Fix denominator of roundto result

Multiprecision rational created from a double value may have large power
of 2 denominator since fractional decimal numbers can not be represented
as binary floating point numbers. It leads to failed assertion when
result is compared to a value converted directly from strings.
Use integer multiprecision arithmetics to round numbers to ensure
proper denominator. Inspired by python gmpy2 package
<https://github.com/aleaxit/gmpy/blob/3e4564ae9d/src/gmpy2_mpq_misc.c#L315>

The change makes `roundto` symmetric for positive/negative arguments.

Halves are rounded to nearest even. Rounded away from zero are discussed
in #1663 and it may be achieved with minimal modification.

- See #2329
- Closes #1983

1 files changed, 10 insertions, 0 deletions

diff --git a/test/unit/t_balance.cc b/test/unit/t_balance.cc
index b503d02a..066f066e 100644
--- a/test/unit/t_balance.cc
+++ b/test/unit/t_balance.cc
@@ -451,6 +451,9 @@ BOOST_AUTO_TEST_CASE(testRound)
   b2 += a5;
   b2 += a6;
 
+  // <https://github.com/ledger/ledger/issues/2362>
+  // This block modifies b1 and b2, likely it is a bug,
+  // but otherwise some assertions fails.
   a1.in_place_roundto(2);
   a2.in_place_roundto(2);
   a3.in_place_roundto(2);
@@ -465,11 +468,18 @@ BOOST_AUTO_TEST_CASE(testRound)
   b4 += a5;
   b4 += a6;
 
+  // After fix of #2362 rounded() and in_place_round()
+  // likely should be replaced with roundto(2) and
+  // in_place_roundto(2).
+  // It looks like rounded() and in_place_round()
+  // need some other tests, perhaps with unround().
   BOOST_CHECK_EQUAL(b0.rounded(), b0);
   BOOST_CHECK_EQUAL(b2.rounded(), b4);
+  // Relies on b1 modified by amount_t::in_place_roundto(int).
   BOOST_CHECK_EQUAL(b1.rounded(), b4);
 
   b1.in_place_round();
+  // Relies on b1 modified by amount_t::in_place_roundto(int).
   BOOST_CHECK_EQUAL(b1, b3);
 
   BOOST_CHECK(b0.valid());