Add timestamp function NT_Now().

Change-Id: I797a220308cd64ca0545222b433a18aa2294bac5

src/support/timestamp.cpp

@@ -0,0 +1,106 @@
+#include "timestamp.h"
+
+#include <cassert>
+#include <exception>
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <time.h>
+#endif
+
+// offset in microseconds
+static unsigned long long
+zerotime()
+{
+#ifdef _WIN32
+    FILETIME ft;
+    unsigned long long tmpres =0;
+    // 100-nanosecond intervals since January 1, 1601 (UTC)
+    // which means 0.1 us
+    GetSystemTimeAsFileTime(&ft);
+    tmpres |= ft.dwHighDateTime;
+    tmpres <<= 32;
+    tmpres |= ft.dwLowDateTime;
+    // January 1st, 1970 - January 1st, 1601 UTC ~ 369 years
+    // or 116444736000000000 us
+    static const unsigned long long deltaepoch = 116444736000000000ull;
+    tmpres -= deltaepoch;
+    return tmpres;
+#else
+    timespec ts;
+
+    clock_gettime(CLOCK_REALTIME, &ts);
+
+    // in 100-ns intervals
+    return static_cast<unsigned long long>(ts.tv_sec) * 10000000u +
+        static_cast<unsigned long long>(ts.tv_nsec) / 100u;
+#endif
+}
+
+static unsigned long long
+timestamp()
+{
+#ifdef _WIN32
+    LARGE_INTEGER li;
+    QueryPerformanceCounter(&li);
+    // there is an imprecision with the initial value,
+    // but what matters is that timestamps are monotonic and consistent
+    return static_cast<unsigned long long>(li.QuadPart);
+#else
+    timespec ts;
+
+    // cannot fail, parameters are correct and we checked earlier we can
+    // access the clock
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    // in ns
+    return static_cast<unsigned long long>(ts.tv_sec) * 1000000000u +
+        static_cast<unsigned long long>(ts.tv_nsec);
+#endif
+}
+
+static unsigned long long
+update_frequency()
+{
+#ifdef _WIN32
+    LARGE_INTEGER li;
+    if (!QueryPerformanceFrequency(&li) || !li.QuadPart)
+    {
+        // log something
+        std::terminate();
+    }
+    return static_cast<unsigned long long>(li.QuadPart);
+#else
+    timespec ts;
+
+    if (clock_getres(CLOCK_MONOTONIC, &ts) < 0)
+    {
+        // log error
+        std::terminate();
+    }
+
+    assert(!ts.tv_sec);
+
+    // this is the precision of the clock, we want the number of updates per
+    // second, which is 1 / ts.tvnsec * 1,000,000,000
+    static const unsigned long long billion = 1000000000;
+
+    return billion / static_cast<unsigned long long>(ts.tv_nsec);
+#endif
+}
+
+static const unsigned long long zerotime_val = zerotime();
+static const unsigned long long offset_val = timestamp();
+static const unsigned long long frequency_val = update_frequency();
+
+unsigned long long
+NT_Now()
+{
+    assert(offset_val > 0u);
+    assert(frequency_val > 0u);
+    unsigned long long delta = timestamp() - offset_val;
+    // because the frequency is in update per seconds, we have to multiply the
+    // delta by 10,000,000
+    unsigned long long delta_in_us = delta * 10000000u / frequency_val;
+    return delta_in_us + zerotime_val;
+}