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Introduce rounding modes for Number:
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You can set a thread-local flag to direct Number how to round
non-exact results with the syntax:

    Number::rounding_mode prev_mode = Number::setround(Number::towards_zero);

This flag will stay in effect for this thread only until another call
to setround.  The previously set rounding mode is returned.

You can also retrieve the current rounding mode with:

    Number::rounding_mode current_mode = Number::getround();

The available rounding modes are:

* to_nearest : Rounds to nearest representable value.  On tie, rounds
               to even.
* towards_zero : Rounds towards zero.
* downward : Rounds towards negative infinity.
* upward : Rounds towards positive infinity.

The default rounding mode is to_nearest.
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@HowardHinnant
HowardHinnant committed Aug 15, 2022
1 parent 1f7be0f commit 574b497
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Showing 3 changed files with 226 additions and 39 deletions.
13 changes: 13 additions & 0 deletions src/ripple/basics/Number.h
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Original file line number Diff line number Diff line change
Expand Up @@ -149,7 +149,17 @@ class Number
return os << to_string(x);
}

// Thread local rounding control. Default is to_nearest
enum rounding_mode { to_nearest, towards_zero, downward, upward };
static rounding_mode
getround();
// Returns previously set mode
static rounding_mode
setround(rounding_mode mode);

private:
static thread_local rounding_mode mode_;

void
normalize();
constexpr bool
Expand Down Expand Up @@ -308,6 +318,9 @@ power(Number const& f, unsigned n);
Number
root(Number f, unsigned d);

Number
root2(Number f);

// Returns f^(n/d)

Number
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53 changes: 46 additions & 7 deletions src/ripple/basics/impl/Number.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@
#include <numeric>
#include <stdexcept>
#include <type_traits>
#include <utility>

#ifdef _MSVC_LANG
#include <boost/multiprecision/cpp_int.hpp>
Expand All @@ -33,6 +34,20 @@ using uint128_t = __uint128_t;

namespace ripple {

thread_local Number::rounding_mode Number::mode_ = Number::to_nearest;

Number::rounding_mode
Number::getround()
{
return mode_;
}

Number::rounding_mode
Number::setround(rounding_mode mode)
{
return std::exchange(mode_, mode);
}

// Guard

// The Guard class is used to tempoarily add extra digits of
Expand Down Expand Up @@ -107,16 +122,40 @@ Number::Guard::pop() noexcept
return d;
}

// Returns:
// -1 if Guard is less than half
// 0 if Guard is exactly half
// 1 if Guard is greater than half
int
Number::Guard::round() noexcept
{
if (digits_ > 0x5000'0000'0000'0000)
return 1;
if (digits_ < 0x5000'0000'0000'0000)
return -1;
if (xbit_)
return 1;
return 0;
auto mode = Number::getround();
switch (mode)
{
case to_nearest:
if (digits_ > 0x5000'0000'0000'0000)
return 1;
if (digits_ < 0x5000'0000'0000'0000)
return -1;
if (xbit_)
return 1;
return 0;
case towards_zero:
return -1;
case downward:
if (sbit_)
{
if (digits_ > 0 || xbit_)
return 1;
}
return -1;
case upward:
if (sbit_)
return -1;
if (digits_ > 0 || xbit_)
return 1;
return -1;
}
}

// Number
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199 changes: 167 additions & 32 deletions src/test/basics/Number_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,24 @@

namespace ripple {

class saveNumberRoundMode
{
Number::rounding_mode mode_;

public:
~saveNumberRoundMode()
{
Number::setround(mode_);
}
explicit saveNumberRoundMode(Number::rounding_mode mode) noexcept
: mode_{mode}
{
}
saveNumberRoundMode(saveNumberRoundMode const&) = delete;
saveNumberRoundMode&
operator=(saveNumberRoundMode const&) = delete;
};

class Number_test : public beast::unit_test::suite
{
public:
Expand Down Expand Up @@ -338,39 +356,156 @@ class Number_test : public beast::unit_test::suite
{
testcase("test_to_integer");
using Case = std::tuple<Number, std::int64_t>;
Case c[]{
{Number{0}, 0},
{Number{1}, 1},
{Number{2}, 2},
{Number{3}, 3},
{Number{-1}, -1},
{Number{-2}, -2},
{Number{-3}, -3},
{Number{10}, 10},
{Number{99}, 99},
{Number{1155}, 1155},
{Number{9'999'999'999'999'999, 0}, 9'999'999'999'999'999},
{Number{9'999'999'999'999'999, 1}, 99'999'999'999'999'990},
{Number{9'999'999'999'999'999, 2}, 999'999'999'999'999'900},
{Number{-9'999'999'999'999'999, 2}, -999'999'999'999'999'900},
{Number{15, -1}, 2},
{Number{14, -1}, 1},
{Number{16, -1}, 2},
{Number{25, -1}, 2},
{Number{6, -1}, 1},
{Number{5, -1}, 0},
{Number{4, -1}, 0},
{Number{-15, -1}, -2},
{Number{-14, -1}, -1},
{Number{-16, -1}, -2},
{Number{-25, -1}, -2},
{Number{-6, -1}, -1},
{Number{-5, -1}, 0},
{Number{-4, -1}, 0}};
for (auto const& [x, y] : c)
saveNumberRoundMode save{Number::setround(Number::to_nearest)};
{
Case c[]{
{Number{0}, 0},
{Number{1}, 1},
{Number{2}, 2},
{Number{3}, 3},
{Number{-1}, -1},
{Number{-2}, -2},
{Number{-3}, -3},
{Number{10}, 10},
{Number{99}, 99},
{Number{1155}, 1155},
{Number{9'999'999'999'999'999, 0}, 9'999'999'999'999'999},
{Number{9'999'999'999'999'999, 1}, 99'999'999'999'999'990},
{Number{9'999'999'999'999'999, 2}, 999'999'999'999'999'900},
{Number{-9'999'999'999'999'999, 2}, -999'999'999'999'999'900},
{Number{15, -1}, 2},
{Number{14, -1}, 1},
{Number{16, -1}, 2},
{Number{25, -1}, 2},
{Number{6, -1}, 1},
{Number{5, -1}, 0},
{Number{4, -1}, 0},
{Number{-15, -1}, -2},
{Number{-14, -1}, -1},
{Number{-16, -1}, -2},
{Number{-25, -1}, -2},
{Number{-6, -1}, -1},
{Number{-5, -1}, 0},
{Number{-4, -1}, 0}};
for (auto const& [x, y] : c)
{
auto j = static_cast<std::int64_t>(x);
BEAST_EXPECT(j == y);
}
}
auto prev_mode = Number::setround(Number::towards_zero);
BEAST_EXPECT(prev_mode == Number::to_nearest);
{
Case c[]{
{Number{0}, 0},
{Number{1}, 1},
{Number{2}, 2},
{Number{3}, 3},
{Number{-1}, -1},
{Number{-2}, -2},
{Number{-3}, -3},
{Number{10}, 10},
{Number{99}, 99},
{Number{1155}, 1155},
{Number{9'999'999'999'999'999, 0}, 9'999'999'999'999'999},
{Number{9'999'999'999'999'999, 1}, 99'999'999'999'999'990},
{Number{9'999'999'999'999'999, 2}, 999'999'999'999'999'900},
{Number{-9'999'999'999'999'999, 2}, -999'999'999'999'999'900},
{Number{15, -1}, 1},
{Number{14, -1}, 1},
{Number{16, -1}, 1},
{Number{25, -1}, 2},
{Number{6, -1}, 0},
{Number{5, -1}, 0},
{Number{4, -1}, 0},
{Number{-15, -1}, -1},
{Number{-14, -1}, -1},
{Number{-16, -1}, -1},
{Number{-25, -1}, -2},
{Number{-6, -1}, 0},
{Number{-5, -1}, 0},
{Number{-4, -1}, 0}};
for (auto const& [x, y] : c)
{
auto j = static_cast<std::int64_t>(x);
BEAST_EXPECT(j == y);
}
}
prev_mode = Number::setround(Number::downward);
BEAST_EXPECT(prev_mode == Number::towards_zero);
{
Case c[]{
{Number{0}, 0},
{Number{1}, 1},
{Number{2}, 2},
{Number{3}, 3},
{Number{-1}, -1},
{Number{-2}, -2},
{Number{-3}, -3},
{Number{10}, 10},
{Number{99}, 99},
{Number{1155}, 1155},
{Number{9'999'999'999'999'999, 0}, 9'999'999'999'999'999},
{Number{9'999'999'999'999'999, 1}, 99'999'999'999'999'990},
{Number{9'999'999'999'999'999, 2}, 999'999'999'999'999'900},
{Number{-9'999'999'999'999'999, 2}, -999'999'999'999'999'900},
{Number{15, -1}, 1},
{Number{14, -1}, 1},
{Number{16, -1}, 1},
{Number{25, -1}, 2},
{Number{6, -1}, 0},
{Number{5, -1}, 0},
{Number{4, -1}, 0},
{Number{-15, -1}, -2},
{Number{-14, -1}, -2},
{Number{-16, -1}, -2},
{Number{-25, -1}, -3},
{Number{-6, -1}, -1},
{Number{-5, -1}, -1},
{Number{-4, -1}, -1}};
for (auto const& [x, y] : c)
{
auto j = static_cast<std::int64_t>(x);
BEAST_EXPECT(j == y);
}
}
prev_mode = Number::setround(Number::upward);
BEAST_EXPECT(prev_mode == Number::downward);
{
auto j = static_cast<std::int64_t>(x);
BEAST_EXPECT(j == y);
Case c[]{
{Number{0}, 0},
{Number{1}, 1},
{Number{2}, 2},
{Number{3}, 3},
{Number{-1}, -1},
{Number{-2}, -2},
{Number{-3}, -3},
{Number{10}, 10},
{Number{99}, 99},
{Number{1155}, 1155},
{Number{9'999'999'999'999'999, 0}, 9'999'999'999'999'999},
{Number{9'999'999'999'999'999, 1}, 99'999'999'999'999'990},
{Number{9'999'999'999'999'999, 2}, 999'999'999'999'999'900},
{Number{-9'999'999'999'999'999, 2}, -999'999'999'999'999'900},
{Number{15, -1}, 2},
{Number{14, -1}, 2},
{Number{16, -1}, 2},
{Number{25, -1}, 3},
{Number{6, -1}, 1},
{Number{5, -1}, 1},
{Number{4, -1}, 1},
{Number{-15, -1}, -1},
{Number{-14, -1}, -1},
{Number{-16, -1}, -1},
{Number{-25, -1}, -2},
{Number{-6, -1}, 0},
{Number{-5, -1}, 0},
{Number{-4, -1}, 0}};
for (auto const& [x, y] : c)
{
auto j = static_cast<std::int64_t>(x);
BEAST_EXPECT(j == y);
}
}
bool caught = false;
try
Expand Down

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