Enable filtering of podcast items using a predicate (#90)

This change enables a Predicate to be passed to the generatePodcastFeed
publishing step, which lets the user conditionally decide which items
to include in the feed. This feature was previously added to standard
RSS feeds, but now podcast feeds also have the same capability.

Sources/Publish/API/PublishingStep.swift

@@ -400,17 +400,21 @@ public extension PublishingStep where Site.ItemMetadata: PodcastCompatibleWebsit
     /// Note that all of the items within the given section must define `podcast`
     /// and `audio` metadata, or an error will be thrown.
     /// - parameter section: The section to generate a podcast feed for.
+    /// - parameter itemPredicate: A predicate used to determine whether to
+    ///   include a given item within the generated feed (default: include all).
     /// - parameter config: The configuration to use when generating the feed.
     /// - parameter date: The date that should act as the build and publishing
     ///   date for the generated feed (default: the current date).
     static func generatePodcastFeed(
         for section: Site.SectionID,
+        itemPredicate: Predicate<Item<Site>>? = nil,
         config: PodcastFeedConfiguration<Site>,
         date: Date = Date()
     ) -> Self {
         step(named: "Generate podcast feed") { context in
             let generator = PodcastFeedGenerator(
                 sectionID: section,
+                itemPredicate: itemPredicate,
                 config: config,
                 context: context,
                 date: date

Sources/Publish/Internal/PodcastFeedGenerator.swift

@@ -9,6 +9,7 @@ import Plot
 
 internal struct PodcastFeedGenerator<Site: Website> where Site.ItemMetadata: PodcastCompatibleWebsiteItemMetadata {
     let sectionID: Site.SectionID
+    let itemPredicate: Predicate<Item<Site>>?
     let config: PodcastFeedConfiguration<Site>
     let context: PublishingContext<Site>
     let date: Date
@@ -18,7 +19,11 @@ internal struct PodcastFeedGenerator<Site: Website> where Site.ItemMetadata: Pod
         let cacheFile = try context.cacheFile(named: "feed")
         let oldCache = try? cacheFile.read().decoded() as Cache
         let section = context.sections[sectionID]
-        let items = section.items.sorted(by: { $0.date > $1.date })
+        var items = section.items.sorted(by: { $0.date > $1.date })
+
+        if let predicate = itemPredicate?.inverse() {
+            items.removeAll(where: predicate.matches)
+        }
 
         if let date = context.lastGenerationDate, let cache = oldCache {
             if cache.config == config, cache.itemCount == items.count {

Tests/PublishTests/Tests/PodcastFeedGenerationTests.swift

@@ -25,6 +25,26 @@ final class PodcastFeedGenerationTests: PublishTestCase {
         XCTAssertFalse(feed.contains("Not included"))
     }
 
+    func testOnlyIncludingItemsMatchingPredicate() throws {
+        let folder = try Folder.createTemporary()
+
+        try generateFeed(
+            in: folder,
+            itemPredicate: \.path == "one/a",
+            content: [
+                "one/a.md": """
+                \(makeStubbedAudioMetadata())
+                # Included
+                """,
+                "one/b.md": "# Not included"
+            ]
+        )
+
+        let feed = try folder.file(at: "Output/feed.rss").readAsString()
+        XCTAssertTrue(feed.contains("Included"))
+        XCTAssertFalse(feed.contains("Not included"))
+    }
+
     func testConvertingRelativeLinksToAbsolute() throws {
         let folder = try Folder.createTemporary()
 
@@ -148,6 +168,7 @@ extension PodcastFeedGenerationTests {
     static var allTests: Linux.TestList<PodcastFeedGenerationTests> {
         [
             ("testOnlyIncludingSpecifiedSection", testOnlyIncludingSpecifiedSection),
+            ("testOnlyIncludingItemsMatchingPredicate", testOnlyIncludingItemsMatchingPredicate),
             ("testConvertingRelativeLinksToAbsolute", testConvertingRelativeLinksToAbsolute),
             ("testItemPrefixAndSuffix", testItemPrefixAndSuffix),
             ("testReusingPreviousFeedIfNoItemsWereModified", testReusingPreviousFeedIfNoItemsWereModified),
@@ -190,6 +211,7 @@ private extension PodcastFeedGenerationTests {
     func generateFeed(
         in folder: Folder,
         config: Configuration? = nil,
+        itemPredicate: Predicate<Item<Site>>? = nil,
         generationSteps: [PublishingStep<Site>] = [
             .addMarkdownFiles()
         ],
@@ -200,6 +222,7 @@ private extension PodcastFeedGenerationTests {
             .group(generationSteps),
             .generatePodcastFeed(
                 for: .one,
+                itemPredicate: itemPredicate,
                 config: config ?? makeConfigStub(),
                 date: date
             )