Remove the kMaxRep+1 rounding tests

- Use more of the available range of the uint128 operands.
- Also refactor Number::Guard::round() to return an enum.

include/xrpl/basics/Number.h

@@ -51,37 +51,43 @@ namespace detail {
  * compile time. Doing it at runtime would be pretty wasteful and
  * inefficient.
  */
-constexpr std::size_t kInt64Digits = 20;
-consteval std::array<std::uint64_t, kInt64Digits>
+constexpr std::size_t kUint64Digits = 20;
+constexpr std::size_t kUint128Digits = 39;
+
+template <typename T, std::size_t digits>
+consteval std::array<T, digits>
 buildPowersOfTen()
 {
-    std::array<std::uint64_t, kInt64Digits> result{};
+    std::array<T, digits> result{};
 
-    std::uint64_t power = 1;
+    T power = 1;
     std::size_t exponent = 0;
     // end the loop early so it doesn't overflow;
     for (; exponent < result.size() - 1; ++exponent, power *= 10)
     {
         result[exponent] = power;
-        if (power > std::numeric_limits<std::uint64_t>::max() / 10)
+        if (power > std::numeric_limits<T>::max() / 10)
             throw std::logic_error("Power of 10 table is too big");
     }
     result[exponent] = power;
-    if (power < std::numeric_limits<std::uint64_t>::max() / 10)
-        throw std::logic_error("Power of 10 table is not big enough for the uint64_t type");
+    if (power < std::numeric_limits<T>::max() / 10)
+        throw std::logic_error("Power of 10 table is not big enough for the given type");
 
     return result;
 }
 
 }  // namespace detail
 
-constexpr std::array<std::uint64_t, detail::kInt64Digits> kPowerOfTen = detail::buildPowersOfTen();
+template <typename T = std::uint64_t, std::size_t digits = detail::kUint64Digits>
+constexpr std::array<T, digits> kPowerOfTenImpl = detail::buildPowersOfTen<T, digits>();
+
+constexpr auto kPowerOfTen = kPowerOfTenImpl<std::uint64_t, detail::kUint64Digits>;
 
 static_assert(kPowerOfTen[0] == 1);
 static_assert(kPowerOfTen[1] == 10);
 static_assert(kPowerOfTen[10] == 10'000'000'000);
 static_assert(
-    isPowerOfTen(kPowerOfTen.back()) && *logTen(kPowerOfTen.back()) == detail::kInt64Digits - 1);
+    isPowerOfTen(kPowerOfTen.back()) && *logTen(kPowerOfTen.back()) == detail::kUint64Digits - 1);
 
 /** MantissaRange defines a range for the mantissa of a normalized Number.
  *

src/libxrpl/basics/Number.cpp

@@ -206,10 +206,16 @@ public:
     void
     doDropDigit(T& mantissa, int& exponent) noexcept;
 
+    enum class Round {
+        Down = -1,
+        Even = 0,
+        Up = 1,
+    };
+
     // Indicate round direction:  1 is up, -1 is down, 0 is even
     // This enables the client to round towards nearest, and on
     // tie, round towards even.
-    [[nodiscard]] int
+    [[nodiscard]] Round
     round() const noexcept;
 
     // Modify the result to the correctly rounded value
@@ -314,41 +320,41 @@ Number::Guard::doDropDigit<uint128_t>(uint128_t& mantissa, int& exponent) noexce
 //     -1 if Guard is less than half
 //      0 if Guard is exactly half
 //      1 if Guard is greater than half
-int
+Number::Guard::Round
 Number::Guard::round() const noexcept
 {
     auto mode = Number::getround();
 
     if (mode == RoundingMode::TowardsZero)
-        return -1;
+        return Round::Down;
 
     if (mode == RoundingMode::Downward)
     {
         if (sbit_)
         {
             if (digits_ > 0 || xbit_)
-                return 1;
+                return Round::Up;
         }
-        return -1;
+        return Round::Down;
     }
 
     if (mode == RoundingMode::Upward)
     {
         if (sbit_)
-            return -1;
+            return Round::Down;
         if (digits_ > 0 || xbit_)
-            return 1;
-        return -1;
+            return Round::Up;
+        return Round::Down;
     }
 
     // assume round to nearest if mode is not one of the predefined values
     if (digits_ > 0x5000'0000'0000'0000)
-        return 1;
+        return Round::Up;
     if (digits_ < 0x5000'0000'0000'0000)
-        return -1;
+        return Round::Down;
     if (xbit_)
-        return 1;
-    return 0;
+        return Round::Up;
+    return Round::Even;
 }
 
 template <UnsignedMantissa T>
@@ -388,7 +394,7 @@ Number::Guard::doRoundUp(
     std::string location)
 {
     auto r = round();
-    if (r == 1 || (r == 0 && (mantissa & 1) == 1))
+    if (r == Round::Up || (r == Round::Even && (mantissa & 1) == 1))
     {
         auto const safeToIncrement = [&maxMantissa](auto const& mantissa) {
             return mantissa < maxMantissa && mantissa < kMaxRep;
@@ -454,7 +460,7 @@ Number::Guard::doRoundDown(
     internalrep const& minMantissa)
 {
     auto r = round();
-    if (r == 1 || (r == 0 && (mantissa & 1) == 1))
+    if (r == Round::Up || (r == Round::Even && (mantissa & 1) == 1))
     {
         --mantissa;
         if (mantissa < minMantissa)
@@ -471,7 +477,7 @@ void
 Number::Guard::doRound(rep& drops, std::string location) const
 {
     auto r = round();
-    if (r == 1 || (r == 0 && (drops & 1) == 1))
+    if (r == Round::Up || (r == Round::Even && (drops & 1) == 1))
     {
         if (drops >= kMaxRep)
         {
@@ -720,26 +726,54 @@ Number::operator+=(Number const& y)
     uint128_t ym = y.mantissa_;
     auto ye = y.exponent_;
     Guard g;
+
+    auto const& range = kRange.get();
+
+    // Bring the exponents of both values into agreement, so the mantissas are on the same scale
+    //   and can be added directly together
+    // expandM / expandE: First try to expand the mantissa and bring the exponent down
+    // shringM / shrinkE: Then shrink the mantissa and bring the exponent up, if necessary
+    auto const adjust = [&g, &range](
+                            uint128_t& expandM, int& expandE, uint128_t& shrinkM, int& shrinkE) {
+        constexpr uint128_t kSafeLimit = kPowerOfTenImpl<uint128_t, detail::kUint128Digits>[37];
+
+        if (range.cuspRoundingFixEnabled == MantissaRange::CuspRoundingFix::Enabled)
+        {
+            while (shrinkE < expandE && shrinkM % 10 == 0)
+            {
+                g.doDropDigit(shrinkM, shrinkE);
+            }
+
+            // We've got 128 bits of mantissa to work with here. Don't throw away data unless we
+            // have to
+            while (shrinkE < expandE && expandE > kMinExponent && expandM < kSafeLimit)
+            {
+                expandM *= 10;
+                --expandE;
+            }
+        }
+
+        while (shrinkE < expandE)
+        {
+            g.doDropDigit(shrinkM, shrinkE);
+        }
+    };
+
     if (xe < ye)
     {
         if (xn)
             g.setNegative();
-        do
-        {
-            g.doDropDigit(xm, xe);
-        } while (xe < ye);
+
+        adjust(ym, ye, xm, xe);
     }
     else if (xe > ye)
     {
         if (yn)
             g.setNegative();
-        do
-        {
-            g.doDropDigit(ym, ye);
-        } while (xe > ye);
+
+        adjust(xm, xe, ym, ye);
     }
 
-    auto const& range = kRange.get();
     auto const& minMantissa = range.min;
     auto const& maxMantissa = range.max;
     auto const cuspRoundingFixEnabled = range.cuspRoundingFixEnabled;
@@ -747,9 +781,19 @@ Number::operator+=(Number const& y)
     if (xn == yn)
     {
         xm += ym;
-        if (xm > maxMantissa || xm > kMaxRep)
+        if (range.cuspRoundingFixEnabled == MantissaRange::CuspRoundingFix::Enabled)
         {
-            g.doDropDigit(xm, xe);
+            while (xm > maxMantissa || xm > kMaxRep)
+            {
+                g.doDropDigit(xm, xe);
+            }
+        }
+        else
+        {
+            if (xm > maxMantissa || xm > kMaxRep)
+            {
+                g.doDropDigit(xm, xe);
+            }
         }
         g.doRoundUp(
             xn,
@@ -779,6 +823,25 @@ Number::operator+=(Number const& y)
             --xe;
         }
         g.doRoundDown(xn, xm, xe, minMantissa);
+        if (range.cuspRoundingFixEnabled == MantissaRange::CuspRoundingFix::Enabled && xm != 0)
+        {
+            // make a new guard
+            Guard g;
+            if (xn)
+                g.setNegative();
+            while (xm > maxMantissa || xm > kMaxRep)
+            {
+                g.doDropDigit(xm, xe);
+            }
+            g.doRoundUp(
+                xn,
+                xm,
+                xe,
+                minMantissa,
+                maxMantissa,
+                cuspRoundingFixEnabled,
+                "Number::addition overflow");
+        }
     }
 
     negative_ = xn;

src/test/basics/Number_test.cpp

@@ -43,6 +43,15 @@ class Number_test : public beast::unit_test::Suite
         return out;
     }
 
+    static BigInt
+    toBigInt(Number const& n)
+    {
+        BigInt v = n.mantissa();
+        for (int i = 0; i < n.exponent(); ++i)
+            v *= 10;
+        return v;
+    }
+
     using dec = boost::multiprecision::cpp_dec_float_50;
 
     template <class T = dec>
@@ -169,28 +178,35 @@ public:
         auto const scale = Number::getMantissaScale();
         testcase << "test_add " << to_string(scale);
 
-        using Case = std::tuple<Number, Number, Number>;
-        auto const cSmall = std::to_array<Case>(
-            {{Number{1'000'000'000'000'000, -15},
-              Number{6'555'555'555'555'555, -29},
-              Number{1'000'000'000'000'066, -15}},
-             {Number{-1'000'000'000'000'000, -15},
-              Number{-6'555'555'555'555'555, -29},
-              Number{-1'000'000'000'000'066, -15}},
-             {Number{-1'000'000'000'000'000, -15},
-              Number{6'555'555'555'555'555, -29},
-              Number{-9'999'999'999'999'344, -16}},
-             {Number{-6'555'555'555'555'555, -29},
-              Number{1'000'000'000'000'000, -15},
-              Number{9'999'999'999'999'344, -16}},
-             {Number{}, Number{5}, Number{5}},
-             {Number{5}, Number{}, Number{5}},
-             {Number{5'555'555'555'555'555, -32768},
-              Number{-5'555'555'555'555'554, -32768},
-              Number{0}},
-             {Number{-9'999'999'999'999'999, -31},
-              Number{1'000'000'000'000'000, -15},
-              Number{9'999'999'999'999'990, -16}}});
+        using Case = std::tuple<Number, Number, Number, int>;
+        auto const cSmall = std::to_array<Case>({
+            {Number{1'000'000'000'000'000, -15},
+             Number{6'555'555'555'555'555, -29},
+             Number{1'000'000'000'000'066, -15},
+             __LINE__},
+            {Number{-1'000'000'000'000'000, -15},
+             Number{-6'555'555'555'555'555, -29},
+             Number{-1'000'000'000'000'066, -15},
+             __LINE__},
+            {Number{-1'000'000'000'000'000, -15},
+             Number{6'555'555'555'555'555, -29},
+             Number{-9'999'999'999'999'344, -16},
+             __LINE__},
+            {Number{-6'555'555'555'555'555, -29},
+             Number{1'000'000'000'000'000, -15},
+             Number{9'999'999'999'999'344, -16},
+             __LINE__},
+            {Number{}, Number{5}, Number{5}, __LINE__},
+            {Number{5}, Number{}, Number{5}, __LINE__},
+            {Number{5'555'555'555'555'555, -32768},
+             Number{-5'555'555'555'555'554, -32768},
+             Number{0},
+             __LINE__},
+            {Number{-9'999'999'999'999'999, -31},
+             Number{1'000'000'000'000'000, -15},
+             Number{9'999'999'999'999'990, -16},
+             __LINE__},
+        });
         auto const cLarge = std::to_array<Case>(
             // Note that items with extremely large mantissas need to be
             // calculated, because otherwise they overflow uint64. Items from C
@@ -198,45 +214,57 @@ public:
             {
                 {Number{1'000'000'000'000'000, -15},
                  Number{6'555'555'555'555'555, -29},
-                 Number{1'000'000'000'000'065'556, -18}},
+                 Number{1'000'000'000'000'065'556, -18},
+                 __LINE__},
                 {Number{-1'000'000'000'000'000, -15},
                  Number{-6'555'555'555'555'555, -29},
-                 Number{-1'000'000'000'000'065'556, -18}},
+                 Number{-1'000'000'000'000'065'556, -18},
+                 __LINE__},
                 {Number{-1'000'000'000'000'000, -15},
                  Number{6'555'555'555'555'555, -29},
-                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
+                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
                 {Number{-6'555'555'555'555'555, -29},
                  Number{1'000'000'000'000'000, -15},
-                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
-                {Number{}, Number{5}, Number{5}},
-                {Number{5}, Number{}, Number{5}},
+                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
+                {Number{}, Number{5}, Number{5}, __LINE__},
+                {Number{5}, Number{}, Number{5}, __LINE__},
                 {Number{5'555'555'555'555'555'000, -32768},
                  Number{-5'555'555'555'555'554'000, -32768},
-                 Number{0}},
+                 Number{0},
+                 __LINE__},
                 {Number{-9'999'999'999'999'999, -31},
                  Number{1'000'000'000'000'000, -15},
-                 Number{9'999'999'999'999'990, -16}},
+                 Number{9'999'999'999'999'990, -16},
+                 __LINE__},
                 // Items from cSmall expanded for the larger mantissa
                 {Number{1'000'000'000'000'000'000, -18},
                  Number{6'555'555'555'555'555'555, -35},
-                 Number{1'000'000'000'000'000'066, -18}},
+                 Number{1'000'000'000'000'000'066, -18},
+                 __LINE__},
                 {Number{-1'000'000'000'000'000'000, -18},
                  Number{-6'555'555'555'555'555'555, -35},
-                 Number{-1'000'000'000'000'000'066, -18}},
+                 Number{-1'000'000'000'000'000'066, -18},
+                 __LINE__},
                 {Number{-1'000'000'000'000'000'000, -18},
                  Number{6'555'555'555'555'555'555, -35},
-                 Number{true, 9'999'999'999'999'999'344ULL, -19, Number::Normalized{}}},
+                 Number{true, 9'999'999'999'999'999'344ULL, -19, Number::Normalized{}},
+                 __LINE__},
                 {Number{-6'555'555'555'555'555'555, -35},
                  Number{1'000'000'000'000'000'000, -18},
-                 Number{false, 9'999'999'999'999'999'344ULL, -19, Number::Normalized{}}},
-                {Number{}, Number{5}, Number{5}},
+                 Number{false, 9'999'999'999'999'999'344ULL, -19, Number::Normalized{}},
+                 __LINE__},
+                {Number{}, Number{5}, Number{5}, __LINE__},
                 {Number{5'555'555'555'555'555'555, -32768},
                  Number{-5'555'555'555'555'555'554, -32768},
-                 Number{0}},
+                 Number{0},
+                 __LINE__},
                 {Number{true, 9'999'999'999'999'999'999ULL, -37, Number::Normalized{}},
                  Number{1'000'000'000'000'000'000, -18},
-                 Number{false, 9'999'999'999'999'999'990ULL, -19, Number::Normalized{}}},
-                {Number{Number::kMaxRep - 1}, Number{1, 0}, Number{Number::kMaxRep}},
+                 Number{false, 9'999'999'999'999'999'990ULL, -19, Number::Normalized{}},
+                 __LINE__},
+                {Number{Number::kMaxRep - 1}, Number{1, 0}, Number{Number::kMaxRep}, __LINE__},
                 // Test extremes
                 {
                     // Each Number operand rounds up, so the actual mantissa is
@@ -244,6 +272,7 @@ public:
                     Number{false, 9'999'999'999'999'999'999ULL, 0, Number::Normalized{}},
                     Number{false, 9'999'999'999'999'999'999ULL, 0, Number::Normalized{}},
                     Number{2, 19},
+                    __LINE__,
                 },
                 {
                     // Does not round. Mantissas are going to be > kMaxRep, so if
@@ -254,21 +283,25 @@ public:
                     Number{false, 9'999'999'999'999'999'990ULL, 0, Number::Normalized{}},
                     Number{false, 9'999'999'999'999'999'990ULL, 0, Number::Normalized{}},
                     Number{false, 1'999'999'999'999'999'998ULL, 1, Number::Normalized{}},
+                    __LINE__,
                 },
             });
         auto const cLargeLegacy = std::to_array<Case>({
-            {Number{Number::kMaxRep}, Number{6, -1}, Number{Number::kMaxRep / 10, 1}},
+            {Number{Number::kMaxRep}, Number{6, -1}, Number{Number::kMaxRep / 10, 1}, __LINE__},
         });
         auto const cLargeCorrected = std::to_array<Case>({
-            {Number{Number::kMaxRep}, Number{6, -1}, Number{(Number::kMaxRep / 10) + 1, 1}},
+            {Number{Number::kMaxRep},
+             Number{6, -1},
+             Number{(Number::kMaxRep / 10) + 1, 1},
+             __LINE__},
         });
         auto test = [this](auto const& c) {
-            for (auto const& [x, y, z] : c)
+            for (auto const& [x, y, z, line] : c)
             {
                 auto const result = x + y;
                 std::stringstream ss;
                 ss << x << " + " << y << " = " << result << ". Expected: " << z;
-                BEAST_EXPECTS(result == z, ss.str());
+                expect(result == z, ss.str(), __FILE__, line);
             }
         };
         if (scale == MantissaRange::MantissaScale::Small)
@@ -308,21 +341,28 @@ public:
         auto const scale = Number::getMantissaScale();
         testcase << "test_sub " << to_string(scale);
 
-        using Case = std::tuple<Number, Number, Number>;
+        using Case = std::tuple<Number, Number, Number, int>;
         auto const cSmall = std::to_array<Case>(
             {{Number{1'000'000'000'000'000, -15},
               Number{6'555'555'555'555'555, -29},
-              Number{9'999'999'999'999'344, -16}},
+              Number{9'999'999'999'999'344, -16},
+              __LINE__},
              {Number{6'555'555'555'555'555, -29},
               Number{1'000'000'000'000'000, -15},
-              Number{-9'999'999'999'999'344, -16}},
-             {Number{1'000'000'000'000'000, -15}, Number{1'000'000'000'000'000, -15}, Number{0}},
+              Number{-9'999'999'999'999'344, -16},
+              __LINE__},
+             {Number{1'000'000'000'000'000, -15},
+              Number{1'000'000'000'000'000, -15},
+              Number{0},
+              __LINE__},
              {Number{1'000'000'000'000'000, -15},
               Number{1'000'000'000'000'001, -15},
-              Number{-1'000'000'000'000'000, -30}},
+              Number{-1'000'000'000'000'000, -30},
+              __LINE__},
              {Number{1'000'000'000'000'001, -15},
               Number{1'000'000'000'000'000, -15},
-              Number{1'000'000'000'000'000, -30}}});
+              Number{1'000'000'000'000'000, -30},
+              __LINE__}});
         auto const cLarge = std::to_array<Case>(
             // Note that items with extremely large mantissas need to be
             // calculated, because otherwise they overflow uint64. Items from C
@@ -330,49 +370,63 @@ public:
             {
                 {Number{1'000'000'000'000'000, -15},
                  Number{6'555'555'555'555'555, -29},
-                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
+                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
                 {Number{6'555'555'555'555'555, -29},
                  Number{1'000'000'000'000'000, -15},
-                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
-                {Number{1'000'000'000'000'000, -15}, Number{1'000'000'000'000'000, -15}, Number{0}},
+                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
+                {Number{1'000'000'000'000'000, -15},
+                 Number{1'000'000'000'000'000, -15},
+                 Number{0},
+                 __LINE__},
                 {Number{1'000'000'000'000'000, -15},
                  Number{1'000'000'000'000'001, -15},
-                 Number{-1'000'000'000'000'000, -30}},
+                 Number{-1'000'000'000'000'000, -30},
+                 __LINE__},
                 {Number{1'000'000'000'000'001, -15},
                  Number{1'000'000'000'000'000, -15},
-                 Number{1'000'000'000'000'000, -30}},
+                 Number{1'000'000'000'000'000, -30},
+                 __LINE__},
                 // Items from cSmall expanded for the larger mantissa
                 {Number{1'000'000'000'000'000'000, -18},
                  Number{6'555'555'555'555'555'555, -32},
-                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
+                 Number{false, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
                 {Number{6'555'555'555'555'555'555, -32},
                  Number{1'000'000'000'000'000'000, -18},
-                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}}},
+                 Number{true, 9'999'999'999'999'344'444ULL, -19, Number::Normalized{}},
+                 __LINE__},
                 {Number{1'000'000'000'000'000'000, -18},
                  Number{1'000'000'000'000'000'000, -18},
-                 Number{0}},
+                 Number{0},
+                 __LINE__},
                 {Number{1'000'000'000'000'000'000, -18},
                  Number{1'000'000'000'000'000'001, -18},
-                 Number{-1'000'000'000'000'000'000, -36}},
+                 Number{-1'000'000'000'000'000'000, -36},
+                 __LINE__},
                 {Number{1'000'000'000'000'000'001, -18},
                  Number{1'000'000'000'000'000'000, -18},
-                 Number{1'000'000'000'000'000'000, -36}},
-                {Number{Number::kMaxRep}, Number{6, -1}, Number{Number::kMaxRep - 1}},
+                 Number{1'000'000'000'000'000'000, -36},
+                 __LINE__},
+                {Number{Number::kMaxRep}, Number{6, -1}, Number{Number::kMaxRep - 1}, __LINE__},
                 {Number{false, Number::kMaxRep + 1, 0, Number::Normalized{}},
                  Number{1, 0},
-                 Number{(Number::kMaxRep / 10) + 1, 1}},
+                 Number{(Number::kMaxRep / 10) + 1, 1},
+                 __LINE__},
                 {Number{false, Number::kMaxRep + 1, 0, Number::Normalized{}},
                  Number{3, 0},
-                 Number{Number::kMaxRep}},
-                {power(2, 63), Number{3, 0}, Number{Number::kMaxRep}},
+                 Number{Number::kMaxRep},
+                 __LINE__},
+                {power(2, 63), Number{3, 0}, Number{Number::kMaxRep}, __LINE__},
             });
         auto test = [this](auto const& c) {
-            for (auto const& [x, y, z] : c)
+            for (auto const& [x, y, z, line] : c)
             {
                 auto const result = x - y;
                 std::stringstream ss;
                 ss << x << " - " << y << " = " << result << ". Expected: " << z;
-                BEAST_EXPECTS(result == z, ss.str());
+                expect(result == z, ss.str(), __FILE__, line);
             }
         };
         if (scale == MantissaRange::MantissaScale::Small)
@@ -1644,9 +1698,7 @@ public:
             BigInt const exactProduct = BigInt(kAValue) * BigInt(kBValue);
 
             // What Number actually stored.
-            BigInt storedValue = BigInt(product.mantissa());
-            for (int i = 0; i < product.exponent(); ++i)
-                storedValue *= 10;
+            BigInt storedValue = toBigInt(product);
 
             BigInt const signedDifference = storedValue - exactProduct;
 
@@ -1869,6 +1921,49 @@ public:
                     break;
             }
         }
+
+        {
+            testcase << "operator+ TowardsZero rounds away from zero " << to_string(scale);
+
+            Number const a{1LL, 20};
+            Number const b{-1'000'000'000'000'000'001LL};
+
+            BEAST_EXPECT(toBigInt(a) == BigInt{"100000000000000000000"});
+            if (scale != MantissaRange::MantissaScale::Small)
+                BEAST_EXPECT(toBigInt(b) == BigInt{"-1000000000000000001"});
+            else
+                BEAST_EXPECT(toBigInt(b) == BigInt{"-1000000000000000000"});
+
+            Number sum;
+            {
+                NumberRoundModeGuard const roundGuard{Number::RoundingMode::TowardsZero};
+                sum = a + b;
+            }
+
+            BigInt const exact = toBigInt(a) + toBigInt(b);
+            BigInt const stored = toBigInt(sum);
+            BigInt const diff = stored - exact;
+
+            log << "\n              a = " << a << "\n              b = " << b
+                << "\n    exact a + b = " << exact.str() << "\n    TowardsZero = " << stored.str()
+                << "\n     difference = " << diff.str() << "\n";
+            log.flush();
+
+            if (scale == MantissaRange::MantissaScale::Small)
+            {
+                BEAST_EXPECT(stored == exact);
+            }
+            else if (scale == MantissaRange::MantissaScale::LargeLegacy)
+            {
+                BEAST_EXPECT(stored > exact);
+            }
+            else
+            {
+                BEAST_EXPECT(stored < exact);
+                BEAST_EXPECT(diff < 0);
+                BEAST_EXPECT(-diff < pow10<BigInt>(sum.exponent()));
+            }
+        }
     }
 
     void