clock_gettime/win: first round clock_gettime for windows

deps/rmonotonic/src/clock_gettime_win.c

+#if defined(_WIN32) || defined(_WIN64)
+#include <windows.h>
+#include <time.h>
+
+#define MS_PER_SEC      1000ULL     // MS = milliseconds
+#define US_PER_MS       1000ULL     // US = microseconds
+#define HNS_PER_US      10ULL       // HNS = hundred-nanoseconds (e.g., 1 hns = 100 ns)
+#define NS_PER_US       1000ULL
+
+#define HNS_PER_SEC     (MS_PER_SEC * US_PER_MS * HNS_PER_US)
+#define NS_PER_HNS      (100ULL)    // NS = nanoseconds
+#define NS_PER_SEC      (MS_PER_SEC * US_PER_MS * NS_PER_US)
+
+int clock_gettime_monotonic(struct timespec *tv)
+{
+    static LARGE_INTEGER ticksPerSec;
+    LARGE_INTEGER ticks;
+    double seconds;
+
+    if (!ticksPerSec.QuadPart) {
+        QueryPerformanceFrequency(&ticksPerSec);
+        if (!ticksPerSec.QuadPart) {
+            errno = ENOTSUP;
+            return -1;
+        }
+    }
+
+    QueryPerformanceCounter(&ticks);
+
+    seconds = (double) ticks.QuadPart / (double) ticksPerSec.QuadPart;
+    tv->tv_sec = (time_t)seconds;
+    tv->tv_nsec = (long)((ULONGLONG)(seconds * NS_PER_SEC) % NS_PER_SEC);
+
+    return 0;
+}
+
+int clock_gettime_realtime(struct timespec *tv)
+{
+    FILETIME ft;
+    ULARGE_INTEGER hnsTime;
+
+    GetSystemTimeAsFileTime(&ft);
+
+    hnsTime.LowPart = ft.dwLowDateTime;
+    hnsTime.HighPart = ft.dwHighDateTime;
+
+    // To get POSIX Epoch as baseline, subtract the number of hns intervals from Jan 1, 1601 to Jan 1, 1970.
+    hnsTime.QuadPart -= (11644473600ULL * HNS_PER_SEC);
+
+    // modulus by hns intervals per second first, then convert to ns, as not to lose resolution
+    tv->tv_nsec = (long) ((hnsTime.QuadPart % HNS_PER_SEC) * NS_PER_HNS);
+    tv->tv_sec = (long) (hnsTime.QuadPart / HNS_PER_SEC);
+
+    return 0;
+}
+
+int clock_gettime(clockid_t type, struct timespec *tp)
+{
+    if (type == CLOCK_MONOTONIC)
+    {
+        return clock_gettime_monotonic(tp);
+    }
+    else if (type == CLOCK_REALTIME)
+    {
+        return clock_gettime_realtime(tp);
+    }
+
+    errno = ENOTSUP;
+    return -1;
+}
+
+#endif