From 4441427943db72f03a660e2ceb6b8e781c43739f Mon Sep 17 00:00:00 2001
From: Geoffrey McRae <geoff@hostfission.com>
Date: Fri, 22 May 2020 20:45:59 +1000
Subject: [PATCH] [client] implemented better clock drift correction

---
 VERSION                      |  2 +-
 client/src/main.c            | 42 +++++++++++-------------------------
 common/include/common/time.h | 36 +++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 30 deletions(-)

diff --git a/VERSION b/VERSION
index 3ef28929..f0c899ed 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-B2-rc1-6-g60f665a65c+1
\ No newline at end of file
+B2-rc1-7-gf5da432d38+1
\ No newline at end of file
diff --git a/client/src/main.c b/client/src/main.c
index 9512c325..c73d271a 100644
--- a/client/src/main.c
+++ b/client/src/main.c
@@ -154,28 +154,23 @@ static int renderThread(void * unused)
 
   while(state.running)
   {
-    // if our clock is too far out of sync, resync it
-    // this can happen when switching to/from a TTY, or due to clock drift
-    // we only check this once every 100 frames
-    if (state.frameTime > 0 && ++resyncCheck == 100)
+    if (state.frameTime > 0)
     {
-      resyncCheck = 0;
+      tsAdd(&time, state.frameTime);
 
-      struct timespec tmp;
-      clock_gettime(CLOCK_REALTIME, &tmp);
-      if (tmp.tv_nsec - time.tv_nsec < 0)
+      // if our clock is too far out of sync, resync it
+      // this can happen when switching to/from a TTY, or due to clock drift
+      // we only check this once every 100 frames
+      if (++resyncCheck == 100)
       {
-        tmp.tv_sec -= time.tv_sec - 1;
-        tmp.tv_nsec = 1000000000 + tmp.tv_nsec - time.tv_nsec;
-      }
-      else
-      {
-        tmp.tv_sec  -= time.tv_sec;
-        tmp.tv_nsec -= time.tv_nsec;
-      }
+        resyncCheck = 0;
 
-      if (tmp.tv_sec > 1)
-        clock_gettime(CLOCK_REALTIME, &time);
+        struct timespec end, diff;
+        clock_gettime(CLOCK_REALTIME, &end);
+        tsDiff(&diff, &time, &end);
+        if (diff.tv_sec > 0 || diff.tv_nsec > 1000000000) // 100ms
+          clock_gettime(CLOCK_REALTIME, &time);
+      }
     }
 
     if (state.lgrResize)
@@ -218,18 +213,7 @@ static int renderThread(void * unused)
     }
 
     if (state.frameTime > 0)
-    {
-      uint64_t nsec = time.tv_nsec + state.frameTime;
-      if(nsec > 1e9)
-      {
-        time.tv_nsec = nsec - 1e9;
-        ++time.tv_sec;
-      }
-      else
-        time.tv_nsec = nsec;
-
       lgWaitEventAbs(e_frame, &time);
-    }
   }
 
   state.running = false;
diff --git a/common/include/common/time.h b/common/include/common/time.h
index b1b0d9d0..7abada18 100644
--- a/common/include/common/time.h
+++ b/common/include/common/time.h
@@ -66,6 +66,42 @@ static inline void nsleep(uint64_t ns)
   };
   nanosleep(&ts, NULL);
 }
+
+static inline void tsDiff(struct timespec *diff, const struct timespec *left,
+              const struct timespec *right)
+{
+  diff->tv_sec = left->tv_sec - right->tv_sec;
+  diff->tv_nsec = left->tv_nsec - right->tv_nsec;
+  if (diff->tv_nsec < 0)
+  {
+    --diff->tv_sec;
+    diff->tv_nsec += 1000000000;
+  }
+}
+
+static inline uint32_t __iter_div_u64_rem(uint64_t dividend, uint32_t divisor, uint64_t *remainder)
+{
+  uint32_t ret = 0;
+
+  while (dividend >= divisor) {
+    /* The following asm() prevents the compiler from
+       optimising this loop into a modulo operation.  */
+    asm("" : "+rm"(dividend));
+
+    dividend -= divisor;
+    ret++;
+  }
+
+  *remainder = dividend;
+
+  return ret;
+}
+
+static inline void tsAdd(struct timespec *a, uint64_t ns)
+{
+  a->tv_sec  += __iter_div_u64_rem(a->tv_nsec + ns, 1000000000L, &ns);
+  a->tv_nsec = ns;
+}
 #endif
 
 typedef bool (*LGTimerFn)(void * udata);