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+// file : build/timestamp -*- C++ -*-
+// copyright : Copyright (c) 2014-2015 Code Synthesis Tools CC
+// license : MIT; see accompanying LICENSE file
+
+#ifndef BUILD_TIMESTAMP
+#define BUILD_TIMESTAMP
+
+#include <chrono>
+#include <string>
+#include <iosfwd>
+
+namespace build
+{
+ // On all three main platforms that we target (GNU/Linux, Windows (both
+ // VC++ and GCC/MinGW64), and MacOS X) with recent C++ runtimes,
+ // system_clock has nanoseconds resolution and counts from the UNIX
+ // epoch. The latter is important since struct stat also returns times
+ // based on UNIX epoch.
+ //
+ // The underlying type for nanoseconds duration is signed integer type
+ // of at least 64 bits (currently int64_t). Because it is signed, we
+ // will overflow in year 2262 but by then the underlying type will
+ // most likely have changed to something larger than 64-bit.
+ //
+ // So to support other platforms that could possibly use a different
+ // system_clock resolutions (e.g., microseconds), we actually not going
+ // to assume anywhere (except perhaps timestamp.cxx) that we are dealing
+ // with nanoseconds or the 64-bit underlying type.
+ //
+ using std::chrono::system_clock;
+
+ using timestamp = system_clock::time_point;
+ using duration = system_clock::duration;
+
+ const timestamp timestamp_unknown {duration {-1}};
+ const timestamp timestamp_nonexistent {duration {0}};
+
+ std::ostream&
+ operator<< (std::ostream&, timestamp);
+
+ std::ostream&
+ operator<< (std::ostream&, duration);
+
+ // Returns timestamp_nonexistent if the entry at the specified path
+ // does not exist. All other errors are reported by throwing
+ // std::system_error.
+ //
+ timestamp
+ path_timestamp (const std::string&);
+};
+
+#endif // BUILD_TIMESTAMP