Refactor modules into contexts (#78)

* [WIP] break out a few contexts, start refactoring fast index modules

* [WIP] more contexts, this time around slow indexing and downloads

* [WIP] got all tests passing

* [WIP] Added moduledocs

* Built a genserver to rename old jobs on boot

* Added a module naming check; moved things around

* Fixed specs

lib/pinchflat/downloading/output_path/parser.ex

@@ -0,0 +1,42 @@
+defmodule Pinchflat.Downloading.OutputPath.Parser do
+  @moduledoc """
+  Parses liquid-ish-style strings into a rendered string
+
+  Used for turning filepath templates into real filepaths
+  """
+
+  use Pinchflat.Downloading.OutputPath.Base
+
+  @doc """
+  Parses a string into a rendered string, using the provided variables. Optionally
+  takes a custom fetcher function for handling missing variables.
+
+  Variable identifiers are surrounded by {{ and }}. The variable keys MUST be strings.
+  If an identifier is not found in the provided variables, it will be removed from the string.
+
+  Returns `{:ok, binary()}` or `{:error, binary()}`.
+  """
+  def parse(string, variables, value_fetch_fn \\ &default_fetcher/2) do
+    # `do_parse` comes from `RenderedString.Base`
+    case do_parse(string) do
+      {:ok, parsed, _, _, _, _} ->
+        {:ok, build_string(parsed, variables, value_fetch_fn)}
+
+      {:error, message, _, _, _, _} ->
+        {:error, message}
+    end
+  end
+
+  defp build_string(parsed, variables, value_fetch_fn) do
+    Enum.reduce(parsed, "", fn element, acc ->
+      case element do
+        {:text, text} -> acc <> text
+        {:interpolation, {:identifier, identifier}} -> acc <> value_fetch_fn.(identifier, variables)
+      end
+    end)
+  end
+
+  def default_fetcher(identifier, variables) do
+    Map.get(variables, identifier, "")
+  end
+end