feat(filters): add convert_to filter for converting feed format

fixtures/minimal_rss_20.xml

@@ -0,0 +1,14 @@
+<rss version="2.0">
+  <channel>
+    <title>Test</title>
+    <link>http://example.com</link>
+    <description>Test description</description>
+
+    <item>
+      <title>Item 1</title>
+      <guid isPermaLink="false">http://example.com/item1</guid>
+      <link>http://example.com/item1</link>
+      <description>Item 1 description</description>
+    </item>
+  </channel>
+</rss>

fixtures/sample_atom.xml

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="utf-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+  <!-- Example taken from https://www.ibm.com/docs/en/baw/22.x?topic=formats-atom-feed-format -->
+  <title>Example Feed</title>
+  <subtitle>A subtitle.</subtitle>
+  <link href="http://example.org/feed/" rel="self"/>
+  <link href="http://example.org/"/>
+  <updated>2003-12-13T18:30:02Z</updated>
+  <author>
+    <name>John Doe</name>
+    <email>johndoe@example.com</email>
+  </author>
+  <id>urn:uuid:60a76c80-d399-11d9-b91C-0003939e0af6</id>
+  <entry>
+    <content type="text/xml">
+      <p:Customer xmlns:p="http://www.ibm.com/crm" xmlns="http://www.ibm.com/crm">
+        <id>10</id>
+      </p:Customer>
+    </content>
+    <title>Atom-Powered Robots Run Amok</title>
+    <link href="http://example.org/2003/12/13/atom03"/>
+    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
+    <updated>2003-12-13T18:30:02Z</updated>
+    <summary>Some text.</summary>
+  </entry>
+  <entry>
+    <content>
+      <content type="text/json">
+        {"firstName"="John","lastName"="Doe","id"="10"}
+      </content>
+      <type>text/json</type>
+    </content>
+    <title>Atom-Powered Robots Run Amok</title>
+    <link href="http://example.org/2003/12/13/atom03"/>
+    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
+    <updated>2003-12-13T18:30:02Z</updated>
+    <summary>Some text.</summary>
+  </entry>
+</feed>

src/feed.rs

@@ -1,3 +1,5 @@
+mod conversion;
+
 use paste::paste;
 use schemars::JsonSchema;
 use serde::Deserialize;
@@ -10,15 +12,15 @@ use crate::source::FromScratch;
 use crate::util::Error;
 use crate::util::Result;
 
-#[derive(Serialize, Deserialize, Clone, Debug)]
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
 #[serde(untagged)]
 pub enum Feed {
   Rss(rss::Channel),
   Atom(atom_syndication::Feed),
 }
 
 #[derive(
-  JsonSchema, Serialize, Deserialize, Clone, Debug, PartialEq, Eq, Hash,
+  JsonSchema, Serialize, Deserialize, Clone, Debug, PartialEq, Eq, Hash, Copy,
 )]
 #[serde(rename_all = "lowercase")]
 pub enum FeedFormat {
@@ -29,15 +31,33 @@ pub enum FeedFormat {
 }
 
 impl Feed {
-  // currently only used in tests.
-  #[cfg(test)]
   pub fn format(&self) -> FeedFormat {
     match self {
       Feed::Rss(_) => FeedFormat::Rss,
       Feed::Atom(_) => FeedFormat::Atom,
     }
   }
 
+  pub fn into_format(self, format: FeedFormat) -> Self {
+    use conversion::W;
+
+    if self.format() == format {
+      return self;
+    }
+
+    match (self, format) {
+      (Feed::Rss(channel), FeedFormat::Atom) => {
+        let feed: atom_syndication::Feed = W(channel).into();
+        Feed::Atom(feed)
+      }
+      (Feed::Atom(feed), FeedFormat::Rss) => {
+        let channel: rss::Channel = W(feed).into();
+        Feed::Rss(channel)
+      }
+      (original_self, _) => original_self,
+    }
+  }
+
   pub fn from_rss_content(content: &[u8]) -> Result<Self> {
     let cursor = std::io::Cursor::new(content);
     let channel = rss::Channel::read_from(cursor)?;
@@ -190,6 +210,30 @@ impl Feed {
   }
 }
 
+#[cfg(test)]
+impl TryFrom<Feed> for rss::Channel {
+  type Error = ();
+
+  fn try_from(feed: Feed) -> Result<Self, Self::Error> {
+    match feed {
+      Feed::Rss(channel) => Ok(channel),
+      _ => Err(()),
+    }
+  }
+}
+
+#[cfg(test)]
+impl TryFrom<Feed> for atom_syndication::Feed {
+  type Error = ();
+
+  fn try_from(feed: Feed) -> Result<Self, Self::Error> {
+    match feed {
+      Feed::Atom(feed) => Ok(feed),
+      _ => Err(()),
+    }
+  }
+}
+
 impl From<&FromScratch> for Feed {
   fn from(config: &FromScratch) -> Self {
     use FeedFormat::*;