Rounding errors could make time_alloc imprecise

mrbgems/mruby-time/src/time.c

@@ -6,6 +6,7 @@
 
 #include <stdio.h>
 #include <time.h>
+#include <math.h>
 #include "mruby.h"
 #include "mruby/class.h"
 #include "mruby/data.h"
@@ -208,12 +209,12 @@ time_alloc(mrb_state *mrb, double sec, double usec, enum mrb_timezone timezone)
   out_of_range:
     mrb_raisef(mrb, E_ARGUMENT_ERROR, "%S out of Time range", mrb_float_value(mrb, sec));
   }
-  tm->usec = (time_t)((sec - tm->sec) * 1.0e6 + usec);
+  tm->usec = (time_t)llrint((sec - tm->sec) * 1.0e6 + usec);
   while (tm->usec < 0) {
     tm->sec--;
     tm->usec += 1000000;
   }
-  while (tm->usec > 1000000) {
+  while (tm->usec >= 1000000) {
     tm->sec++;
     tm->usec -= 1000000;
   }

mrbgems/mruby-time/test/time.rb

@@ -203,3 +203,11 @@ assert('day of week methods') do
   assert_false t.friday?
   assert_false t.saturday?
 end
+
+assert('2000 times 500us make a second') do
+  t = Time.utc 2015
+  2000.times do
+    t += 0.0005
+  end
+  t.usec == 0
+end