pw_chrono: Improve SystemClock C API

Changes the SystemClock C Api to:
1) Use a pw_chrono_SystemClock_Duration struct instead of aliasing
   an int64_t under pw_chrono_SystemClock_TickCount which could
   accidentally permit direct tick usage.
2) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H} and
   PW_SYSTEM_CLOCK_{MS,S,MIN,H}_CEIL to permit C API users to
   create durations which round up to the nearest tick for deadlines
   and timeouts, mirroring std::chrono::ceil.
3) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H}_FLOOR to permit C API users to
   create durations which round down to the nearest tick for oddball
   corner cases, mirroring std::chrono::floor.
4) In order to enable said macros, the system_clock_config.h backend
   config was changed to require the clock period as a preprocessor
   defines instead of a std::ratio<>.
5) Renames pw_chrono_SystemClock_TimeDelta to
   pw_chrono_SystemClock_TimeElapsed to make the argument ordering
   make more sense.
6) Changes existing std::chrono::duration_cast usage to
   std::chrono::ceil and std::chrono:floor to set a good example
   to be explicit on rounding.
7) Renames pw_chrono_SystemClock_TickCountsToNsTruncate accordingly to
   pw_chrono_SystemClock_DurationToNsFloor.

Requires: pigweed-internal:9340
Change-Id: Ia628dceac53f964eda7c4aacd3d790b8b0e92207
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31280
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/pw_chrono_stl/public/pw_chrono_stl/system_clock_config.h b/pw_chrono_stl/public/pw_chrono_stl/system_clock_config.h
index 05b1c712b..9766fa244 100644
--- a/pw_chrono_stl/public/pw_chrono_stl/system_clock_config.h
+++ b/pw_chrono_stl/public/pw_chrono_stl/system_clock_config.h
@@ -13,15 +13,19 @@
 // the License.
 #pragma once
 
-#include <chrono>
+// Ideally we'd use std::chrono::steady_clock::period, however this is not
+// something we can expose to the C API. Instead we assume it has nanosecond
+// compatibility and we rely on implicit conversion to tell us at compile time
+// whether this is incompatible.
+#define PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_NUMERATOR 1
+#define PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_DENOMINATOR 100'000'0000
+
+#ifdef __cplusplus
 
 #include "pw_chrono/epoch.h"
 
 namespace pw::chrono::backend {
 
-// Provide the native std::chrono::steady_clock period.
-using SystemClockPeriodSecondsRatio = std::chrono::steady_clock::period;
-
 // The std::chrono::steady_clock does not have a defined epoch.
 constexpr inline Epoch kSystemClockEpoch = pw::chrono::Epoch::kUnknown;
 
@@ -32,3 +36,5 @@ constexpr inline bool kSystemClockNmiSafe = true;
 constexpr inline bool kSystemClockFreeRunning = true;
 
 }  // namespace pw::chrono::backend
+
+#endif  // __cplusplus