + 1) < precision)
+    {
+        // If we have fixed precision, and we don't have enough digits after the decimal yet
+        // insert a sufficient amount of zeros
+        const auto remaining_zeros = precision - (buffer - (decimal_dot_pos + 1));
+        BOOST_CHARCONV_ASSERT(remaining_zeros > 0);
+        std::memset(buffer, '0', static_cast<std::size_t>(remaining_zeros));
+        buffer += remaining_zeros;
+    }
 
     return {buffer, std::errc()};
 

test/Jamfile

@@ -70,4 +70,5 @@ run github_issue_166.cpp ;
 run github_issue_166_float128.cpp ;
 run github_issue_186.cpp ;
 run github_issue_212.cpp ;
+run github_issue_266.cpp ;
 run github_issue_267.cpp ;

test/github_issue_266.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/266
+
+#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(123456789012345.6789012345678901, 5, "123456789012345.67188");
+    test(1234567890123456.789012345678901, 5, "1234567890123456.75000");
+    test(12345678901234567.89012345678901, 5, "12345678901234568.00000");
+    test(123456789012345678.9012345678901, 5, "123456789012345680.00000");
+    test(1234567890123456789.012345678901, 5, "1234567890123456768.00000");
+
+    test(123456789012345678901234567.8901, 5, "123456789012345678152597504.00000");
+    test(12345678901234567890123456.78901, 5, "12345678901234568244756480.00000");
+    test(1234567890123456789012345.678901, 5, "1234567890123456824475648.00000");
+    test(123456789012345678901234.5678901, 5, "123456789012345685803008.00000");
+    test(12345678901234567890123.45678901, 5, "12345678901234567741440.00000");
+    test(123456789012345678901.2345678901, 5, "123456789012345683968.00000");
+
+    return boost::report_errors();
+}