Implement some formatting options in Grisu

This commit is contained in:
Victor Zverovich 2018-08-26 09:51:49 -07:00
parent f51080916e
commit e483a01a0f
2 changed files with 77 additions and 20 deletions

View File

@ -364,12 +364,14 @@ FMT_FUNC fp get_cached_power(int min_exponent, int &pow10_exponent) {
return fp(data::POW10_SIGNIFICANDS[index], data::POW10_EXPONENTS[index]);
}
// Writes the exponent exp in the form "[-]d{1,3}" to buffer.
// Writes the exponent exp in the form "[+-]d{1,3}" to buffer.
FMT_FUNC char *write_exponent(char *buffer, int exp) {
FMT_ASSERT(-1000 < exp && exp < 1000, "exponent out of range");
if (exp < 0) {
*buffer++ = '-';
exp = -exp;
} else {
*buffer++ = '+';
}
if (exp >= 100) {
*buffer++ = '0' + static_cast<char>(exp / 100);
@ -446,25 +448,58 @@ FMT_FUNC void grisu2_gen_digits(
// Prettifies the output of the Grisu2 algorithm.
// The number is given as v = buffer * 10^exp.
FMT_FUNC void grisu2_prettify(char *buffer, size_t &size, int exp) {
FMT_FUNC void grisu2_prettify(char *buffer, size_t &size, int exp, char type,
size_t precision, bool print_decimal_point) {
int int_size = static_cast<int>(size);
// 10^(full_exp - 1) <= v <= 10^full_exp.
int full_exp = static_cast<int>(size) + exp;
// Insert a decimal point after the first digit and add an exponent.
std::memmove(buffer + 2, buffer + 1, size - 1);
buffer[1] = '.';
char *p = buffer + size + 1;
*p++ = 'e';
size = to_unsigned(write_exponent(p, full_exp - 1) - buffer);
int full_exp = int_size + exp;
if (int_size <= full_exp && full_exp <= 21) {
// 1234e7 -> 12340000000
std::uninitialized_fill_n(buffer + int_size, full_exp - int_size, '0');
char *p = buffer + full_exp;
if (print_decimal_point && size < precision) {
*p++ = '.';
auto fill_size = precision - size;
std::uninitialized_fill_n(p, fill_size, '0');
p += fill_size;
}
size = to_unsigned(p - buffer);
} else if (0 < full_exp && full_exp <= 21) {
// 1234e-2 -> 12.34
size_t fractional_size = to_unsigned(int_size - full_exp);
std::memmove(buffer + full_exp + 1, buffer + full_exp, fractional_size);
buffer[full_exp] = '.';
if (type == 'f' && fractional_size < precision) {
size_t num_zeros = precision - fractional_size;
std::uninitialized_fill_n(buffer + size + 1, num_zeros, '0');
size += num_zeros;
}
++size;
} else if (-6 < full_exp && full_exp <= 0) {
// 1234e-6 -> 0.001234
int offset = 2 - full_exp;
std::memmove(buffer + offset, buffer, size);
buffer[0] = '0';
buffer[1] = '.';
std::uninitialized_fill_n(buffer + 2, -full_exp, '0');
size = to_unsigned(int_size + offset);
} else {
// Insert a decimal point after the first digit and add an exponent.
std::memmove(buffer + 2, buffer + 1, size - 1);
buffer[1] = '.';
char *p = buffer + size + 1;
*p++ = 'e';
size = to_unsigned(write_exponent(p, full_exp - 1) - buffer);
}
}
// Formats value using Grisu2 algorithm. Grisu2 doesn't give any guarantees on
// the shortness of the result.
FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size) {
FMT_FUNC void grisu2_format_positive(double value, char *buffer, size_t &size,
int &dec_exp) {
FMT_ASSERT(value > 0, "value is nonpositive");
fp fp_value(value);
fp lower, upper; // w^- and w^+ in the Grisu paper.
fp_value.compute_boundaries(lower, upper);
// Find a cached power of 10 close to 1 / upper.
int dec_exp = 0; // K in Grisu.
const int min_exp = -60; // alpha in Grisu.
auto dec_pow = get_cached_power( // \tilde{c}_{-k} in Grisu.
min_exp - (upper.e + fp::significand_size), dec_exp);
@ -477,7 +512,28 @@ FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size) {
--scaled_upper.f; // \tilde{M}^+ - 1 ulp -> M^+_{\downarrow}.
uint64_t delta = scaled_upper.f - scaled_lower.f;
grisu2_gen_digits(scaled_value, scaled_upper, delta, buffer, size, dec_exp);
grisu2_prettify(buffer, size, dec_exp);
}
// Formats value using Grisu2 algorithm. Grisu2 doesn't give any guarantees on
// the shortness of the result.
FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size, char type,
int precision, bool print_decimal_point) {
FMT_ASSERT(value >= 0, "value is negative");
int dec_exp = 0; // K in Grisu.
if (value > 0) {
grisu2_format_positive(value, buffer, size, dec_exp);
} else {
*buffer = '0';
size = 1;
}
size_t unsigned_precision = precision >= 0 ? precision : 6;
if (size > unsigned_precision) {
// TODO: round instead of truncating
dec_exp += size - unsigned_precision;
size = unsigned_precision;
}
grisu2_prettify(buffer, size, dec_exp, type, unsigned_precision,
print_decimal_point);
}
} // namespace internal

View File

@ -367,7 +367,8 @@ FMT_API fp get_cached_power(int min_exponent, int &pow10_exponent);
// Formats value using Grisu2 algorithm:
// https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
FMT_API void grisu2_format(double value, char *buffer, size_t &size);
FMT_API void grisu2_format(double value, char *buffer, size_t &size, char type,
int precision, bool print_decimal_point);
template <typename Allocator>
typename Allocator::value_type *allocate(Allocator& alloc, std::size_t n) {
@ -2949,12 +2950,12 @@ void basic_writer<Range>::write_double(T value, const format_specs &spec) {
basic_memory_buffer<char_type> buffer;
if (internal::const_check(FMT_USE_GRISU && sizeof(T) <= sizeof(double) &&
std::numeric_limits<double>::is_iec559)) {
// The max size = 10 (hi) + 20 (lo) + 5 (exp).
enum { BUF_SIZE = 35 };
char buf[BUF_SIZE];
char buf[100]; // TODO: correct buffer size
size_t size = 0;
internal::grisu2_format(static_cast<double>(value), buf, size);
FMT_ASSERT(size <= BUF_SIZE, "buffer overflow");
internal::grisu2_format(
static_cast<double>(value), buf, size, static_cast<char>(spec.type()),
spec.precision(), spec.flag(HASH_FLAG));
FMT_ASSERT(size <= 100, "buffer overflow");
buffer.append(buf, buf + size); // TODO: avoid extra copy
} else {
format_specs normalized_spec(spec);