Merge branch 'fix_relative_position_calculation' into 'master'

.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

@@ -2,16 +2,14 @@ module RelativePositioning
   extend ActiveSupport::Concern
 
   MIN_POSITION = 0
+  START_POSITION = Gitlab::Database::MAX_INT_VALUE / 2
   MAX_POSITION = Gitlab::Database::MAX_INT_VALUE
+  IDEAL_DISTANCE = 500
 
   included do
     after_save :save_positionable_neighbours
   end
 
-  def min_relative_position
-    self.class.in_projects(project.id).minimum(:relative_position)
-  end
-
   def max_relative_position
     self.class.in_projects(project.id).maximum(:relative_position)
   end
@@ -26,7 +24,7 @@ module RelativePositioning
         maximum(:relative_position)
     end
 
-    prev_pos || MIN_POSITION
+    prev_pos
   end
 
   def next_relative_position
@@ -39,55 +37,95 @@ module RelativePositioning
         minimum(:relative_position)
     end
 
-    next_pos || MAX_POSITION
+    next_pos
   end
 
   def move_between(before, after)
     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 = before.next_relative_position
+
+    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]
+
+      pos_after = issue_to_move.relative_position
+    end
+
+    self.relative_position = position_between(pos_before, pos_after)
+  end
+
+  def move_before(after = self)
     pos_after = after.relative_position
+    pos_before = after.prev_relative_position
 
-    if pos_after && (pos_before == pos_after)
-      self.relative_position = pos_before
-      before.move_before(self)
-      after.move_after(self)
+    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]
 
-      @positionable_neighbours = [before, after]
-    else
-      self.relative_position = position_between(pos_before, pos_after)
+      pos_before = issue_to_move.relative_position
     end
+
+    self.relative_position = position_between(pos_before, pos_after)
   end
 
-  def move_before(after)
-    self.relative_position = position_between(after.prev_relative_position, after.relative_position)
+  def move_to_end
+    self.relative_position = position_between(max_relative_position || START_POSITION, MAX_POSITION)
   end
 
-  def move_after(before)
-    self.relative_position = position_between(before.relative_position, before.next_relative_position)
+  # 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
 
-  def move_to_end
-    self.relative_position = position_between(max_relative_position, MAX_POSITION)
+  # 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 some random position between them. The range is huge as
-  # the maximum integer value is 2147483647. Ideally, the calculated value would be
-  # exactly between those terminating values, but this will introduce possibility of a race condition
-  # so two or more issues can get the same value, we want to avoid that and we also want to avoid
-  # using a lock here. If we have two issues with distance more than one thousand, we are OK.
-  # Given the huge range of possible values that integer can fit we shoud never face a problem.
+  # calculates the position between them. The range is huge as
+  # 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
 
-    rand(pos_before.next..pos_after.pred)
+    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 - IDEAL_DISTANCE
+      elsif pos_after == MAX_POSITION
+        pos_before + IDEAL_DISTANCE
+      else
+        halfway
+      end
+    end
   end
 
   def save_positionable_neighbours

db/post_migrate/20170309171644_reset_relative_position_for_issue.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ResetRelativePositionForIssue < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    update_column_in_batches(:issues, :relative_position, nil) do |table, query|
+      query.where(table[:relative_position].not_eq(nil))
+    end
+  end
+
+  def down
+  end
+end

spec/models/concerns/relative_positioning_spec.rb

@@ -12,12 +12,6 @@ describe Issue, 'RelativePositioning' do
     end
   end
 
-  describe '#min_relative_position' do
-    it 'returns maximum position' do
-      expect(issue.min_relative_position).to eq issue.relative_position
-    end
-  end
-
   describe '#max_relative_position' do
     it 'returns maximum position' do
       expect(issue.max_relative_position).to eq issue1.relative_position
@@ -29,8 +23,8 @@ describe Issue, 'RelativePositioning' do
       expect(issue1.prev_relative_position).to eq issue.relative_position
     end
 
-    it 'returns minimum position if there is no issue above' do
-      expect(issue.prev_relative_position).to eq RelativePositioning::MIN_POSITION
+    it 'returns nil if there is no issue above' do
+      expect(issue.prev_relative_position).to eq nil
     end
   end
 
@@ -39,8 +33,8 @@ describe Issue, 'RelativePositioning' do
       expect(issue.next_relative_position).to eq issue1.relative_position
     end
 
-    it 'returns next position if there is no issue below' do
-      expect(issue1.next_relative_position).to eq RelativePositioning::MAX_POSITION
+    it 'returns nil if there is no issue below' do
+      expect(issue1.next_relative_position).to eq nil
     end
   end
 
@@ -72,6 +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)
@@ -100,5 +122,83 @@ describe Issue, 'RelativePositioning' do
       expect(new_issue.relative_position).to be > issue.relative_position
       expect(issue.relative_position).to be < issue1.relative_position
     end
+
+    it 'positions issues between other two if distance is 1' do
+      issue1.update relative_position: issue.relative_position + 1
+
+      new_issue.move_between(issue, issue1)
+
+      expect(new_issue.relative_position).to be > issue.relative_position
+      expect(issue.relative_position).to be < issue1.relative_position
+    end
+
+    it 'positions issue in the middle of other two if distance is big enough' do
+      issue.update relative_position: 6000
+      issue1.update relative_position: 10000
+
+      new_issue.move_between(issue, issue1)
+
+      expect(new_issue.relative_position).to eq(8000)
+    end
+
+    it 'positions issue closer to the middle if we are at the very top' do
+      issue1.update relative_position: 6000
+
+      new_issue.move_between(nil, issue1)
+
+      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
+      issue.update relative_position: 6000
+      issue1.update relative_position: nil
+
+      new_issue.move_between(issue, nil)
+
+      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
+      issue.update relative_position: 100
+      issue1.update relative_position: 400
+
+      new_issue.move_between(issue, issue1)
+
+      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