1 files changed, 64 insertions, 26 deletions

diff --git a/app/models/concerns/relative_positioning.rb b/app/models/concerns/relative_positioning.rb
index 603f2dd7e5d..f1d8532a6d6 100644
--- a/app/models/concerns/relative_positioning.rb
+++ b/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