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Implement Grisu rounding

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This commit is contained in:
Victor Zverovich 2018-10-13 07:44:34 -07:00
parent 4bb76ef0c7
commit 699297520a
1 changed files with 19 additions and 4 deletions
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23
include/fmt/format-inl.h
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@ -461,19 +461,30 @@ FMT_FUNC fp get_cached_power(int min_exponent, int &pow10_exponent) {
return fp(data::POW10_SIGNIFICANDS[index], data::POW10_EXPONENTS[index]);
}
FMT_FUNC void grisu2_round(char *buffer, size_t size, uint64_t delta,
uint64_t remainder, uint64_t exp, uint64_t diff) {
while (remainder < diff && delta - remainder >= exp &&
(remainder + exp < diff || diff - remainder > remainder + exp - diff)) {
--buffer[size - 1];
remainder += exp;
}
}
// Generates output using Grisu2 digit-gen algorithm.
FMT_FUNC void grisu2_gen_digits(
const fp &scaled_value, const fp &scaled_upper, uint64_t delta,
char *buffer, size_t &size, int &dec_exp) {
internal::fp one(1ull << -scaled_upper.e, scaled_upper.e);
internal::fp diff = scaled_upper - scaled_value; // wp_w in Grisu.
// hi (p1 in Grisu) contains the most significant digits of scaled_upper.
// hi = floor(scaled_upper / one).
uint32_t hi = static_cast<uint32_t>(scaled_upper.f >> -one.e);
// lo (p2 in Grisu) contains the least significants digits of scaled_upper.
// lo = scaled_upper mod 1.
// lo = scaled_upper % one.
uint64_t lo = scaled_upper.f & (one.f - 1);
size = 0;
auto exp = count_digits(hi); // kappa in Grisu.
// Generate digits for the most significant part (hi).
while (exp > 0) {
uint32_t digit = 0;
// This optimization by miloyip reduces the number of integer divisions by
@ -498,11 +509,13 @@ FMT_FUNC void grisu2_gen_digits(
uint64_t remainder = (static_cast<uint64_t>(hi) << -one.e) + lo;
if (remainder <= delta) {
dec_exp += exp;
// TODO: use scaled_value
(void)scaled_value;
grisu2_round(buffer, size, delta, remainder,
static_cast<uint64_t>(data::POWERS_OF_10_32[exp]) << -one.e,
diff.f);
return;
}
}
// Generate digits for the least significant part (lo).
for (;;) {
lo *= 10;
delta *= 10;
@ -513,6 +526,8 @@ FMT_FUNC void grisu2_gen_digits(
--exp;
if (lo < delta) {
dec_exp += exp;
grisu2_round(buffer, size, delta, lo, one.f,
diff.f * data::POWERS_OF_10_32[-exp]);
return;
}
}
@ -552,7 +567,7 @@ FMT_FUNC void round(char *buffer, size_t &size, int &exp,
}
}
// Writes the exponent exp in the form "[+-]d{1,3}" to buffer.
// Writes the exponent exp in the form "[+-]d{2,3}" to buffer.
FMT_FUNC char *write_exponent(char *buffer, int exp) {
FMT_ASSERT(-1000 < exp && exp < 1000, "exponent out of range");
if (exp < 0) {