More proactive rebalancing to ensure there's always a gap for insertions

lib/ecto_ranked.ex

@@ -41,27 +41,40 @@ defmodule EctoRanked do
     case position do
       pos when pos in ["first", :first] ->
         first = get_current_first(cs, options)
-        rank_between(cs, options, @min, first || @max)
+
+        if first do
+          rank_between(cs, options, pos, @min, first || @max)
+        else
+          update_index_from_position(cs, options, :middle)
+        end
+
+      pos when pos in ["middle", :middle] ->
+        rank_between(cs, options, pos, @min, @max)
 
       pos when pos in ["last", :last] ->
         last = get_current_last(cs, options)
-        rank_between(cs, options, last || @min, @max)
+
+        if last do
+          rank_between(cs, options, pos, last || @min, @max)
+        else
+          update_index_from_position(cs, options, :middle)
+        end
 
       pos when pos in ["up", :up] ->
         case find_prev_two(cs, options) do
           nil -> cs
-          {min, max} -> rank_between(cs, options, min, max)
+          {min, max} -> rank_between(cs, options, pos, min, max)
         end
 
       pos when pos in ["down", :down] ->
         case find_next_two(cs, options) do
           nil -> cs
-          {min, max} -> rank_between(cs, options, min, max)
+          {min, max} -> rank_between(cs, options, pos, min, max)
         end
 
       number when is_integer(number) ->
         {min, max} = find_neighbors(cs, options, number)
-        rank_between(cs, options, min, max)
+        rank_between(cs, options, number, min, max)
 
       nil ->
         if get_field(cs, options.rank_field) &&
@@ -87,9 +100,13 @@ defmodule EctoRanked do
     get_change(cs, scope_field)
   end
 
-  defp rank_between(cs, options, min, max) do
-    rank = EctoRanked.Utils.ceiling((max - min) / 2) + min
-    rank_at(cs, options, rank)
+  defp rank_between(cs, options, position, min, max) do
+    if max - min <= 1 do
+      cs |> rebalance_ranks(options) |> update_index_from_position(options, position)
+    else
+      rank = EctoRanked.Utils.ceiling((max - min) / 2) + min
+      rank_at(cs, options, rank)
+    end
   end
 
   defp rank_at(cs, options, rank) do

test/ranked_test.exs

@@ -61,6 +61,29 @@ defmodule EctoRanked.RankedTest do
         Model.changeset(%Model{}, %{my_position: "wrong"}) |> Repo.insert!()
       end
     end
+
+    test "appending to list when @max is being used rebalances instead of shifting" do
+      for _i <- 1..31 do
+        %Model{} |> Model.changeset(%{}) |> Repo.insert!()
+      end
+
+      # Confirm the first 31 items perfectly spread between 0 and @max - 1
+      models = Model |> order_by(asc: :my_rank) |> Repo.all()
+      assert List.first(models).my_rank == 0
+      assert List.last(models).my_rank == @max - 1
+
+      # The 32nd item would leave no gap, so rebalance
+      %Model{} |> Model.changeset(%{}) |> Repo.insert!()
+      models = Model |> order_by(asc: :my_rank) |> Repo.all()
+      assert List.first(models).my_rank == -2_017_333_123
+      assert List.last(models).my_rank == 2_017_333_123
+
+      # Adding another item now fills the newly-created gap
+      %Model{} |> Model.changeset(%{}) |> Repo.insert!()
+      models = Model |> order_by(asc: :my_rank) |> Repo.all()
+      assert List.first(models).my_rank == -2_017_333_123
+      assert List.last(models).my_rank == 2_082_408_385
+    end
   end
 
   describe "updates" do