[lib] Fix time_elapsed()

The previous implementation was very naive and incorrect.

libowlps/libowlps.c

@@ -250,20 +250,63 @@ void owl_timestamp_to_string(char *const dst, const owl_timestamp src)
 uint_fast32_t owl_time_elapsed_ms(const owl_timestamp d1,
                                   const owl_timestamp d2)
 {
-  return owl_timestamp_to_ms(owl_time_elapsed(d1, d2)) ;
+  owl_timestamp elapsed ;
+  owl_time_elapsed(&d1, &d2, &elapsed) ;
+  return owl_timestamp_to_ms(elapsed) ;
 }
 
 
 /*
- * Returns a owl_timestamp storing the time between two dates.
+ * Computes the time difference between two owl_timestamp 'd1' and
+ * 'd2'.
+ * Note that it is a delay, not a simple substraction, therefore the
+ * result is always positive. The result is stored in the 'elapsed'
+ * argument.
  */
-owl_timestamp owl_time_elapsed(const owl_timestamp d1,
-                               const owl_timestamp d2)
+void owl_time_elapsed(const owl_timestamp *const d1,
+                      const owl_timestamp *const d2,
+                      owl_timestamp *const elapsed)
 {
-  owl_timestamp elapsed ;
-  elapsed.tv_sec = abs(d1.tv_sec - d2.tv_sec) ;
-  elapsed.tv_nsec = abs(d1.tv_nsec - d2.tv_nsec) ;
-  return elapsed ;
+  int_fast32_t sec, nsec ;
+
+  assert(d1) ;
+  assert(d2) ;
+  assert(elapsed) ;
+
+  sec = (int_fast64_t) d1->tv_sec - d2->tv_sec ;
+  nsec = (int_fast64_t) d1->tv_nsec - d2->tv_nsec ;
+
+  if (sec == 0)
+    {
+      elapsed->tv_sec = 0 ;
+      elapsed->tv_nsec = abs(nsec) ;
+    }
+  else if (sec > 0)
+    {
+      if (nsec >= 0)
+        {
+          elapsed->tv_sec = sec ;
+          elapsed->tv_nsec = nsec ;
+        }
+      else // nsec < 0
+        {
+          elapsed->tv_sec = sec - 1 ;
+          elapsed->tv_nsec = nsec + 1000000000ul ;
+        }
+    }
+  else // sec < 0
+    {
+      if (nsec > 0)
+        {
+          elapsed->tv_sec = abs(sec) - 1 ;
+          elapsed->tv_nsec = 1000000000ul - nsec ;
+        }
+      else // nsec <= 0
+        {
+          elapsed->tv_sec = abs(sec) ;
+          elapsed->tv_nsec = abs(nsec) ;
+        }
+    }
 }
 
 

libowlps/owlps.h

@@ -221,8 +221,9 @@ uint64_t owl_timestamp_to_ms(const owl_timestamp d) ;
 void owl_timestamp_to_string(char *const dst, const owl_timestamp src) ;
 uint_fast32_t owl_time_elapsed_ms(const owl_timestamp d1,
                                   const owl_timestamp d2) ;
-owl_timestamp owl_time_elapsed(const owl_timestamp d1,
-                               const owl_timestamp d2) ;
+void owl_time_elapsed(const owl_timestamp *const d1,
+                      const owl_timestamp *const d2,
+                      owl_timestamp *const elapsed) ;
 
 // Endianess
 owl_timestamp owl_hton_timestamp(const owl_timestamp d) ;