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Add FormatInt for fast stand-alone integer formatting.

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This commit is contained in:
Victor Zverovich 2013-09-09 06:51:03 -07:00
parent 75daf81895
commit 65d47e5e09
2 changed files with 58 additions and 6 deletions
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12
format.cc
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@ -108,14 +108,14 @@ struct CharTraits<wchar_t> {
swprintf(buffer, size, format, width, precision, value);
}
};
}
const char DIGITS[] =
const char fmt::internal::DIGITS[] =
"0001020304050607080910111213141516171819"
"2021222324252627282930313233343536373839"
"4041424344454647484950515253545556575859"
"6061626364656667686970717273747576777879"
"8081828384858687888990919293949596979899";
}
void fmt::internal::ReportUnknownType(char code, const char *type) {
if (std::isprint(static_cast<unsigned char>(code))) {
@ -154,8 +154,8 @@ void fmt::BasicWriter<Char>::FormatDecimal(
// "Three Optimization Tips for C++". See speed-test for a comparison.
unsigned index = (value % 100) * 2;
value /= 100;
buffer[num_digits] = DIGITS[index + 1];
buffer[num_digits - 1] = DIGITS[index];
buffer[num_digits] = internal::DIGITS[index + 1];
buffer[num_digits - 1] = internal::DIGITS[index];
num_digits -= 2;
}
if (value < 10) {
@ -163,8 +163,8 @@ void fmt::BasicWriter<Char>::FormatDecimal(
return;
}
unsigned index = static_cast<unsigned>(value * 2);
buffer[1] = DIGITS[index + 1];
buffer[0] = DIGITS[index];
buffer[1] = internal::DIGITS[index + 1];
buffer[0] = internal::DIGITS[index];
}
template <typename Char>

52
format.h
View File

@ -188,6 +188,8 @@ inline unsigned CountDigits(uint64_t n) {
}
}
extern const char DIGITS[];
template <typename Char>
class FormatterProxy;
}
@ -1055,6 +1057,56 @@ class Formatter : private Action, public BasicFormatter<Char> {
}
};
/**
Fast integer formatter.
*/
class FormatInt {
private:
// Buffer should be large enough to hold all digits (digits10 + 1),
// a sign and a null character.
enum {BUFFER_SIZE = std::numeric_limits<uint64_t>::digits10 + 3};
char buffer_[BUFFER_SIZE];
char *str_;
// Formats value in reverse and returns the number of digits.
char *FormatDecimal(uint64_t value) {
char *buffer_end = buffer_ + BUFFER_SIZE;
*--buffer_end = '\0';
while (value >= 100) {
// Integer division is slow so do it for a group of two digits instead
// of for every digit. The idea comes from the talk by Alexandrescu
// "Three Optimization Tips for C++". See speed-test for a comparison.
unsigned index = (value % 100) * 2;
value /= 100;
*--buffer_end = internal::DIGITS[index + 1];
*--buffer_end = internal::DIGITS[index];
}
if (value < 10) {
*--buffer_end = static_cast<char>('0' + value);
return buffer_end;
}
unsigned index = static_cast<unsigned>(value * 2);
*--buffer_end = internal::DIGITS[index + 1];
*--buffer_end = internal::DIGITS[index];
return buffer_end;
}
public:
explicit FormatInt(int value) {
uint64_t abs_value = value;
bool negative = value < 0;
if (negative)
abs_value = 0 - value;
str_ = FormatDecimal(abs_value);
if (negative)
*--str_ = '-';
}
explicit FormatInt(unsigned value) : str_(FormatDecimal(value)) {}
const char *c_str() const { return str_; }
std::string str() const { return str_; }
};
/**
\rst
Formats a string similarly to Python's `str.format