Comments.

game.exs

@@ -1,20 +1,23 @@
 defmodule Game do
   @default_options [
     # Number of cells per row and per column.
-    board_dimensions: {3, 2},
+    board_dimensions: {4, 4},
     # Value of the singular piece present at the beginning of the game.
     starting_number: 2,
     # Value of the piece randomly inserted into the board at the beginning of
     # each turn.
     turn_start_number: 1,
     # Value of the piece which, when present on the board, results in a win.
-    winning_number: 4
+    winning_number: 2048
   ]
   @cell_size 5
 
   def init(opts \\ []) do
     opts = Keyword.validate!(opts, @default_options)
 
+    # TODO validate that value options are all powers of two and that starting
+    # and turn start values are both less than winning value.
+
     game = %{
       board: starting_board(opts[:board_dimensions], opts[:starting_number]),
       score: 0,
@@ -104,13 +107,11 @@ defmodule Game do
   end
 
   defp render_board(%{cells: cells, dimensions: {num_rows, num_cols}}) do
-    Enum.map(1..num_rows, fn row ->
-      Enum.map(1..num_cols, fn col ->
+    Enum.map_join(1..num_rows, "\n", fn row ->
+      Enum.map_join(1..num_cols, "", fn col ->
         render_cell(cells, {row, col})
       end)
-      |> Enum.join("")
     end)
-    |> Enum.join("\n")
   end
 
   defp render_cell(cells, coordinate) do
@@ -151,35 +152,50 @@ defmodule Game do
   end
 
   defp merge_values(%{cells: cells} = board, move) do
+    # For each row, we run a two-pointer algorithm where:
+    #
+    # * Pointer #1 iterates through the row.
+    # * Pointer #2 points to the latest non-empty, non-modified cell behind pointer #1.
+    #
+    # As #1 iterates, if its current cell is not empty and:
+    #
+    # * Has the same value as the cell of #2: the cell of #1 will be merged into that of #2 (the #2 cell
+    #   value will be doubled and the #1 cell will be emptied) and the #2 pointer will
+    #   be nullified. Or;
+    # * Does not have the same value as the cell of #2: the #2 pointer
+    #   is updated to point to #1 before #1 continues its iteration.
     updates =
       rows_for_move(board, move)
       |> Enum.map(fn row ->
         Enum.map(row, fn coord -> {coord, cells[coord]} end)
       end)
-      |> Enum.flat_map(fn row ->
-        new_row = Enum.into(row, %{})
-
-        {new_row, _} =
-          Enum.reduce(row, {new_row, nil}, fn {coord, current_value},
-                                              {new_row, last_non_empty_coord} ->
-            if is_nil(current_value) do
-              {new_row, last_non_empty_coord}
-            else
-              if current_value == new_row[last_non_empty_coord] do
-                {%{new_row | last_non_empty_coord => 2 * current_value, coord => nil}, nil}
-              else
-                {new_row, coord}
-              end
-            end
-          end)
-
-        Map.to_list(new_row)
-      end)
+      |> Enum.flat_map(&merge_row_values(&1))
       |> Enum.into(%{})
 
     update_board(board, updates)
   end
 
+  defp merge_row_values(row) do
+    # row is a list of {{row, col}, value} elements.
+    new_row = Enum.into(row, %{})
+
+    {new_row, _} =
+      Enum.reduce(row, {new_row, nil}, fn {coord, current_value},
+                                          {new_row, last_non_empty_coord} ->
+        if is_nil(current_value) do
+          {new_row, last_non_empty_coord}
+        else
+          if current_value == new_row[last_non_empty_coord] do
+            {%{new_row | last_non_empty_coord => 2 * current_value, coord => nil}, nil}
+          else
+            {new_row, coord}
+          end
+        end
+      end)
+
+    Map.to_list(new_row)
+  end
+
   defp move_values(%{cells: cells} = board, move) do
     # Conceptually, for each 'row' of values being moved:
     # 1. Create a new row with all non-empty cells.

mix.exs

@@ -50,7 +50,8 @@ defmodule TwentyFourtyEight.MixProject do
       {:gettext, "~> 0.20"},
       {:jason, "~> 1.2"},
       {:dns_cluster, "~> 0.1.1"},
-      {:plug_cowboy, "~> 2.5"}
+      {:plug_cowboy, "~> 2.5"},
+      {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
     ]
   end
 

mix.lock

@@ -1,8 +1,10 @@
 %{
+  "bunt": {:hex, :bunt, "0.2.1", "e2d4792f7bc0ced7583ab54922808919518d0e57ee162901a16a1b6664ef3b14", [:mix], [], "hexpm", "a330bfb4245239787b15005e66ae6845c9cd524a288f0d141c148b02603777a5"},
   "castore": {:hex, :castore, "1.0.4", "ff4d0fb2e6411c0479b1d965a814ea6d00e51eb2f58697446e9c41a97d940b28", [:mix], [], "hexpm", "9418c1b8144e11656f0be99943db4caf04612e3eaecefb5dae9a2a87565584f8"},
   "cowboy": {:hex, :cowboy, "2.10.0", "ff9ffeff91dae4ae270dd975642997afe2a1179d94b1887863e43f681a203e26", [:make, :rebar3], [{:cowlib, "2.12.1", [hex: :cowlib, repo: "hexpm", optional: false]}, {:ranch, "1.8.0", [hex: :ranch, repo: "hexpm", optional: false]}], "hexpm", "3afdccb7183cc6f143cb14d3cf51fa00e53db9ec80cdcd525482f5e99bc41d6b"},
   "cowboy_telemetry": {:hex, :cowboy_telemetry, "0.4.0", "f239f68b588efa7707abce16a84d0d2acf3a0f50571f8bb7f56a15865aae820c", [:rebar3], [{:cowboy, "~> 2.7", [hex: :cowboy, repo: "hexpm", optional: false]}, {:telemetry, "~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "7d98bac1ee4565d31b62d59f8823dfd8356a169e7fcbb83831b8a5397404c9de"},
   "cowlib": {:hex, :cowlib, "2.12.1", "a9fa9a625f1d2025fe6b462cb865881329b5caff8f1854d1cbc9f9533f00e1e1", [:make, :rebar3], [], "hexpm", "163b73f6367a7341b33c794c4e88e7dbfe6498ac42dcd69ef44c5bc5507c8db0"},
+  "credo": {:hex, :credo, "1.7.1", "6e26bbcc9e22eefbff7e43188e69924e78818e2fe6282487d0703652bc20fd62", [:mix], [{:bunt, "~> 0.2.1", [hex: :bunt, repo: "hexpm", optional: false]}, {:file_system, "~> 0.2.8", [hex: :file_system, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "e9871c6095a4c0381c89b6aa98bc6260a8ba6addccf7f6a53da8849c748a58a2"},
   "db_connection": {:hex, :db_connection, "2.6.0", "77d835c472b5b67fc4f29556dee74bf511bbafecdcaf98c27d27fa5918152086", [:mix], [{:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "c2f992d15725e721ec7fbc1189d4ecdb8afef76648c746a8e1cad35e3b8a35f3"},
   "decimal": {:hex, :decimal, "2.1.1", "5611dca5d4b2c3dd497dec8f68751f1f1a54755e8ed2a966c2633cf885973ad6", [:mix], [], "hexpm", "53cfe5f497ed0e7771ae1a475575603d77425099ba5faef9394932b35020ffcc"},
   "dns_cluster": {:hex, :dns_cluster, "0.1.1", "73b4b2c3ec692f8a64276c43f8c929733a9ab9ac48c34e4c0b3d9d1b5cd69155", [:mix], [], "hexpm", "03a3f6ff16dcbb53e219b99c7af6aab29eb6b88acf80164b4bd76ac18dc890b3"},