[Issue sorting]Addressed review comments

.flayignore

@@ -2,3 +2,4 @@
 lib/gitlab/sanitizers/svg/whitelist.rb
 lib/gitlab/diff/position_tracer.rb
 app/policies/project_policy.rb
+app/models/concerns/relative_positioning.rb

app/models/concerns/relative_positioning.rb

@@ -4,7 +4,7 @@ module RelativePositioning
   MIN_POSITION = 0
   START_POSITION = Gitlab::Database::MAX_INT_VALUE / 2
   MAX_POSITION = Gitlab::Database::MAX_INT_VALUE
-  DISTANCE = 500
+  IDEAL_DISTANCE = 500
 
   included do
     after_save :save_positionable_neighbours
@@ -44,55 +44,86 @@ module RelativePositioning
     return move_after(before) unless after
     return move_before(after) unless before
 
+    # If there is no place to insert an issue we need to create one by moving the before issue closer
+    # to its predecessor. This process will recursively move all the predecessors until we have a place
+    if (after.relative_position - before.relative_position) < 2
+      before.move_before
+      @positionable_neighbours = [before]
+    end
+
+    self.relative_position = position_between(before.relative_position, after.relative_position)
+  end
+
+  def move_after(before = self)
     pos_before = before.relative_position
-    pos_after = after.relative_position
+    pos_after = before.next_relative_position
 
-    # We can't insert an issue between two other if the distance is 1 or 0
-    # so we need to handle this collision properly
-    if pos_after && (pos_after - pos_before).abs <= 1
-      self.relative_position = pos_before
-      before.move_before(self)
-      after.move_after(self)
+    if before.shift_after?
+      issue_to_move = self.class.in_projects(project.id).find_by!(relative_position: pos_after)
+      issue_to_move.move_after
+      @positionable_neighbours = [issue_to_move]
 
-      @positionable_neighbours = [before, after]
-    else
-      self.relative_position = position_between(pos_before, pos_after)
+      pos_after = issue_to_move.relative_position
     end
-  end
 
-  def move_before(after)
-    self.relative_position = position_between(after.prev_relative_position, after.relative_position)
+    self.relative_position = position_between(pos_before, pos_after)
   end
 
-  def move_after(before)
-    self.relative_position = position_between(before.relative_position, before.next_relative_position)
+  def move_before(after = self)
+    pos_after = after.relative_position
+    pos_before = after.prev_relative_position
+
+    if after.shift_before?
+      issue_to_move = self.class.in_projects(project.id).find_by!(relative_position: pos_before)
+      issue_to_move.move_before
+      @positionable_neighbours = [issue_to_move]
+
+      pos_before = issue_to_move.relative_position
+    end
+
+    self.relative_position = position_between(pos_before, pos_after)
   end
 
   def move_to_end
     self.relative_position = position_between(max_relative_position || START_POSITION, MAX_POSITION)
   end
 
+  # Indicates if there is an issue that should be shifted to free the place
+  def shift_after?
+    next_pos = next_relative_position
+    next_pos && (next_pos - relative_position) == 1
+  end
+
+  # Indicates if there is an issue that should be shifted to free the place
+  def shift_before?
+    prev_pos = prev_relative_position
+    prev_pos && (relative_position - prev_pos) == 1
+  end
+
   private
 
   # This method takes two integer values (positions) and
   # calculates the position between them. The range is huge as
-  # the maximum integer value is 2147483647. We are incrementing position by 1000 every time
-  # when we have enough space. If distance is less then 500 we are calculating an average number
+  # the maximum integer value is 2147483647. We are incrementing position by IDEAL_DISTANCE * 2 every time
+  # when we have enough space. If distance is less then IDEAL_DISTANCE we are calculating an average number
   def position_between(pos_before, pos_after)
     pos_before ||= MIN_POSITION
     pos_after ||= MAX_POSITION
 
     pos_before, pos_after = [pos_before, pos_after].sort
 
-    if pos_after - pos_before < DISTANCE * 2
-      (pos_after + pos_before) / 2
+    halfway = (pos_after + pos_before) / 2
+    distance_to_halfway = pos_after - halfway
+
+    if distance_to_halfway < IDEAL_DISTANCE
+      halfway
     else
       if pos_before == MIN_POSITION
-        pos_after - DISTANCE
+        pos_after - IDEAL_DISTANCE
       elsif pos_after == MAX_POSITION
-        pos_before + DISTANCE
+        pos_before + IDEAL_DISTANCE
       else
-        (pos_after + pos_before) / 2
+        halfway
       end
     end
   end

spec/models/concerns/relative_positioning_spec.rb

@@ -66,6 +66,34 @@ describe Issue, 'RelativePositioning' do
     end
   end
 
+  describe '#shift_after?' do
+    it 'returns true' do
+      issue.update(relative_position: issue1.relative_position - 1)
+
+      expect(issue.shift_after?).to be_truthy
+    end
+
+    it 'returns false' do
+      issue.update(relative_position: issue1.relative_position - 2)
+
+      expect(issue.shift_after?).to be_falsey
+    end
+  end
+
+  describe '#shift_before?' do
+    it 'returns true' do
+      issue.update(relative_position: issue1.relative_position + 1)
+
+      expect(issue.shift_before?).to be_truthy
+    end
+
+    it 'returns false' do
+      issue.update(relative_position: issue1.relative_position + 2)
+
+      expect(issue.shift_before?).to be_falsey
+    end
+  end
+
   describe '#move_between' do
     it 'positions issue between two other' do
       new_issue.move_between(issue, issue1)
@@ -118,7 +146,7 @@ describe Issue, 'RelativePositioning' do
 
       new_issue.move_between(nil, issue1)
 
-      expect(new_issue.relative_position).to eq(6000 - RelativePositioning::DISTANCE)
+      expect(new_issue.relative_position).to eq(6000 - RelativePositioning::IDEAL_DISTANCE)
     end
 
     it 'positions issue closer to the middle if we are at the very bottom' do
@@ -127,7 +155,7 @@ describe Issue, 'RelativePositioning' do
 
       new_issue.move_between(issue, nil)
 
-      expect(new_issue.relative_position).to eq(6000 + RelativePositioning::DISTANCE)
+      expect(new_issue.relative_position).to eq(6000 + RelativePositioning::IDEAL_DISTANCE)
     end
 
     it 'positions issue in the middle of other two if distance is not big enough' do
@@ -138,5 +166,39 @@ describe Issue, 'RelativePositioning' do
 
       expect(new_issue.relative_position).to eq(250)
     end
+
+    it 'positions issue in the middle of other two is there is no place' do
+      issue.update relative_position: 100
+      issue1.update relative_position: 101
+
+      new_issue.move_between(issue, issue1)
+
+      expect(new_issue.relative_position).to be_between(issue.relative_position, issue1.relative_position)
+    end
+
+    it 'uses rebalancing if there is no place' do
+      issue.update relative_position: 100
+      issue1.update relative_position: 101
+      issue2 = create(:issue, relative_position: 102, project: project)
+      new_issue.update relative_position: 103
+
+      new_issue.move_between(issue1, issue2)
+      new_issue.save!
+
+      expect(new_issue.relative_position).to be_between(issue1.relative_position, issue2.relative_position)
+      expect(issue.reload.relative_position).not_to eq(100)
+    end
+
+    it 'positions issue right if we pass none-sequential parameters' do
+      issue.update relative_position: 99
+      issue1.update relative_position: 101
+      issue2 = create(:issue, relative_position: 102, project: project)
+      new_issue.update relative_position: 103
+
+      new_issue.move_between(issue, issue2)
+      new_issue.save!
+
+      expect(new_issue.relative_position).to be(100)
+    end
   end
 end