Windows support for tbxx::time_accu

tbxx/time_accu.hpp

@@ -1,8 +1,60 @@
 #ifndef TBXX_TIME_ACCU_HPP
 #define TBXX_TIME_ACCU_HPP
 
+#ifdef _MSC_VER
+#include <winsock2.h>
+
+inline void timeradd(timeval *a, timeval *b, timeval *res) {
+  res->tv_sec = a->tv_sec + b->tv_sec;
+  res->tv_usec = a->tv_usec + b->tv_usec;
+  if(res->tv_usec > 1000000) {
+    ++res->tv_sec;
+    res->tv_usec -= 1000000;
+  }
+}
+
+inline void timersub(timeval *a, timeval *b, timeval *res) {
+  res->tv_sec = a->tv_sec - b->tv_sec;
+  res->tv_usec = a->tv_usec - b->tv_usec;
+  if(res->tv_usec < 0) {
+    --res->tv_sec;
+    res->tv_usec += 1000000;
+  }
+}
+
+/* C.f. the following pieces of official Microsoft documentation:
+
+GetSystemTimeAsFileTime function:
+https://msdn.microsoft.com/en-us/library/windows/desktop/ms724397(v=vs.85).aspx
+
+FILETIME structure:
+https://msdn.microsoft.com/en-us/library/windows/desktop/ms724284(v=vs.85).aspx
+
+Conversion from Windows begining of time to Unix epoch:
+http://stackoverflow.com/a/6161842/1428153
+
+Note: time zone not supported but we don't need it for the purpose
+of time_accu below.
+*/
+inline int gettimeofday(struct timeval *tv, void *tz) {
+  union {
+    // time since 1 Jan 1601 in 10-th of micro-seconds (usec)
+    unsigned long long t;
+    FILETIME ft;
+  };
+  GetSystemTimeAsFileTime (&ft);
+  t /= 10LL; //in usec
+  tv->tv_usec = (long)(t % 1000000);
+  tv->tv_sec  = (long)(t / 1000000 - 11644473600LL);
+  return 0;
+}
+
+#else
+
 #include <sys/time.h>
 
+#endif
+
 namespace tbxx {
 
   struct time_accu