add detec_format and validate functions

lib/feed_reader.ex

@@ -2,7 +2,35 @@ defmodule FeedReader do
   @moduledoc """
   Standalone feed fetching and parsing library.
 
-  The public parsing and fetching APIs will be added incrementally. This
-  module is the stable namespace for the package.
+  This module exposes the public format detection and validation helpers.
   """
+
+  @typedoc "`:rss`, `:atom`, `:rdf`, `:unknown`, or a custom format's atom ID."
+  @type format_id :: atom()
+
+  @doc """
+  Detects a format from a bounded binary prefix.
+
+  ## Options
+  - `:max_detection_bytes` — `pos_integer()`; default `65_536`; invalid values yield `:unknown`.
+  - `:formats` — list of modules, checked after RSS, Atom, and RDF in list order; duplicates removed.
+    Custom modules provide `matches_prefix?/1` and `id/0`.
+  """
+  @spec detect_format(binary(), keyword()) :: format_id()
+  def detect_format(content, opts \\ []) do
+    FeedReader.FormatDetector.detect(content, opts)
+  end
+
+  @doc """
+  Checks only the bounded-prefix signature for a format ID or module.
+
+  ## Options
+  - `:max_detection_bytes` — `pos_integer()`, default `65_536`; invalid values yield `false`.
+  - `:formats` — custom modules used to resolve custom format IDs. Modules provide
+    `matches_prefix?/1` and `id/0`; passing the format module directly also works.
+  """
+  @spec validate(binary(), atom() | module(), keyword()) :: boolean()
+  def validate(content, format, opts \\ []) do
+    FeedReader.FormatDetector.validate(content, format, opts)
+  end
 end