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Fix %S formatting for chrono durations with leading zeroes #3814

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merged 7 commits into from
Feb 7, 2024
Merged

Fix %S formatting for chrono durations with leading zeroes #3814

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8227a2f
fix chrono duration rounding with leading zeroes
js324 Jan 16, 2024
7e96e39
fix type
js324 Jan 16, 2024
e81ef54
add tests and single digit case
js324 Jan 17, 2024
2ce9f9a
cmake change
js324 Jan 17, 2024
2ccc7aa
remove comment
js324 Jan 17, 2024
4d38a66
formatting
js324 Jan 17, 2024
f8b9e79
truncation instead of rounding
js324 Feb 6, 2024
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19 changes: 12 additions & 7 deletions include/fmt/chrono.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1151,18 +1151,23 @@ void write_fractional_seconds(OutputIt& out, Duration d, int precision = -1) {
out = std::fill_n(out, leading_zeroes, '0');
out = format_decimal<Char>(out, n, num_digits).end;
}
} else {
} else if (precision > 0) {
*out++ = '.';
leading_zeroes = (std::min)(leading_zeroes, precision);
out = std::fill_n(out, leading_zeroes, '0');
int remaining = precision - leading_zeroes;
if (remaining != 0 && remaining < num_digits) {
n /= to_unsigned(detail::pow10(to_unsigned(num_digits - remaining)));
out = format_decimal<Char>(out, n, remaining).end;
out = std::fill_n(out, leading_zeroes, '0');
if (remaining < num_digits) {
int num_truncated_digits = num_digits - remaining;
n /= to_unsigned(detail::pow10(to_unsigned(num_truncated_digits)));
if (n) {
out = format_decimal<Char>(out, n, remaining).end;
}
return;
}
out = format_decimal<Char>(out, n, num_digits).end;
remaining -= num_digits;
if (n) {
out = format_decimal<Char>(out, n, num_digits).end;
remaining -= num_digits;
}
out = std::fill_n(out, remaining, '0');
}
}
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14 changes: 14 additions & 0 deletions test/chrono-test.cc
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Expand Up @@ -791,6 +791,20 @@ TEST(chrono_test, cpp20_duration_subsecond_support) {
"01.234000");
EXPECT_EQ(fmt::format("{:.6%S}", std::chrono::milliseconds{-1234}),
"-01.234000");
EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{12345}),
"12.34");
EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{12375}),
"12.37");
EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{-12375}),
"-12.37");
EXPECT_EQ(fmt::format("{:.0%S}", std::chrono::milliseconds{12054}),
"12");
EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{99999}),
"39.99");
Comment on lines +802 to +803
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If we are rounding than this should probably be 40.00.

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Thinking more of it, we should check what the standard has to say about rounding in this case and, if necessary, open an issue for clarification.

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From what I have read in the standard, there doesn't seem to be anything related to rounding rules for these cases (unless I missed something). There possibly isn't any because allowing a precision modifier with %S for a duration with an integral rep is unique to fmt/not part of the standard?

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The same applies to the floating-point representation and the standard is unclear what to do about it. libc++ currently ignores precision: https://www.godbolt.org/z/7Gxj5hG8v. I asked for clarification, let's see how it goes.

One problem with rounding is that if we round 59.999 seconds we'll get 00 seconds and will have to increment minutes, and so on. I.e. rounding should apply to the argument, not just %S.

So I see two options:

  • Truncate
  • Round the argument such that all chrono specifiers see the rounded value

In any case we shouldn't round in some cases and truncate in other. I am leaning to the second option since it is consistent with floating-point formatting.

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Sounds good, I have a fix for the rounding ready but I will wait until there is confirmation which way (round/truncate) to go

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In the meantime I suggest going with truncation (round towards zero) since that's the default for duration_cast: https://stackoverflow.com/q/36122428/471164.

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Just wondering, is there a reason why we should follow duration_cast specifically? Because in the case of round (https://en.cppreference.com/w/cpp/chrono/duration/round), it follows the round half to even method. IMO, I feel like rounding makes more sense so it doesn't lose "precision" of the original number.

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@vitaut vitaut Feb 6, 2024 •
edited
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The idea was that duration_cast is the closest we can get to some sort of a default but thinking more of it there is nothing special about it compared to other functions. I don't have strong opinion about whether to round or truncate but truncation seems simpler because you don't need to bump minutes etc. Neither method looses precision more than the other.

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Sure that's fair, I pushed the changes to make the rounding into truncation/towards zero now.

EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{1000}),
"01.00");
EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::milliseconds{1}),
"00.001");
EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::seconds{1234}), "34.000");
EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::hours{1234}), "00.000");
EXPECT_EQ(fmt::format("{:.5%S}", dms(1.234)), "00.00123");
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