feat(batch): Impls batch merging

lib/rox/batch.ex

@@ -73,4 +73,18 @@ defmodule Rox.Batch do
     |> :lists.reverse
     |> Native.batch_write(db)
   end
+
+
+  @doc """
+  Merges a list of `Batch.t` into a single `Batch.t`.
+
+  """
+  @spec merge([t]) :: t
+  def merge(batches) do
+    batches
+    |> Enum.reduce(Batch.new, fn %Batch{operations: ops}, %Batch{operations: merge_ops} = acc ->
+      %{acc | operations: Enum.concat(ops, merge_ops)}
+    end)
+  end
+
 end

test/rox/batch_test.exs

@@ -0,0 +1,39 @@
+defmodule Rox.BatchTest do
+  use ExUnit.Case, async: true
+
+  alias Rox.Batch
+
+  describe "merge/1" do
+    test "stores operations in reverse order" do
+      batches = [
+        %Batch{operations: [put: {"key_a", "value_a"}]},
+        %Batch{operations: [put: {"key_b", "value_b"}]},
+        %Batch{operations: [delete: "key_a"]},
+      ]
+
+      assert Batch.merge(batches) == %Batch{
+        operations: [
+          delete: "key_a",
+          put: {"key_b", "value_b"},
+          put: {"key_a", "value_a"},
+        ]
+      }
+    end
+
+    test "works when a single batch contains multiple operations" do
+      batches = [
+        %Batch{operations: [put: {"key_b", "value_b"}, put: {"key_a", "value_a"}]},
+        %Batch{operations: [delete: "key_a"]},
+      ]
+
+      assert Batch.merge(batches) == %Batch{
+        operations: [
+          delete: "key_a",
+          put: {"key_b", "value_b"},
+          put: {"key_a", "value_a"},
+        ]
+      }
+    end
+  end
+
+end