[gnc-numeric.hpp] Keep the sign and leading zeroes in decimal output

operator<< printed the integer and fractional parts with separate
integer divisions, so 5/100 came out as "0.5", 105/100 as "1.5" and
-1/100 as "0.1": leading zeroes of the fraction were dropped and values
between -1 and 0 lost their sign. Print the magnitude and write each
fractional digit so the output matches the value.

libgnucash/engine/gnc-numeric.hpp

@@ -363,8 +363,19 @@ std::basic_ostream<charT, traits>& operator<<(std::basic_ostream<charT, traits>&
     if (n.denom() == 1)
          ss << n.num();
     else if (n.is_decimal())
-        ss << n.num() / n.denom() << dec_pt
-           << (n.num() > 0 ? n.num() : -n.num()) % n.denom();
+    {
+        /* Use the magnitude so that values between -1 and 0 keep their sign,
+         * and write each fractional digit so that leading zeroes are kept. */
+        auto num = n.num();
+        auto mag = num < 0 ? 0 - static_cast<uint64_t>(num)
+                           : static_cast<uint64_t>(num);
+        auto den = static_cast<uint64_t>(n.denom());
+        if (num < 0)
+            ss << static_cast<charT>('-');
+        ss << mag / den << dec_pt;
+        for (auto place = den / 10; place > 0; place /= 10)
+            ss << static_cast<charT>('0' + mag % den / place % 10);
+    }
     else
         ss << n.num() << "/" << n.denom();
     s << ss.str();

libgnucash/engine/test/gtest-gnc-numeric.cpp

@@ -328,6 +328,29 @@ TEST(gncnumeric_stream, output_stream)
     EXPECT_EQ("123/456", output.str());
 }
 
+TEST(gncnumeric_stream, output_stream_decimal_places)
+{
+    std::ostringstream output;
+    output << GncNumeric(5, 100);
+    EXPECT_EQ("0.05", output.str());
+    output.str("");
+    output << GncNumeric(105, 100);
+    EXPECT_EQ("1.05", output.str());
+    output.str("");
+    output << GncNumeric(-105, 100);
+    EXPECT_EQ("-1.05", output.str());
+    output.str("");
+    output << GncNumeric(-1, 100);
+    EXPECT_EQ("-0.01", output.str());
+    output.str("");
+    output << GncNumeric(-5, 10);
+    EXPECT_EQ("-0.5", output.str());
+    output.str("");
+    output << GncNumeric(INT64_MIN, 1000);
+    EXPECT_EQ("-9223372036854775.808", output.str());
+    EXPECT_EQ("0.007", GncNumeric(7, 1000).to_string());
+}
+
 TEST(gncnumeric_stream, input_stream)
 {
     std::istringstream input("123456");