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Creating More Accuracy in NS_TO_S #460

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196e3bf
Creating more accuracy in NS_TO_S
CursedRock17 Apr 16, 2024
375e2f2
More precise testing
CursedRock17 Apr 17, 2024
431e7f6
Fixing uncrustify
CursedRock17 Apr 18, 2024
627a2ea
Adding custom functions
CursedRock17 Jul 12, 2024
006e414
Nitpiking
CursedRock17 Jul 12, 2024
4f51c49
Adding better definitions
CursedRock17 Jul 23, 2024
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5 changes: 4 additions & 1 deletion include/rcutils/time.h
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,10 @@ extern "C"
#define RCUTILS_US_TO_NS(microseconds) ((microseconds) * 1000LL)

/// Convenience macro to convert nanoseconds to seconds.
#define RCUTILS_NS_TO_S(nanoseconds) ((nanoseconds) / (1000LL * 1000LL * 1000LL))
#define RCUTILS_NS_TO_S(nanoseconds) \
((1e-9 * (double)((int32_t)(nanoseconds % 1000000000l))) + \
((double)((int32_t)((nanoseconds - ((int32_t)(nanoseconds % 1000000000l))) / 1000000000l))))
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@clalancette clalancette Jul 9, 2024

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See my comment in ros2/rmw_fastrtps#755 on why I'm very nervous about making this change. Basically things that were expecting integer division before are now going to get FPU division. While it may be more accurate, it also changes the expectations.

This actually leads to another observation, which is that the type of nanoseconds is ill-defined because this is a macro.

All of that leads me to another thought. Maybe what we should do here is to make a new function called double rcutils_nanoseconds_to_seconds(double). And then we (slowly) convert uses of the macro over to that new function. That way we aren't making this change for things that don't expect it, and we can get the types right on the new function.

Thoughts?

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@CursedRock17 CursedRock17 Jul 9, 2024

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While it may be more accurate, it also changes the expectations.

Yeah as we found in this review, changing the nature of the macro induces errors in testing. To be fair, it's a beneficial change, but if it causes errors in testing there's a high chance it could lead to breakages on the users' end. Though I don't think going the route of new typed function is a better option as it just adds more to the file and alters the way code is written, when I would assume users just want to use a macro, design-wise. Unless most of the macros are rebranded to typed functions, I would feel that it's just an awkward change when it comes to DX.

This actually leads to another observation, which is that the type of nanoseconds is ill-defined because this is a macro.

Yeah while rcutils and dds implementations use int64_t it's not necessarily common knowledge for users.

I would like to create more accuracy, but since there's not a effective way to deprecate macros, I doubt it's worth breaking user behavior to do so, nor is it worth expanding on.

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@mjcarroll mjcarroll Jul 9, 2024

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I would like to create more accuracy, but since there's not a effective way to deprecate macros, I doubt it's worth breaking user behavior to do so, nor is it worth expanding on.

I think that adding more accurate function calls (especially if they are constexpr) and migrating where it makes sense would work?

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@clalancette clalancette Jul 9, 2024

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I think that adding more accurate function calls (especially if they are constexpr) and migrating where it makes sense would work?

Yeah, that's what I'm thinking we should do.

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@CursedRock17 CursedRock17 Jul 9, 2024

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(especially if they are constexpr) and migrating where it makes sense would work?

Seems fair enough to make more calls, but since rcutils is essentially C repository, did we need these split functions in a different repo?

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@clalancette clalancette Jul 9, 2024

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Seems fair enough to make more calls, but since rcutils is essentially C repository, did we need these split functions in a different repo?

Good point. It can't be constexpr here, but it can at least be a function, be const, and use types.

Alternatively, if we wanted to use C++ and constexpr, we could move it to rcpputils, but then there are certain places it can't be used (like rcutils, rcl, rcl_action, and rmw).


/// Convenience macro to convert nanoseconds to milliseconds.
#define RCUTILS_NS_TO_MS(nanoseconds) ((nanoseconds) / (1000LL * 1000LL))
/// Convenience macro to convert nanoseconds to microseconds.
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14 changes: 6 additions & 8 deletions test/test_time.cpp
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Expand Up @@ -91,14 +91,12 @@ TEST_F(TestTimeFixture, test_rcutils_time_conversion_macros) {
9007199254740992.); // value is truncated!

// nanoseconds to seconds
EXPECT_EQ(RCUTILS_NS_TO_S(1000000000ll), 1ll); // int64_t
EXPECT_EQ(RCUTILS_NS_TO_S(1000000042ll), 1ll); // int64_t (truncated)
EXPECT_EQ(RCUTILS_NS_TO_S(-1999999999ll), -1ll); // int64_t (truncated)
EXPECT_EQ(RCUTILS_NS_TO_S(200000000.), 0.2); // double
EXPECT_EQ(RCUTILS_NS_TO_S(1.0 + 1.0), 0.000000002); // sum of doubles
EXPECT_EQ(
RCUTILS_NS_TO_S(9007199254740992.),
9007199.254740992); // maximum precision double (53 bits)
EXPECT_EQ(RCUTILS_NS_TO_S(1000000000ll), 1.0); // int64_t
EXPECT_EQ(RCUTILS_NS_TO_S(1000000042ll), 1.000000042); // int64_t
EXPECT_EQ(RCUTILS_NS_TO_S(-1999999999ll), -1.999999999); // int64_t
EXPECT_EQ(RCUTILS_NS_TO_S(1 + 1), 0.000000002); // sum of two int64_ts
EXPECT_EQ(RCUTILS_NS_TO_S(9007199254740992ll),
9007199.254740992); // maximum precision int64_t

// nanoseconds to milliseconds
EXPECT_EQ(RCUTILS_NS_TO_MS(1000000ll), 1ll); // int64_t
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