From ffc1e5f889a9a3dce91314fed32164de660d1684 Mon Sep 17 00:00:00 2001
From: Caleb Buxton <me@cpb.ca>
Date: Fri, 10 Dec 2021 11:03:04 -0800
Subject: [PATCH] fix: equivalent negative durations add to the same time
 (#43795)

* bug: illustrate negative durations don't add to the same time

* fix: equivalent negative durations add to the same time

Co-authored-by: Caleb <me@cpb.ca>
Co-authored-by: Braden Staudacher <braden.staudacher@chime.com>

* Updates CHANGELOG with fix to `ActiveSupport::Duration.build`
---
 activesupport/CHANGELOG.md                   |  7 +++++++
 activesupport/lib/active_support/duration.rb |  7 ++++---
 activesupport/test/core_ext/duration_test.rb | 11 +++++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 04e45a01e2a..86e54924393 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Fix `ActiveSupport::Duration.build` to support negative values.
 
+    The algorithm to collect the `parts` of the `ActiveSupport::Duration`
+    ignored the sign of the `value` and accumulated incorrect part values. This
+    impacted `ActiveSupport::Duration#sum` (which is dependent on `parts`) but
+    not `ActiveSupport::Duration#eql?` (which is dependent on `value`).
+
+    *Caleb Buxton*, *Braden Staudacher*
 
 Please check [7-0-stable](https://github.com/rails/rails/blob/7-0-stable/activesupport/CHANGELOG.md) for previous changes.
diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index a393c9a07ab..19986d1d0c9 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -191,13 +191,14 @@ module ActiveSupport
         end
 
         parts = {}
-        remainder = value.round(9)
+        remainder_sign = value <=> 0
+        remainder = value.round(9).abs
         variable = false
 
         PARTS.each do |part|
           unless part == :seconds
             part_in_seconds = PARTS_IN_SECONDS[part]
-            parts[part] = remainder.div(part_in_seconds)
+            parts[part] = remainder.div(part_in_seconds) * remainder_sign
             remainder %= part_in_seconds
 
             unless parts[part].zero?
@@ -206,7 +207,7 @@ module ActiveSupport
           end
         end unless value == 0
 
-        parts[:seconds] = remainder
+        parts[:seconds] = remainder * remainder_sign
 
         new(value, parts, variable)
       end
diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 5f38ccf2905..f957daa8948 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -752,6 +752,17 @@ class DurationTest < ActiveSupport::TestCase
     assert (1.day + 12.hours).variable?
   end
 
+  def test_duration_symmetry
+    time = Time.parse("Dec 7, 2021")
+    expected_time = Time.parse("2021-12-06 23:59:59")
+
+    assert_equal expected_time, time + -1.second
+    assert_equal expected_time, time + ActiveSupport::Duration.build(1) * -1
+    assert_equal expected_time, time + -ActiveSupport::Duration.build(1)
+    assert_equal expected_time, time + ActiveSupport::Duration::Scalar.new(-1)
+    assert_equal expected_time, time + ActiveSupport::Duration.build(-1)
+  end
+
   private
     def eastern_time_zone
       if Gem.win_platform?