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Improve accuracy of Number::operator+=

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- Use more of the available range of the uint128 operands.
- Also refactor Number::Guard::round() to return an enum.
This commit is contained in:
Ed Hennis
2026-06-02 15:35:42 -04:00
parent 35bee87909
commit 9e8c3caef4
2 changed files with 67 additions and 46 deletions
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38
src/libxrpl/basics/Number.cpp
View File

@@ -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)
{
@@ -817,7 +823,7 @@ Number::operator+=(Number const& y)
--xe;
}
g.doRoundDown(xn, xm, xe, minMantissa);
if (range.cuspRoundingFixEnabled == MantissaRange::CuspRoundingFix::Enabled)
if (range.cuspRoundingFixEnabled == MantissaRange::CuspRoundingFix::Enabled && xm != 0)
{
// make a new guard
Guard g;

75
src/test/basics/Number_test.cpp
View File

@@ -179,33 +179,34 @@ public:
testcase << "test_add " << to_string(scale);
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 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
@@ -2021,13 +2022,27 @@ public:
BigInt const exact = toBigInt(a) + toBigInt(b);
BigInt const stored = toBigInt(sum);
BigInt const diff = stored - exact;
log << "\n exact a + b = " << exact.str() << "\n TowardsZero = " << stored.str()
<< "\n";
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::LargeLegacy)
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()));
}
}
}