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Optimize %T in tm formatting #2500

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Merged
vitaut merged 2 commits into from
Sep 15, 2021
+17 −1
Merged

Optimize %T in tm formatting #2500

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20c68a6
Optimize %T in tm formatting
phprus Sep 13, 2021
7407f76
Fix wchar_t tm formatting
phprus Sep 13, 2021
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15 changes: 14 additions & 1 deletion include/fmt/chrono.h
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Original file line number Diff line number Diff line change
Expand Up @@ -549,6 +549,7 @@ template <typename Char> struct formatter<std::tm, Char> {
enum class spec {
unknown,
year_month_day,
hh_mm_ss,
};
spec spec_ = spec::unknown;

Expand All @@ -563,7 +564,13 @@ template <typename Char> struct formatter<std::tm, Char> {
while (end != ctx.end() && *end != '}') ++end;
auto size = detail::to_unsigned(end - it);
specs = {it, size};
if (specs == string_view("%F", 2)) spec_ = spec::year_month_day;
// basic_string_view<>::compare isn't constexpr before C++17

@miscco miscco Sep 13, 2021

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There is no basic_string_view<> before C++17.

Could it be that the internal replacement should be updated?

@phprus phprus Sep 13, 2021

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Problem in std::char_traits<>::compare:

fmt/include/fmt/core.h

Lines 472 to 478 in 3d0c7ae

FMT_CONSTEXPR_CHAR_TRAITS auto compare(basic_string_view other) const -> int {
size_t str_size = size_ < other.size_ ? size_ : other.size_;
int result = std::char_traits<Char>::compare(data_, other.data_, str_size);
if (result == 0)
result = size_ == other.size_ ? 0 : (size_ < other.size_ ? -1 : 1);
return result;
}

constexpr std::char_traits<>::compare starts from C++17 only

if (specs.size() == 2 && specs[0] == Char('%')) {
if (specs[1] == Char('F'))
spec_ = spec::year_month_day;
else if (specs[1] == Char('T'))
spec_ = spec::hh_mm_ss;
}
return end;
}

Expand All @@ -578,6 +585,12 @@ template <typename Char> struct formatter<std::tm, Char> {
detail::to_unsigned(tm.tm_mon + 1),
detail::to_unsigned(tm.tm_mday), '-');
return std::copy_n(buf, sizeof(buf), ctx.out());
} else if (spec_ == spec::hh_mm_ss) {
char buf[8];
detail::write_digit2_separated(buf, detail::to_unsigned(tm.tm_hour),
detail::to_unsigned(tm.tm_min),
detail::to_unsigned(tm.tm_sec), ':');
return std::copy_n(buf, sizeof(buf), ctx.out());
}
basic_memory_buffer<Char> tm_format;
tm_format.append(specs.begin(), specs.end());
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1 change: 1 addition & 0 deletions test/chrono-test.cc
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Expand Up @@ -49,6 +49,7 @@ TEST(chrono_test, format_tm) {
EXPECT_EQ(fmt::format("The date is {:%Y-%m-%d %H:%M:%S}.", tm),
"The date is 2016-04-25 11:22:33.");
EXPECT_EQ(fmt::format("{:%F}", tm), "2016-04-25");
EXPECT_EQ(fmt::format("{:%T}", tm), "11:22:33");
}

TEST(chrono_test, grow_buffer) {
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2 changes: 2 additions & 0 deletions test/xchar-test.cc
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Expand Up @@ -261,6 +261,8 @@ TEST(xchar_test, chrono) {
EXPECT_EQ(fmt::format("The date is {:%Y-%m-%d %H:%M:%S}.", tm),
"The date is 2016-04-25 11:22:33.");
EXPECT_EQ(L"42s", fmt::format(L"{}", std::chrono::seconds(42)));
EXPECT_EQ(fmt::format(L"{:%F}", tm), L"2016-04-25");
EXPECT_EQ(fmt::format(L"{:%T}", tm), L"11:22:33");
}

TEST(xchar_test, color) {
Expand Down