Calculate travel_offset to align with the precision of argument to Timecop.travel (#421)

* Calculate travel_offset to align with clock precision

Subtracting two `Time`s returns a `Float`, which may not be accurate
down to subsecond resolution. Because `Float`s are stored as double-
precision values (IEEE 754), they can have resolutions much higher than
the typical minimum clock precision of 10e-9 seconds. Which can result
in two `Time` object not comparing as equal when they are the same down
to the nanosecond, when one has had a travel_offset applied to it.

* Use the realtime clock's resolution when testing travel_offset's alignment

* Improve failure message for travel_offset alignment test

* Test that time_offset keeps precision of very high times passed to Timecop.travel

* Mention this change in History.md

Author: dgholz

History.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Calculate travel_offset to align with the precision of argument to Timecop.travel ([#421](https://github.com/travisjeffery/timecop/pull/421))
+
 ## v0.9.10
 
 - Make Process.clock_gettime configurable and turned off by default (for backwards compatability) ([#427](https://github.com/travisjeffery/timecop/pull/427))

lib/timecop/time_stack_item.rb

@@ -157,7 +157,7 @@ def parse_time(*args)
     end
 
     def compute_travel_offset
-      time - Time.now_without_mock_time
+      time.to_r - Time.now_without_mock_time.to_r
     end
 
     def times_are_equal_within_epsilon t1, t2, epsilon_in_seconds

test/time_stack_item_test.rb

@@ -296,4 +296,22 @@ def test_datetime_timezones
       assert_equal dt, now, "#{dt.to_f}, #{now.to_f}"
     end
   end
+
+  def test_travel_offset_aligns_to_clock
+    t = Time.now
+    stack_item = Timecop::TimeStackItem.new(:travel, t)
+    travel_offset_denom = stack_item.travel_offset.to_r.denominator
+    clock_resolution = Process.clock_getres(:CLOCK_REALTIME, :hertz)
+    assert_equal 0, clock_resolution.modulo(travel_offset_denom),
+      "travel offset precision (#{travel_offset_denom}) does not align with clock resolution (#{clock_resolution})"
+  end
+
+  def test_travel_offset_aligns_to_travel_time
+    t = Time.now + 0.001_002_003_004
+    stack_item = Timecop::TimeStackItem.new(:travel, t)
+    travel_offset_denom = stack_item.travel_offset.to_r.denominator
+    travel_time_denom = t.to_r.denominator
+    assert_equal 0, travel_time_denom.modulo(travel_offset_denom),
+      "travel offset precision (#{travel_offset_denom}) does not align with travel time precision (#{travel_time_denom})"
+  end
 end