Added support for subsecond resolution for time_point #2292

brcha · 2021-05-18T12:47:14Z

This code:

#include "fmt/include/fmt/chrono.h"

int main () {
	auto now = std::chrono::high_resolution_clock::now();
	fmt::print("Time is {:%T}\n", now);
	fmt::print("Time (HH:MM) is {:%H:%M}\n", now);

	std::chrono::system_clock::time_point t{std::chrono::milliseconds{1234}};
	fmt::print("Seconds are {:%S}\n", t.time_since_epoch());
}

now outputs:

tmp/c++11tests/chrono ❯ g++ -std=c++11 -o fmt_test fmt_test.cpp -L ./fmt-build -lfmt                                                                                                                                                           уто 18 14:41:59

tmp/c++11tests/chrono ❯ ./fmt_test                                                                                                                                                                                                             уто 18 14:43:17
Time is 14:43:18.21308033
Time (HH:MM) is 14:43
Seconds are 01.234

…rmats

…erwise

…e, to compare only up to seconds

ecorm · 2021-05-18T16:51:15Z

include/fmt/chrono.h

@@ -412,7 +412,11 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
  auto format(std::chrono::time_point<std::chrono::system_clock> val,


Should be a template accepting any std::chrono::time_point<std::chrono::system_clock, Period>

Or more precisely, the type of the first parameter should be std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<Rep, Period>>.

ecorm · 2021-05-18T16:56:05Z

include/fmt/chrono.h

@@ -412,7 +412,11 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
  auto format(std::chrono::time_point<std::chrono::system_clock> val,
              FormatContext& ctx) -> decltype(ctx.out()) {
    std::tm time = localtime(val);
-    return formatter<std::tm, Char>::format(time, ctx);
+    auto epoch = val.time_since_epoch();
+    auto seconds = std::chrono::duration_cast<std::chrono::milliseconds>(epoch).count() / 1000;


You can just duration_cast<std::chrono::seconds>

ecorm · 2021-05-18T16:57:37Z

include/fmt/chrono.h

+    auto epoch = val.time_since_epoch();
+    auto seconds = std::chrono::duration_cast<std::chrono::milliseconds>(epoch).count() / 1000;
+    auto today = std::chrono::high_resolution_clock::duration(std::chrono::milliseconds(seconds * 1000));
+    auto subseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(epoch - today);


Just do subseconds = epoch - seconds.

Times preceding the epoch (1970-01-01) need to be considered so that subseconds is never negative.

ecorm · 2021-05-18T17:07:50Z

include/fmt/chrono.h

+    auto hasT = std::find(tm_format.begin(), tm_format.end(), 'T') != tm_format.end();
+    auto writeSubseconds = (hasS || hasT) && (subseconds != 0);
+    if (writeSubseconds)
+      tm_format.push_back('.');


Does this need to check locale an insert the preferred decimal separator?

It should check, but I don't know how to get the locale in the chrono formatter. I'll try to grep for anything related to locale.

Locale can be addressed separately, the default is locale-independent anyway.

ecorm · 2021-05-18T17:08:32Z

include/fmt/chrono.h

@@ -449,6 +459,11 @@ template <typename Char> struct formatter<std::tm, Char> {
      const size_t MIN_GROWTH = 10;
      buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
    }
+
+    if (writeSubseconds) {
+      buf.append(std::to_string(subseconds));


This does not add leading zeroes.

Thank you for the comments. I did notice I've overlooked some of those, but I was out. Should be fixed now.

brcha · 2021-05-18T19:37:57Z

Should also test with time preceding the epoch, as well as whole seconds (zero subseconds).

It does test for that, and it works. I've tested it like:

  auto past = std::chrono::system_clock::time_point{-std::chrono::hours{15}};
  fmt::print("15 hours before the start of time is: {:%Y:%m:%d %H:%M:%S}\n", past);

and I get

15 hours before the start of time is: 1969:12:31 10:00:00

ecorm · 2021-05-18T19:47:23Z

Should also test with time preceding the epoch, as well as whole seconds (zero subseconds).

It does test for that, and it works. I've tested it like:
  auto past = std::chrono::system_clock::time_point{-std::chrono::hours{15}};
  fmt::print("15 hours before the start of time is: {:%Y:%m:%d %H:%M:%S}\n", past);
and I get
15 hours before the start of time is: 1969:12:31 10:00:00

I think it would have failed if you tried a time with non-zero subseconds.

ecorm · 2021-05-18T19:56:59Z

See here for a possible implementation of std::chrono::floor: https://en.cppreference.com/w/cpp/chrono/duration/floor

brcha · 2021-05-18T20:02:15Z

Should also test with time preceding the epoch, as well as whole seconds (zero subseconds).

It does test for that, and it works. I've tested it like:
  auto past = std::chrono::system_clock::time_point{-std::chrono::hours{15}};
  fmt::print("15 hours before the start of time is: {:%Y:%m:%d %H:%M:%S}\n", past);
and I get
15 hours before the start of time is: 1969:12:31 10:00:00
I think it would have failed if you tried a time with non-zero subseconds.

You were correct. I've updated the std::tm localtime as well, and set subseconds to be 1s - whatever value they had. Works now for epoch -15h - 15ns as "1969:12:31 09:59:59.999999985".

brcha · 2021-05-18T20:16:38Z

This only works when Period::num is 1, which is not guaranteed. This can be computed at compile time. See how Hinnant does it (it's quite complex, though).

Thanks for the links. I've got some friends coming over soon, so I'll take a look tomorrow (or later). Also a way to handle fractional time.

brcha · 2021-05-18T20:22:05Z

This is computed needlessly when the given time precedes the epoch (it is recomputed again on line 420).

It has to be separately computed because localtime simply truncates the subseconds and returns whichever second was before that. But it shouldn't be computed twice, that's true.

vitaut

Thanks for the PR.

vitaut · 2021-05-21T17:23:50Z

include/fmt/chrono.h

@@ -412,7 +412,11 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
  auto format(std::chrono::time_point<std::chrono::system_clock> val,


Or more precisely, the type of the first parameter should be std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<Rep, Period>>.

vitaut · 2021-05-21T17:25:34Z

include/fmt/chrono.h

+
+    if (subseconds < subseconds.zero()) {
+      time = localtime(val - std::chrono::seconds{1});
+      subseconds = std::chrono::seconds{1} + subseconds;


nit: std::chrono::seconds{1} -> std::chrono::seconds(1) in two places for consistency with the rest of the library.

vitaut · 2021-05-21T17:31:38Z

include/fmt/chrono.h

+      auto width = std::to_string(Period::den).size() - 1;
+      std::basic_stringstream<Char> os;
+      os.fill('0');
+      os.width(width);
+      os << subseconds.count();
+      auto formatted_ss = os.str();


Please use format instead of stringstream, something like

auto formatted_ss format("{:0{}}", subseconds.count(), width); return formatter<std::tm, Char>::format(time, ctx, bytes(formatted_ss));

vitaut · 2021-05-21T17:33:50Z

include/fmt/chrono.h

@@ -449,8 +476,15 @@ template <typename Char> struct formatter<std::tm, Char> {
      const size_t MIN_GROWTH = 10;
      buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
    }
+
+    auto buf_end = buf.end() - 1;
+    if (writeSubseconds) {


writeSubseconds -> write_subseconds (naming convention).

vitaut · 2021-06-01T14:11:32Z

@brcha, do you plan to continue working on this PR or shall we close it for now?

brcha · 2021-06-01T19:03:39Z

@brcha, do you plan to continue working on this PR or shall we close it for now?

Sorry, I was very busy the last week or so. I'll continue working on this PR ASAP, given that now I have a lot more free time.

vitaut · 2021-06-23T17:15:02Z

Closing for now but feel free to submit another PR if and whenever you have time. Thanks!

vitaut · 2021-06-23T17:15:26Z

Or let me know and I'll reopen this one.

kkm000 · 2021-07-18T07:30:18Z

@brcha, @ecorm, if you ever get back to this, I think this hasn't been noticed in the discussion, unless I miss it: the test is one hour off, epoch‒15 hours is 09:00:00, not 10:00:00. At sharp hours, the hour and –(offset) must sum up to 0 mod 24, for both pre- and post-epoch times. For example, epoch‒1 hour is 23:00, ‒(‒1)+23 = 24 ≡ 0 mod 24; epoch+25 hours is 01:00, ‒(+1) + 25 = 24 ≡ 0 mod 24).

zeroxia · 2023-02-03T07:06:38Z

I think making %S to output seconds including fractional part is a breaking change. (strftime defines %S to be decimal number, range is 00 to 60)

Is it possible to make %S accept parameters like floating point, e.g. %.06S would output fractional part in nanoseconds, while %.03S will output milliseconds.

Or keep %S to always output integral seconds, and introduce another formatter char to output subseconds value.

vitaut · 2023-02-03T20:07:07Z

{fmt} follows the C++ standard (#2207) but you can discard the fractional using std::chrono::floor (https://en.cppreference.com/w/cpp/chrono/duration/floor) or other method.

Added support for subsecond resolution for time_point and %T or %S fo…

ac6dd9f

…rmats

brcha mentioned this pull request May 18, 2021

Printed seconds precision does not match C++20 spec #2207

Closed

brcha added 2 commits May 18, 2021 15:05

Added subseconds=0 as default for format(tm…), because tests fail oth…

10591ac

…erwise

Fixed the one chrono test that compared %S of fmt:format with strftim…

e2c2704

…e, to compare only up to seconds

ecorm reviewed May 18, 2021

View reviewed changes

Fixed absolute values, correct subsecond width and zero padding

3bf4aba

Hopefully fixed template error on MSVC

e41100c

Handle subsecond time before epoch start correctly

7a33f5e

Do not compute localtime twice

4b9c9b9

vitaut requested changes May 21, 2021

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vitaut closed this Jun 23, 2021

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Added support for subsecond resolution for time_point #2292

Added support for subsecond resolution for time_point #2292

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vitaut commented Feb 3, 2023

		@@ -412,7 +412,11 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
		auto format(std::chrono::time_point<std::chrono::system_clock> val,

Added support for subsecond resolution for time_point #2292

Added support for subsecond resolution for time_point #2292

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