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feat: add ROUND_HALF_TO_EVEN flag for banker's rounding (fixes #251) #319

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feat: add ROUND_HALF_TO_EVEN flag for banker's rounding (fixes #251) #319

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33 changes: 22 additions & 11 deletions double-conversion/bignum-dtoa.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -76,18 +76,21 @@ static void GenerateShortestDigits(Bignum* numerator, Bignum* denominator,
// Generates 'requested_digits' after the decimal point.
static void BignumToFixed(int requested_digits, int* decimal_point,
Bignum* numerator, Bignum* denominator,
Vector<char> buffer, int* length);
Vector<char> buffer, int* length,
bool round_half_to_even = false);
// Generates 'count' digits of numerator/denominator.
// Once 'count' digits have been produced rounds the result depending on the
// remainder (remainders of exactly .5 round upwards). Might update the
// decimal_point when rounding up (for example for 0.9999).
static void GenerateCountedDigits(int count, int* decimal_point,
Bignum* numerator, Bignum* denominator,
Vector<char> buffer, int* length);
Vector<char> buffer, int* length,
bool round_half_to_even = false);


void BignumDtoa(double v, BignumDtoaMode mode, int requested_digits,
Vector<char> buffer, int* length, int* decimal_point) {
Vector<char> buffer, int* length, int* decimal_point,
bool round_half_to_even) {
DOUBLE_CONVERSION_ASSERT(v > 0);
DOUBLE_CONVERSION_ASSERT(!Double(v).IsSpecial());
uint64_t significand;
Expand Down Expand Up @@ -155,12 +158,14 @@ void BignumDtoa(double v, BignumDtoaMode mode, int requested_digits,
case BIGNUM_DTOA_FIXED:
BignumToFixed(requested_digits, decimal_point,
&numerator, &denominator,
buffer, length);
buffer, length,
round_half_to_even);
break;
case BIGNUM_DTOA_PRECISION:
GenerateCountedDigits(requested_digits, decimal_point,
&numerator, &denominator,
buffer, length);
buffer, length,
round_half_to_even);
break;
default:
DOUBLE_CONVERSION_UNREACHABLE();
Expand Down Expand Up @@ -282,7 +287,8 @@ static void GenerateShortestDigits(Bignum* numerator, Bignum* denominator,
// exponent (decimal_point), when rounding upwards.
static void GenerateCountedDigits(int count, int* decimal_point,
Bignum* numerator, Bignum* denominator,
Vector<char> buffer, int* length) {
Vector<char> buffer, int* length,
bool round_half_to_even) {
DOUBLE_CONVERSION_ASSERT(count >= 0);
if (count <= 0) {
// No digits requested. The "last digit" store below would write buffer[count - 1].
Expand All @@ -302,7 +308,8 @@ static void GenerateCountedDigits(int count, int* decimal_point,
// Generate the last digit.
uint16_t digit;
digit = numerator->DivideModuloIntBignum(*denominator);
if (Bignum::PlusCompare(*numerator, *numerator, *denominator) >= 0) {
int compare = Bignum::PlusCompare(*numerator, *numerator, *denominator);
if (compare > 0 || (compare == 0 && (round_half_to_even ? (digit % 2 != 0) : true))) {
digit++;
}
DOUBLE_CONVERSION_ASSERT(digit <= 10);
Expand Down Expand Up @@ -330,7 +337,8 @@ static void GenerateCountedDigits(int count, int* decimal_point,
// Input verifies: 1 <= (numerator + delta) / denominator < 10.
static void BignumToFixed(int requested_digits, int* decimal_point,
Bignum* numerator, Bignum* denominator,
Vector<char> buffer, int* length) {
Vector<char> buffer, int* length,
bool round_half_to_even) {
// Note that we have to look at more than just the requested_digits, since
// a number could be rounded up. Example: v=0.5 with requested_digits=0.
// Even though the power of v equals 0 we can't just stop here.
Expand All @@ -350,9 +358,11 @@ static void BignumToFixed(int requested_digits, int* decimal_point,
// Initially the fraction lies in range (1, 10]. Multiply the denominator
// by 10 so that we can compare more easily.
denominator->Times10();
if (Bignum::PlusCompare(*numerator, *numerator, *denominator) >= 0) {
int compare = Bignum::PlusCompare(*numerator, *numerator, *denominator);
if (compare > 0 || (compare == 0 && !round_half_to_even)) {
// If the fraction is >= 0.5 then we have to include the rounded
// digit.
// digit. In half-to-even, the preceding digit is 0 (which is even),
// so exact halfway cases round down to 0.
buffer[0] = '1';
*length = 1;
(*decimal_point)++;
Expand All @@ -367,7 +377,8 @@ static void BignumToFixed(int requested_digits, int* decimal_point,
int needed_digits = (*decimal_point) + requested_digits;
GenerateCountedDigits(needed_digits, decimal_point,
numerator, denominator,
buffer, length);
buffer, length,
round_half_to_even);
}
}

Expand Down
3 changes: 2 additions & 1 deletion double-conversion/bignum-dtoa.h
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Original file line number Diff line number Diff line change
Expand Up @@ -77,7 +77,8 @@ enum BignumDtoaMode {
// 'BignumDtoa' expects the given buffer to be big enough to hold all digits
// and a terminating null-character.
void BignumDtoa(double v, BignumDtoaMode mode, int requested_digits,
Vector<char> buffer, int* length, int* point);
Vector<char> buffer, int* length, int* point,
bool round_half_to_even = false);

} // namespace double_conversion

Expand Down
62 changes: 35 additions & 27 deletions double-conversion/double-to-string.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -187,7 +187,8 @@ bool DoubleToStringConverter::ToShortestIeeeNumber(
int decimal_rep_length;

DoubleToAscii(value, mode, 0, decimal_rep, kDecimalRepCapacity,
&sign, &decimal_rep_length, &decimal_point);
&sign, &decimal_rep_length, &decimal_point,
(flags_ & ROUND_HALF_TO_EVEN) != 0);

bool unique_zero = (flags_ & UNIQUE_ZERO) != 0;
if (sign && (value != 0.0 || !unique_zero)) {
Expand Down Expand Up @@ -233,7 +234,8 @@ bool DoubleToStringConverter::ToFixed(double value,
int decimal_rep_length;
DoubleToAscii(value, FIXED, requested_digits,
decimal_rep, kDecimalRepCapacity,
&sign, &decimal_rep_length, &decimal_point);
&sign, &decimal_rep_length, &decimal_point,
(flags_ & ROUND_HALF_TO_EVEN) != 0);

bool unique_zero = ((flags_ & UNIQUE_ZERO) != 0);
if (sign && (value != 0.0 || !unique_zero)) {
Expand Down Expand Up @@ -274,11 +276,13 @@ bool DoubleToStringConverter::ToExponential(
if (requested_digits == -1) {
DoubleToAscii(value, SHORTEST, 0,
decimal_rep, kDecimalRepCapacity,
&sign, &decimal_rep_length, &decimal_point);
&sign, &decimal_rep_length, &decimal_point,
(flags_ & ROUND_HALF_TO_EVEN) != 0);
} else {
DoubleToAscii(value, PRECISION, requested_digits + 1,
decimal_rep, kDecimalRepCapacity,
&sign, &decimal_rep_length, &decimal_point);
&sign, &decimal_rep_length, &decimal_point,
(flags_ & ROUND_HALF_TO_EVEN) != 0);
DOUBLE_CONVERSION_ASSERT(decimal_rep_length <= requested_digits + 1);

for (int i = decimal_rep_length; i < requested_digits + 1; ++i) {
Expand Down Expand Up @@ -322,7 +326,8 @@ bool DoubleToStringConverter::ToPrecision(double value,

DoubleToAscii(value, PRECISION, precision,
decimal_rep, kDecimalRepCapacity,
&sign, &decimal_rep_length, &decimal_point);
&sign, &decimal_rep_length, &decimal_point,
(flags_ & ROUND_HALF_TO_EVEN) != 0);
DOUBLE_CONVERSION_ASSERT(decimal_rep_length <= precision);

bool unique_zero = ((flags_ & UNIQUE_ZERO) != 0);
Expand Down Expand Up @@ -391,7 +396,8 @@ void DoubleToStringConverter::DoubleToAscii(double v,
int buffer_length,
bool* sign,
int* length,
int* point) {
int* point,
bool round_half_to_even) {
Vector<char> vector(buffer, buffer_length);
DOUBLE_CONVERSION_ASSERT(!Double(v).IsSpecial());
DOUBLE_CONVERSION_ASSERT(mode == SHORTEST || mode == SHORTEST_SINGLE || requested_digits >= 0);
Expand Down Expand Up @@ -423,31 +429,33 @@ void DoubleToStringConverter::DoubleToAscii(double v,
return;
}

bool fast_worked;
switch (mode) {
case SHORTEST:
fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST, 0, vector, length, point);
break;
case SHORTEST_SINGLE:
fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST_SINGLE, 0,
vector, length, point);
break;
case FIXED:
fast_worked = FastFixedDtoa(v, requested_digits, vector, length, point);
break;
case PRECISION:
fast_worked = FastDtoa(v, FAST_DTOA_PRECISION, requested_digits,
vector, length, point);
break;
default:
fast_worked = false;
DOUBLE_CONVERSION_UNREACHABLE();
bool fast_worked = false;
if (!round_half_to_even) {
switch (mode) {
case SHORTEST:
fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST, 0, vector, length, point);
break;
case SHORTEST_SINGLE:
fast_worked = FastDtoa(v, FAST_DTOA_SHORTEST_SINGLE, 0,
vector, length, point);
break;
case FIXED:
fast_worked = FastFixedDtoa(v, requested_digits, vector, length, point);
break;
case PRECISION:
fast_worked = FastDtoa(v, FAST_DTOA_PRECISION, requested_digits,
vector, length, point);
break;
default:
fast_worked = false;
DOUBLE_CONVERSION_UNREACHABLE();
}
if (fast_worked) return;
}
if (fast_worked) return;

// If the fast dtoa didn't succeed use the slower bignum version.
BignumDtoaMode bignum_mode = DtoaToBignumDtoaMode(mode);
BignumDtoa(v, bignum_mode, requested_digits, vector, length, point);
BignumDtoa(v, bignum_mode, requested_digits, vector, length, point, round_half_to_even);
vector[*length] = '\0';
}

Expand Down
8 changes: 6 additions & 2 deletions double-conversion/double-to-string.h
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Original file line number Diff line number Diff line change
Expand Up @@ -80,7 +80,8 @@ class DoubleToStringConverter {
UNIQUE_ZERO = 8,
NO_TRAILING_ZERO = 16,
EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL = 32,
EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL = 64
EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL = 64,
ROUND_HALF_TO_EVEN = 128
};

// Flags should be a bit-or combination of the possible Flags-enum.
Expand All @@ -106,6 +107,8 @@ class DoubleToStringConverter {
// - EMIT_TRAILING_ZERO_AFTER_POINT_IN_EXPONENTIAL: in addition to a trailing
// decimal point emits a trailing '0'-character. This flag requires the
// EMIT_TRAILING_DECIMAL_POINT_IN_EXPONENTIAL flag.
// - ROUND_HALF_TO_EVEN: round halfway cases towards the nearest even number
// (banker's rounding) instead of away from zero. Matches printf, std::to_chars, etc.
//
// Infinity symbol and nan_symbol provide the string representation for these
// special values. If the string is NULL and the special value is encountered
Expand Down Expand Up @@ -453,7 +456,8 @@ class DoubleToStringConverter {
int buffer_length,
bool* sign,
int* length,
int* point);
int* point,
bool round_half_to_even = false);

private:
// Implementation for ToShortest and ToShortestSingle.
Expand Down
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