fix: more work on metadata diff

lib/bds/document_fields.ex

@@ -0,0 +1,43 @@
+defmodule BDS.DocumentFields do
+  @moduledoc false
+
+  def get(fields, key, default \\ nil) when is_map(fields) and is_binary(key) do
+    case fetch(fields, key) do
+      {:ok, value} -> value
+      :error -> default
+    end
+  end
+
+  def fetch!(fields, key) when is_map(fields) and is_binary(key) do
+    case fetch(fields, key) do
+      {:ok, value} -> value
+      :error -> raise KeyError, key: key, term: fields
+    end
+  end
+
+  def has_key?(fields, key) when is_map(fields) and is_binary(key) do
+    match?({:ok, _value}, fetch(fields, key))
+  end
+
+  def fetch(fields, key) when is_map(fields) and is_binary(key) do
+    key
+    |> aliases_for()
+    |> Enum.find_value(:error, fn alias_key ->
+      if Map.has_key?(fields, alias_key) do
+        {:ok, Map.get(fields, alias_key)}
+      end
+    end)
+  end
+
+  defp aliases_for(key) do
+    [key, Macro.underscore(key), lower_camelize(key)]
+    |> Enum.uniq()
+  end
+
+  defp lower_camelize(value) do
+    case Macro.camelize(value) do
+      <<first::utf8, rest::binary>> -> String.downcase(<<first::utf8>>) <> rest
+      "" -> ""
+    end
+  end
+end