// Generates 'requested_digits' after the decimal point. It might omit
// trailing '0's. If the input number is too small then no digits at all are
// generated (ex.: 2 fixed digits for 0.00001).
//
// Input verifies: 1 <= (numerator + delta) / denominator < 10.
static void BignumToFixed(
int requested_digits,
ref int decimal_point,
Bignum numerator,
Bignum denominator,
DtoaBuilder buffer)
{
// 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.
if (-(decimal_point) > requested_digits)
{
// The number is definitively too small.
// Ex: 0.001 with requested_digits == 1.
// Set decimal-point to -requested_digits. This is what Gay does.
// Note that it should not have any effect anyways since the string is
// empty.
decimal_point = -requested_digits;
buffer.Reset();
return;
}
if (-decimal_point == requested_digits)
{
// We only need to verify if the number rounds down or up.
// Ex: 0.04 and 0.06 with requested_digits == 1.
Debug.Assert(decimal_point == -requested_digits);
// 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)
{
// If the fraction is >= 0.5 then we have to include the rounded
// digit.
buffer[0] = '1';
decimal_point++;
}
else
{
// Note that we caught most of similar cases earlier.
buffer.Reset();
}
}
else
{
// The requested digits correspond to the digits after the point.
// The variable 'needed_digits' includes the digits before the point.
int needed_digits = (decimal_point) + requested_digits;
GenerateCountedDigits(needed_digits, ref decimal_point, numerator, denominator, buffer);
}
}
// Let v = numerator / denominator < 10.
// Then we generate 'count' digits of d = x.xxxxx... (without the decimal point)
// from left to right. Once 'count' digits have been produced we decide wether
// to round up or down. Remainders of exactly .5 round upwards. Numbers such
// as 9.999999 propagate a carry all the way, and change the
// exponent (decimal_point), when rounding upwards.
static void GenerateCountedDigits(
int count,
ref int decimal_point,
Bignum numerator,
Bignum denominator,
DtoaBuilder buffer)
{
Debug.Assert(count >= 0);
for (int i = 0; i < count - 1; ++i)
{
uint d = numerator.DivideModuloIntBignum(denominator);
Debug.Assert(d <= 9); // digit is a uint and therefore always positive.
// digit = numerator / denominator (integer division).
// numerator = numerator % denominator.
buffer.Append((char)(d + '0'));
// Prepare for next iteration.
numerator.Times10();
}
// Generate the last digit.
uint digit = numerator.DivideModuloIntBignum(denominator);
if (Bignum.PlusCompare(numerator, numerator, denominator) >= 0)
{
digit++;
}
buffer.Append((char)(digit + '0'));
// Correct bad digits (in case we had a sequence of '9's). Propagate the
// carry until we hat a non-'9' or til we reach the first digit.
for (int i = count - 1; i > 0; --i)
{
if (buffer[i] != '0' + 10)
{
break;
}
buffer[i] = '0';
buffer[i - 1]++;
}
if (buffer[0] == '0' + 10)
{
// Propagate a carry past the top place.
buffer[0] = '1';
decimal_point++;
}
}
// This routine multiplies numerator/denominator so that its values lies in the
// range 1-10. That is after a call to this function we have:
// 1 <= (numerator + delta_plus) /denominator < 10.
// Let numerator the input before modification and numerator' the argument
// after modification, then the output-parameter decimal_point is such that
// numerator / denominator * 10^estimated_power ==
// numerator' / denominator' * 10^(decimal_point - 1)
// In some cases estimated_power was too low, and this is already the case. We
// then simply adjust the power so that 10^(k-1) <= v < 10^k (with k ==
// estimated_power) but do not touch the numerator or denominator.
// Otherwise the routine multiplies the numerator and the deltas by 10.
private static void FixupMultiply10(
int estimated_power,
bool is_even,
out int decimal_point,
Bignum numerator,
Bignum denominator,
Bignum delta_minus,
Bignum delta_plus)
{
bool in_range;
if (is_even)
{
in_range = Bignum.PlusCompare(numerator, delta_plus, denominator) >= 0;
}
else
{
in_range = Bignum.PlusCompare(numerator, delta_plus, denominator) > 0;
}
if (in_range)
{
// Since numerator + delta_plus >= denominator we already have
// 1 <= numerator/denominator < 10. Simply update the estimated_power.
decimal_point = estimated_power + 1;
}
else
{
decimal_point = estimated_power;
numerator.Times10();
if (Bignum.Equal(delta_minus, delta_plus))
{
delta_minus.Times10();
delta_plus.AssignBignum(delta_minus);
}
else
{
delta_minus.Times10();
delta_plus.Times10();
}
}
}
// The procedure starts generating digits from the left to the right and stops
// when the generated digits yield the shortest decimal representation of v. A
// decimal representation of v is a number lying closer to v than to any other
// double, so it converts to v when read.
//
// This is true if d, the decimal representation, is between m- and m+, the
// upper and lower boundaries. d must be strictly between them if !is_even.
// m- := (numerator - delta_minus) / denominator
// m+ := (numerator + delta_plus) / denominator
//
// Precondition: 0 <= (numerator+delta_plus) / denominator < 10.
// If 1 <= (numerator+delta_plus) / denominator < 10 then no leading 0 digit
// will be produced. This should be the standard precondition.
private static void GenerateShortestDigits(
Bignum numerator,
Bignum denominator,
Bignum delta_minus,
Bignum delta_plus,
bool is_even,
DtoaBuilder buffer)
{
// Small optimization: if delta_minus and delta_plus are the same just reuse
// one of the two bignums.
if (Bignum.Equal(delta_minus, delta_plus))
{
delta_plus = delta_minus;
}
buffer.Reset();
while (true)
{
uint digit;
digit = numerator.DivideModuloIntBignum(denominator);
// digit = numerator / denominator (integer division).
// numerator = numerator % denominator.
buffer.Append((char)(digit + '0'));
// Can we stop already?
// If the remainder of the division is less than the distance to the lower
// boundary we can stop. In this case we simply round down (discarding the
// remainder).
// Similarly we test if we can round up (using the upper boundary).
bool in_delta_room_minus;
bool in_delta_room_plus;
if (is_even)
{
in_delta_room_minus = Bignum.LessEqual(numerator, delta_minus);
}
else
{
in_delta_room_minus = Bignum.Less(numerator, delta_minus);
}
if (is_even)
{
in_delta_room_plus = Bignum.PlusCompare(numerator, delta_plus, denominator) >= 0;
}
else
{
in_delta_room_plus = Bignum.PlusCompare(numerator, delta_plus, denominator) > 0;
}
if (!in_delta_room_minus && !in_delta_room_plus)
{
// Prepare for next iteration.
numerator.Times10();
delta_minus.Times10();
// We optimized delta_plus to be equal to delta_minus (if they share the
// same value). So don't multiply delta_plus if they point to the same
// object.
if (delta_minus != delta_plus)
{
delta_plus.Times10();
}
}
else if (in_delta_room_minus && in_delta_room_plus)
{
// Let's see if 2*numerator < denominator.
// If yes, then the next digit would be < 5 and we can round down.
int compare = Bignum.PlusCompare(numerator, numerator, denominator);
if (compare < 0)
{
// Remaining digits are less than .5. -> Round down (== do nothing).
}
else if (compare > 0)
{
// Remaining digits are more than .5 of denominator. . Round up.
// Note that the last digit could not be a '9' as otherwise the whole
// loop would have stopped earlier.
// We still have an assert here in case the preconditions were not
// satisfied.
buffer[buffer.Length - 1]++;
}
else
{
// Halfway case.
// TODO(floitsch): need a way to solve half-way cases.
// For now let's round towards even (since this is what Gay seems to
// do).
if ((buffer[buffer.Length - 1] - '0') % 2 == 0)
{
// Round down => Do nothing.
}
else
{
buffer[buffer.Length - 1]++;
}
}
return;
}
else if (in_delta_room_minus)
{
// Round down (== do nothing).
return;
}
else
{
// in_delta_room_plus
// Round up.
// Note again that the last digit could not be '9' since this would have
// stopped the loop earlier.
// We still have an DCHECK here, in case the preconditions were not
// satisfied.
buffer[buffer.Length - 1]++;
return;
}
}
}