More accurately determine where to place tiles after Undo

Author: m1foley

Diff for: TODO.md

@@ -2,7 +2,7 @@
 
 - CD: https://fly.io/docs/app-guides/continuous-deployment-with-github-actions/
 - Put everyone's discards in front of their hand (suggested by Mom)
-- Bug: Can't undo to get first discarded tile of the game from bot (human went last, missed a discarded tile, can't go back to beginning of game). If there are no human actions to go back to, Undo should return to beginning of game.
+- If no undoable moves, Undo button should reset the game to get first discarded tile of the game from bot (human went last, missed a discarded tile, can't go back to beginning of game).
 - Bug: Undoing a win after a bot drew a tile hung bot (workaround: paused & unpaused bots)
 - Shrink tile sizes on mobile
 - Lobby not receiving bot player added
@@ -33,7 +33,7 @@
 - Display everyone's wind direction instead of staircase
 - Animate deal: 4 at a time goes into people's hand
 - Sanity check for (possible race condition) bug: rearranging hand leads to duplicate tiles. Last time it happened after peek tile got put into hand, and user drag & dropped that new tile.
-- Remember sorted hand so it doesn't get reshuffled when losing internet connection
+- Remember sorted hand in browser so it doesn't get reshuffled when losing internet connection
 
 ## Future rule enforcement
 - Winning tile was the last one picked up

Diff for: lib/mjw/core/seat.ex

@@ -211,16 +211,36 @@ defmodule Mjw.Seat do
       Enum.filter([seat.peektile, seat.wintile], & &1)
   end
 
-  # Doesn't check wintile because it can never be populated in an undo_state
+  defp restore_to_hand(%__MODULE__{} = seat, %__MODULE__{} = undo_state_seat, tile)
+       when undo_state_seat.peektile == tile do
+    %{seat | peektile: tile}
+  end
+
+  # When restoring a tile to a hand during an Undo, the hand probably changed
+  # since then so it's not always easy to determine the ideal position. We can
+  # assume players sort their hands, so attempt to restore it next to the tile
+  # that would follow it by sort order. If not available it goes to the end.
   defp restore_to_hand(%__MODULE__{} = seat, %__MODULE__{} = undo_state_seat, tile) do
-    if undo_state_seat.peektile == tile do
-      %{seat | peektile: tile}
-    else
-      removed_from_list =
-        [:exposed, :concealed, :hiddengongs]
-        |> Enum.find(fn list -> tile in Map.get(undo_state_seat, list) end)
+    # Doesn't check wintile because it can never be populated in an undo_state
+    removed_from_list =
+      [:exposed, :concealed, :hiddengongs]
+      |> Enum.find(fn list -> tile in Map.get(undo_state_seat, list) end)
 
-      seat |> Map.update!(removed_from_list, fn list -> list ++ [tile] end)
-    end
+    seat
+    |> Map.update!(removed_from_list, fn list ->
+      sorted_list = (list ++ [tile]) |> Mjw.Tile.sort()
+      sorted_list_index = sorted_list |> Enum.find_index(&(&1 == tile))
+      insert_before_tile = sorted_list |> Enum.at(sorted_list_index + 1)
+
+      insert_before_index =
+        if insert_before_tile do
+          list |> Enum.find_index(&(&1 == insert_before_tile))
+        else
+          length(list)
+        end
+
+      Enum.slice(list, 0, insert_before_index) ++
+        [tile] ++ Enum.slice(list, insert_before_index..-1//1)
+    end)
   end
 end

Diff for: lib/mjw/core/tile.ex

@@ -5,7 +5,7 @@ defmodule Mjw.Tile do
   @tile_format_regex ~r/^[ncbwd][a-z1-9]-[0-3]$/
 
   @doc """
-  Sort according to beauty, with special tiles last
+  Sort by suit and then number
   """
   def sort(tiles) do
     Enum.sort_by(tiles, fn tile ->

Diff for: test/mjw/core/seat_test.exs

@@ -587,7 +587,7 @@ defmodule Mjw.SeatTest do
 
       result = Mjw.Seat.merge_for_undo(seat, undo_state_seat)
 
-      assert result == %{seat | hiddengongs: ["n3-0", "n4-0", "n3-1"]}
+      assert result == %{seat | hiddengongs: ["n3-0", "n3-1", "n4-0"]}
     end
 
     test "restores a tile removed from the peektile by the undoable action" do

Diff for: test/mjw/core/tile_test.exs

@@ -27,6 +27,7 @@ defmodule Mjw.TileTest do
           "we-0",
           "we-1",
           "b9-0",
+          "b8-0",
           "b9-1",
           "n1-3",
           "dp-0",
@@ -40,6 +41,7 @@ defmodule Mjw.TileTest do
                "n1-3",
                "c1-0",
                "b1-3",
+               "b8-0",
                "b9-0",
                "b9-1",
                "we-0",