time/FileTime: move code to SystemClock.hxx

src/time/FileTime.hxx

 /*
- * Copyright 2013-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2013-2021 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -30,6 +30,8 @@
 #ifndef FILE_TIME_HXX
 #define FILE_TIME_HXX
 
+#include "SystemClock.hxx"
+
 #include <fileapi.h>
 
 #include <chrono>
@@ -108,15 +110,7 @@ FileTimeToUnixEpochDuration(FILETIME ft) noexcept
 inline std::chrono::system_clock::time_point
 FileTimeToChrono(FILETIME ft) noexcept
 {
-	/* this is guaranteed to be 0 in C++20 */
-	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
-
-	const auto windows_duration = FileTimeToUnixEpochDuration(ft);
-	const auto sys_duration =
-		std::chrono::duration_cast<std::chrono::system_clock::duration>
-		(windows_duration);
-
-	return unix_epoch + sys_duration;
+	return TimePointAfterUnixEpoch(FileTimeToUnixEpochDuration(ft));
 }
 
 constexpr FILETIME
@@ -143,10 +137,7 @@ UnixEpochDurationToFileTime(FileTimeDuration d) noexcept
 inline FILETIME
 ChronoToFileTime(std::chrono::system_clock::time_point tp) noexcept
 {
-	/* this is guaranteed to be 0 in C++20 */
-	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
-
-	const auto since_unix_epoch = tp - unix_epoch;
+	const auto since_unix_epoch = DurationSinceUnixEpoch(tp);
 	const auto ft_since_unix_epoch =
 		std::chrono::duration_cast<FileTimeDuration>(since_unix_epoch);
 

src/time/SystemClock.hxx

+/*
+ * Copyright 2021 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <chrono>
+
+/**
+ * Calculate a std::chrono::system_clock::time_point from a duration
+ * relative to the UNIX epoch (1970-01-01T00:00Z).
+ */
+template<class Rep, class Period>
+[[gnu::const]]
+std::chrono::system_clock::time_point
+TimePointAfterUnixEpoch(const std::chrono::duration<Rep,Period> &d) noexcept
+{
+	/* this is guaranteed to be 0 in C++20 */
+	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
+
+	return unix_epoch +
+		std::chrono::duration_cast<std::chrono::system_clock::duration>(d);
+}
+
+/**
+ * Calculate the duration that has passed since the UNIX epoch
+ * (1970-01-01T00:00Z).
+ */
+[[gnu::const]]
+inline std::chrono::system_clock::duration
+DurationSinceUnixEpoch(const std::chrono::system_clock::time_point &tp) noexcept
+{
+	/* this is guaranteed to be 0 in C++20 */
+	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
+
+	return tp - unix_epoch;
+}