From cabe26374ef047c93bcebed83d70923631910fc0 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Mon, 21 Sep 2026 18:24:16 +0000
Subject: Actually resolve shared links, and guard against dead code

Bug report from the phone: pasting a link produced "Could not parse feed:
Unexpected <! (position:START_DOCUMENT...)" -- the XML parser meeting
<!DOCTYPE html>. The cause was mine. SubscriptionResolver.findFeedLink and
applePodcastId were written, unit-tested, documented as how the share sheet
would work, and had zero call sites. The raw URL went straight to the parser.

FeedResolver now does what I said the app already did: a direct feed resolves
to itself, a show's web page resolves through its link rel=alternate, an Apple
Podcasts link goes through the iTunes lookup, and a YouTube channel or
playlist uses YouTube's own RSS endpoint. Both entry points -- the in-app add
box and the share sheet -- go through it, so they cannot drift apart again.

Verified on-device against real sites: thisamericanlife.org resolved to
/podcast/rss.xml, a direct megaphone feed passed through unchanged, and a
YouTube handle failed with an explanation rather than opaquely.

Errors are now written for a person. "Unexpected <!" told the user nothing;
"That's a web page, not a podcast feed, and it doesn't link to one" tells them
what to do. A test asserts parser internals never reach the UI.

Fixing the class, not just the instance:

- A reachability test fails when a helper object's function has no production
  caller. Its first version passed on the very code it was written to catch,
  because a bare name also matches a KDoc cross-reference -- it now requires a
  call-shaped match. Verified by reintroducing the original bug and watching
  it fail.
- That audit found a real functional gap, not just unused code:
  QueueRepository.advance had no caller, so an episode ending never advanced
  the queue. Playback now marks the episode played, advances, and starts the
  next one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B
---
 .../java/org/terst/listen/data/Repositories.kt     |   3 +
 .../java/org/terst/listen/feed/FeedResolver.kt     | 115 ++++++++++++++
 .../org/terst/listen/playback/PlayerConnection.kt  |  21 ++-
 .../java/org/terst/listen/ui/ListenViewModel.kt    |  36 ++---
 .../java/org/terst/listen/ui/PlayerViewModel.kt    |  22 ++-
 .../org/terst/listen/ui/ShareReceiverActivity.kt   |  40 +++--
 .../test/java/org/terst/listen/ReachabilityTest.kt |  92 +++++++++++
 .../java/org/terst/listen/feed/FeedResolverTest.kt | 175 +++++++++++++++++++++
 8 files changed, 463 insertions(+), 41 deletions(-)
 create mode 100644 app/src/main/java/org/terst/listen/feed/FeedResolver.kt
 create mode 100644 app/src/test/java/org/terst/listen/ReachabilityTest.kt
 create mode 100644 app/src/test/java/org/terst/listen/feed/FeedResolverTest.kt

(limited to 'app/src')

diff --git a/app/src/main/java/org/terst/listen/data/Repositories.kt b/app/src/main/java/org/terst/listen/data/Repositories.kt
index cae5edf..fb869d8 100644
--- a/app/src/main/java/org/terst/listen/data/Repositories.kt
+++ b/app/src/main/java/org/terst/listen/data/Repositories.kt
@@ -5,6 +5,7 @@ import okhttp3.OkHttpClient
 import org.terst.listen.download.ChaquopyYouTubeSource
 import org.terst.listen.download.EpisodeDownloader
 import org.terst.listen.feed.FeedFetcher
+import org.terst.listen.feed.FeedResolver
 import org.terst.listen.queue.QueueOrdering
 import org.terst.listen.queue.RetentionPolicy
 import java.io.File
@@ -44,6 +45,8 @@ class AppGraph(private val context: Context) {
         File(context.filesDir, "audio").also { it.mkdirs() }
     }
 
+    val resolver: FeedResolver by lazy { FeedResolver(client) }
+
     val repository: SubscriptionRepository by lazy {
         SubscriptionRepository(subscriptions, episodes, FeedFetcher(client))
     }
diff --git a/app/src/main/java/org/terst/listen/feed/FeedResolver.kt b/app/src/main/java/org/terst/listen/feed/FeedResolver.kt
new file mode 100644
index 0000000..42253d3
--- /dev/null
+++ b/app/src/main/java/org/terst/listen/feed/FeedResolver.kt
@@ -0,0 +1,115 @@
+package org.terst.listen.feed
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import java.io.IOException
+
+/**
+ * Turns whatever the user shared into a feed URL.
+ *
+ * Discovery was cut from scope, so the share sheet and OPML are the only ways to subscribe --
+ * which means anything that fails to resolve is a show the user simply cannot add.
+ *
+ * This exists because [SubscriptionResolver.findFeedLink] was written, tested, and then never
+ * called: the raw shared URL went straight to the XML parser, so sharing a show's web page
+ * produced "Could not parse feed: Unexpected <!" -- the parser meeting `<!DOCTYPE html>`.
+ */
+class FeedResolver(
+    private val client: OkHttpClient,
+    private val lookupBase: String = "https://itunes.apple.com/lookup",
+) {
+
+    sealed interface Resolution {
+        data class Feed(val url: String) : Resolution
+        data class Failed(val message: String) : Resolution
+    }
+
+    suspend fun resolve(sharedText: String): Resolution {
+        val url = SubscriptionResolver.extractUrl(sharedText)
+            ?: sharedText.trim().takeIf { it.startsWith("http", ignoreCase = true) }
+            ?: return Resolution.Failed("No link found in that text.")
+
+        if (SubscriptionResolver.isYouTube(url)) return resolveYouTube(url)
+        SubscriptionResolver.applePodcastId(url)?.let { return resolveApple(it) }
+        return resolveWeb(url)
+    }
+
+    private fun resolveYouTube(url: String): Resolution =
+        SubscriptionResolver.youtubeFeedUrl(url)?.let { Resolution.Feed(it) }
+        // A handle or /c/ URL carries no channel id. Saying so beats failing opaquely, since
+        // the fix is entirely in the user's hands.
+            ?: Resolution.Failed(
+                "That YouTube link doesn't contain a channel id. Open the channel and use the " +
+                    "URL with /channel/ in it, or share a playlist."
+            )
+
+    private suspend fun resolveApple(id: String): Resolution = withContext(Dispatchers.IO) {
+        val response = runCatching { fetch("$lookupBase?id=$id&entity=podcast") }
+            .getOrElse { return@withContext Resolution.Failed("Could not reach Apple: ${it.message}") }
+
+        val body = response ?: return@withContext Resolution.Failed("Apple returned nothing for that link.")
+        val feedUrl = runCatching {
+            Json { ignoreUnknownKeys = true }.decodeFromString<LookupResponse>(body)
+                .results.firstNotNullOfOrNull { it.feedUrl }
+        }.getOrNull()
+
+        feedUrl?.let { Resolution.Feed(it) }
+            ?: Resolution.Failed("Apple has no podcast feed for that link.")
+    }
+
+    /**
+     * Fetches the URL and decides what it is.
+     *
+     * Sniffing the body rather than trusting Content-Type: plenty of hosts serve feeds as
+     * text/html or application/octet-stream, and rejecting those would fail on real feeds.
+     */
+    private suspend fun resolveWeb(url: String): Resolution = withContext(Dispatchers.IO) {
+        val body = try {
+            fetch(url) ?: return@withContext Resolution.Failed("That address returned nothing.")
+        } catch (e: IOException) {
+            return@withContext Resolution.Failed(e.message ?: "Could not reach that address.")
+        }
+
+        if (looksLikeFeed(body)) return@withContext Resolution.Feed(url)
+
+        SubscriptionResolver.findFeedLink(body, url)?.let { return@withContext Resolution.Feed(it) }
+
+        Resolution.Failed(
+            "That's a web page, not a podcast feed, and it doesn't link to one. Look for an " +
+                "RSS link on the page and share that instead."
+        )
+    }
+
+    /** Parses as a feed if it yields a title or any episodes; see FeedFetcher for the same rule. */
+    private fun looksLikeFeed(body: String): Boolean {
+        val parsed = runCatching { PodcastFeedParser.parse(body.byteInputStream()) }.getOrNull()
+            ?: return false
+        return parsed.title != null || parsed.episodes.isNotEmpty()
+    }
+
+    /** Returns the body, or throws IOException with a message already fit to show a person. */
+    private fun fetch(url: String): String? {
+        val request = Request.Builder().url(url).header("User-Agent", USER_AGENT).build()
+        client.newCall(request).execute().use { response ->
+            if (!response.isSuccessful) {
+                throw IOException("That address returned HTTP ${response.code}.")
+            }
+            return response.body?.string()
+        }
+    }
+
+    @Serializable
+    private data class LookupResponse(val results: List<LookupResult> = emptyList())
+
+    @Serializable
+    private data class LookupResult(@SerialName("feedUrl") val feedUrl: String? = null)
+
+    private companion object {
+        const val USER_AGENT = "Listen/1.0 (Android podcast client)"
+    }
+}
diff --git a/app/src/main/java/org/terst/listen/playback/PlayerConnection.kt b/app/src/main/java/org/terst/listen/playback/PlayerConnection.kt
index af9cc14..df756ca 100644
--- a/app/src/main/java/org/terst/listen/playback/PlayerConnection.kt
+++ b/app/src/main/java/org/terst/listen/playback/PlayerConnection.kt
@@ -33,6 +33,15 @@ class PlayerConnection(private val context: Context) {
 
     private var controller: MediaController? = null
 
+    /**
+     * Fired when an episode plays to its end, with that episode's id.
+     *
+     * Exposed as a callback rather than handled here because what happens next -- mark played,
+     * advance the queue, start the next one -- is repository work this class has no business
+     * knowing about.
+     */
+    var onEpisodeFinished: ((String) -> Unit)? = null
+
     private val _nowPlaying = MutableStateFlow(
         NowPlaying(null, null, isPlaying = false, positionMs = 0, durationMs = null, speed = 1.0f)
     )
@@ -61,7 +70,17 @@ class PlayerConnection(private val context: Context) {
     private val listener = object : Player.Listener {
         override fun onIsPlayingChanged(isPlaying: Boolean) = publish()
         override fun onMediaItemTransition(mediaItem: MediaItem?, reason: Int) = publish()
-        override fun onPlaybackStateChanged(playbackState: Int) = publish()
+
+        override fun onPlaybackStateChanged(playbackState: Int) {
+            if (playbackState == Player.STATE_ENDED) {
+                // Read before anything else runs: advancing the queue replaces the media item,
+                // so the id is gone by the time the callback returns.
+                controller?.currentMediaItem?.mediaId?.let { finished ->
+                    onEpisodeFinished?.invoke(finished)
+                }
+            }
+            publish()
+        }
     }
 
     /**
diff --git a/app/src/main/java/org/terst/listen/ui/ListenViewModel.kt b/app/src/main/java/org/terst/listen/ui/ListenViewModel.kt
index c7a27e5..5d483d6 100644
--- a/app/src/main/java/org/terst/listen/ui/ListenViewModel.kt
+++ b/app/src/main/java/org/terst/listen/ui/ListenViewModel.kt
@@ -14,6 +14,7 @@ import org.terst.listen.data.Graph
 import org.terst.listen.data.RefreshOutcome
 import org.terst.listen.data.SubscriptionEntity
 import org.terst.listen.data.SubscriptionKind
+import org.terst.listen.feed.FeedResolver
 import org.terst.listen.feed.SubscriptionResolver
 import android.net.Uri
 import org.terst.listen.opml.Opml
@@ -108,26 +109,25 @@ class ListenViewModel(app: Application) : AndroidViewModel(app) {
      * identically.
      */
     fun subscribe(rawInput: String) {
-        val url = SubscriptionResolver.extractUrl(rawInput) ?: rawInput.trim()
-        if (url.isEmpty()) {
-            _banner.value = Banner("Nothing to subscribe to", isError = true)
-            return
-        }
-
         viewModelScope.launch {
-            val kind = if (SubscriptionResolver.isYouTube(url)) {
-                SubscriptionKind.YOUTUBE_CHANNEL
-            } else {
-                SubscriptionKind.PODCAST
+            when (val resolved = graph.resolver.resolve(rawInput)) {
+                is FeedResolver.Resolution.Failed ->
+                    _banner.value = Banner(resolved.message, isError = true)
+
+                is FeedResolver.Resolution.Feed -> {
+                    val kind = if (SubscriptionResolver.isYouTube(resolved.url)) {
+                        SubscriptionKind.YOUTUBE_CHANNEL
+                    } else {
+                        SubscriptionKind.PODCAST
+                    }
+                    graph.repository.subscribe(resolved.url, kind).fold(
+                        onSuccess = { _banner.value = Banner("Subscribed to ${it.title}") },
+                        onFailure = {
+                            _banner.value = Banner(it.message ?: "Could not subscribe", isError = true)
+                        },
+                    )
+                }
             }
-            val feedUrl = SubscriptionResolver.youtubeFeedUrl(url) ?: url
-
-            graph.repository.subscribe(feedUrl, kind).fold(
-                onSuccess = { _banner.value = Banner("Subscribed to ${it.title}") },
-                // The reason, not a generic failure: "couldn't subscribe" gives the user nothing
-                // to act on.
-                onFailure = { _banner.value = Banner(it.message ?: "Could not subscribe", isError = true) },
-            )
         }
     }
 
diff --git a/app/src/main/java/org/terst/listen/ui/PlayerViewModel.kt b/app/src/main/java/org/terst/listen/ui/PlayerViewModel.kt
index 350d3a0..e112837 100644
--- a/app/src/main/java/org/terst/listen/ui/PlayerViewModel.kt
+++ b/app/src/main/java/org/terst/listen/ui/PlayerViewModel.kt
@@ -32,7 +32,12 @@ class PlayerViewModel(app: Application) : AndroidViewModel(app) {
         _banner.value = null
     }
 
-    private val connection = PlayerConnection(app).also { it.connect() }
+    private val connection = PlayerConnection(app).also { connection ->
+        connection.connect()
+        // Without this the queue never moves on -- QueueRepository.advance existed with no
+        // caller, which is exactly how a queue stops being a queue.
+        connection.onEpisodeFinished = { episodeId -> onFinished(episodeId) }
+    }
     val nowPlaying: StateFlow<NowPlaying> = connection.nowPlaying
 
     init {
@@ -63,6 +68,21 @@ class PlayerViewModel(app: Application) : AndroidViewModel(app) {
         }
     }
 
+    /**
+     * An episode reached its end: record it, drop it from the queue, start whatever is next.
+     *
+     * Marking played before advancing matters -- advance rewrites the queue, and doing it the
+     * other way round loses the id.
+     */
+    private fun onFinished(episodeId: String) {
+        viewModelScope.launch {
+            graph.episodes.markPlayed(episodeId, System.currentTimeMillis())
+            val nextId = graph.queue.advance()
+            val next = nextId?.let { graph.episodes.getById(it) } ?: return@launch
+            play(next)
+        }
+    }
+
     fun togglePlayPause() = connection.togglePlayPause()
 
     fun skipBack() = connection.seekBy(-SKIP_BACK_MS)
diff --git a/app/src/main/java/org/terst/listen/ui/ShareReceiverActivity.kt b/app/src/main/java/org/terst/listen/ui/ShareReceiverActivity.kt
index 2ee9418..d9d80fe 100644
--- a/app/src/main/java/org/terst/listen/ui/ShareReceiverActivity.kt
+++ b/app/src/main/java/org/terst/listen/ui/ShareReceiverActivity.kt
@@ -8,6 +8,7 @@ import androidx.lifecycle.lifecycleScope
 import kotlinx.coroutines.launch
 import org.terst.listen.data.Graph
 import org.terst.listen.data.SubscriptionKind
+import org.terst.listen.feed.FeedResolver
 import org.terst.listen.feed.SubscriptionResolver
 
 /**
@@ -29,30 +30,27 @@ class ShareReceiverActivity : ComponentActivity() {
             ?.getStringExtra(Intent.EXTRA_TEXT)
             .orEmpty()
 
-        val url = SubscriptionResolver.extractUrl(shared)
-        if (url == null) {
-            reportAndFinish("No link found in that share")
-            return
-        }
-
-        // Toast only -- finishing here would cancel lifecycleScope and take the subscribe with
-        // it, so the activity stays alive until the network call completes.
-        toast("Resolving…")
+        toast("Resolving\u2026")
         lifecycleScope.launch {
             val graph = Graph.get(applicationContext)
-            val kind = if (SubscriptionResolver.isYouTube(url)) {
-                SubscriptionKind.YOUTUBE_CHANNEL
-            } else {
-                SubscriptionKind.PODCAST
-            }
-            val feedUrl = SubscriptionResolver.youtubeFeedUrl(url) ?: url
 
-            graph.repository.subscribe(feedUrl, kind).fold(
-                onSuccess = { reportAndFinish("Subscribed to ${it.title}") },
-                // The actual reason. "Couldn't subscribe" leaves the user with nothing to act on,
-                // and this is the only feedback a share ever gets.
-                onFailure = { reportAndFinish(it.message ?: "Could not subscribe") },
-            )
+            // The same resolution the in-app add box uses, so a shared show page or Apple
+            // Podcasts link behaves identically in both places.
+            when (val resolved = graph.resolver.resolve(shared)) {
+                is FeedResolver.Resolution.Failed -> reportAndFinish(resolved.message)
+
+                is FeedResolver.Resolution.Feed -> {
+                    val kind = if (SubscriptionResolver.isYouTube(resolved.url)) {
+                        SubscriptionKind.YOUTUBE_CHANNEL
+                    } else {
+                        SubscriptionKind.PODCAST
+                    }
+                    graph.repository.subscribe(resolved.url, kind).fold(
+                        onSuccess = { reportAndFinish("Subscribed to ${it.title}") },
+                        onFailure = { reportAndFinish(it.message ?: "Could not subscribe") },
+                    )
+                }
+            }
         }
     }
 
diff --git a/app/src/test/java/org/terst/listen/ReachabilityTest.kt b/app/src/test/java/org/terst/listen/ReachabilityTest.kt
new file mode 100644
index 0000000..4eaed9e
--- /dev/null
+++ b/app/src/test/java/org/terst/listen/ReachabilityTest.kt
@@ -0,0 +1,92 @@
+package org.terst.listen
+
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.io.File
+
+/**
+ * Fails when a function on a helper object has no caller in production code.
+ *
+ * Written after `SubscriptionResolver.findFeedLink` was built, unit-tested, described in the
+ * design notes, and never called -- so sharing a show's web page produced a raw XML parser
+ * exception. Tests passing said nothing, because a tested function with no call site is still
+ * dead code.
+ *
+ * Scoped to `object` declarations on purpose: that is where pure helpers live here, and it is
+ * the shape where unreachable code hides without a compiler warning.
+ */
+class ReachabilityTest {
+
+    /**
+     * Functions that are deliberately not called from production.
+     *
+     * Every entry needs a reason. An allowlist without justification is just a way of turning
+     * the check off one line at a time.
+     */
+    private val allowed = mapOf(
+        "YouTubeAudio.ytDlpVersion" to
+            "Called from YouTubeAudioTest, which fails on a stale bundled yt-dlp. That guard " +
+                "is the whole point; it has no job in production.",
+        "YouTubeAudio.probe" to
+            "Metadata without downloading. Unused while the only YouTube path is download, but " +
+                "the resolver will want it for episode durations.",
+    )
+
+    private val mainDir = File("src/main/java/org/terst/listen")
+
+    private val objectDecl = Regex("""^(?:internal )?object (\w+)""", RegexOption.MULTILINE)
+
+    /** Top-level members of an object are indented exactly four spaces. */
+    private val memberFun = Regex("""^    (?:suspend )?fun (\w+)""", RegexOption.MULTILINE)
+
+    /** Any declaration starting at column zero ends the preceding object's body. */
+    private val nextTopLevel = Regex(
+        """^(?:@|private |internal |abstract |sealed |data )*(?:class|object|interface|fun|val) """,
+        RegexOption.MULTILINE,
+    )
+
+    /** Unreferenced functions declared on one object. */
+    private fun deadIn(
+        declaration: MatchResult,
+        file: File,
+        bodies: Map<File, String>,
+    ): List<String> {
+        val body = bodies.getValue(file)
+        val objectName = declaration.groupValues[1]
+        // Only this object's own body. A file often holds an object and a class, and scanning
+        // the whole file attributes the class's methods to the object.
+        val slice = body.substring(declaration.range.first).let { rest ->
+            val next = nextTopLevel.find(rest, startIndex = 1)?.range?.first ?: rest.length
+            rest.substring(0, next)
+        }
+
+        return memberFun.findAll(slice)
+            .map { "$objectName.${it.groupValues[1]}" }
+            .filterNot { it in allowed }
+            // The open paren matters: a bare name also matches a KDoc cross-reference like
+            // [SubscriptionResolver.findFeedLink], which made an earlier version of this check
+            // pass on the very dead code it was written to catch.
+            .filterNot { qualified -> bodies.any { (other, b) -> other != file && b.contains("$qualified(") } }
+            .toList()
+    }
+
+    @Test
+    fun `every helper object function has a caller`() {
+        val sources = mainDir.walkTopDown().filter { it.extension == "kt" }.toList()
+        assertTrue("could not find sources under ${mainDir.absolutePath}", sources.isNotEmpty())
+
+        val bodies = sources.associateWith { it.readText() }
+
+        val dead = bodies.flatMap { (file, body) ->
+            objectDecl.findAll(body).flatMap { declaration ->
+                deadIn(declaration, file, bodies)
+            }
+        }
+
+        assertTrue(
+            "No production caller for: ${dead.sorted()}. Either wire it up, delete it, or add " +
+                "it to the allowlist in this test with a reason.",
+            dead.isEmpty(),
+        )
+    }
+}
diff --git a/app/src/test/java/org/terst/listen/feed/FeedResolverTest.kt b/app/src/test/java/org/terst/listen/feed/FeedResolverTest.kt
new file mode 100644
index 0000000..55e33b2
--- /dev/null
+++ b/app/src/test/java/org/terst/listen/feed/FeedResolverTest.kt
@@ -0,0 +1,175 @@
+package org.terst.listen.feed
+
+import kotlinx.coroutines.test.runTest
+import okhttp3.OkHttpClient
+import okhttp3.mockwebserver.MockResponse
+import okhttp3.mockwebserver.MockWebServer
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+
+/**
+ * Turning whatever was shared into a feed URL.
+ *
+ * Written after a real bug report: pasting a show's web page produced
+ * "Could not parse feed: Unexpected <!" -- the raw URL went straight to the XML parser, which
+ * choked on <!DOCTYPE html>. SubscriptionResolver.findFeedLink existed and was tested but had
+ * zero call sites.
+ */
+class FeedResolverTest {
+
+    private lateinit var server: MockWebServer
+    private lateinit var resolver: FeedResolver
+
+    private val feedXml = """<?xml version="1.0"?>
+        <rss version="2.0"><channel>
+          <title>A Show</title><link>https://example.com</link><description>d</description>
+          <item><title>One</title><guid>g1</guid>
+            <enclosure url="https://cdn/1.mp3" length="10" type="audio/mpeg"/></item>
+        </channel></rss>""".trimIndent()
+
+    @Before
+    fun setUp() {
+        server = MockWebServer()
+        server.start()
+        resolver = FeedResolver(OkHttpClient())
+    }
+
+    @After
+    fun tearDown() = server.shutdown()
+
+    private fun url(path: String) = server.url(path).toString()
+
+    private suspend fun resolve(input: String) = resolver.resolve(input)
+
+    @Test
+    fun `a direct feed url resolves to itself`() = runTest {
+        server.enqueue(MockResponse().setBody(feedXml))
+        val result = resolve(url("/feed.xml"))
+        assertEquals(FeedResolver.Resolution.Feed(url("/feed.xml")), result)
+    }
+
+    /** The reported bug: a show's web page, not a feed. */
+    @Test
+    fun `a show page resolves through its feed link`() = runTest {
+        val html = """
+            <!DOCTYPE html><html><head>
+              <link rel="alternate" type="application/rss+xml" href="${url("/rss")}">
+            </head><body>The Show</body></html>
+        """.trimIndent()
+        server.enqueue(MockResponse().setHeader("Content-Type", "text/html").setBody(html))
+
+        val result = resolve(url("/show"))
+        assertEquals(FeedResolver.Resolution.Feed(url("/rss")), result)
+    }
+
+    @Test
+    fun `a relative feed link resolves against the page`() = runTest {
+        val html = """<!DOCTYPE html><html><head>
+            <link rel="alternate" type="application/rss+xml" href="/rss"></head></html>"""
+        server.enqueue(MockResponse().setHeader("Content-Type", "text/html").setBody(html))
+
+        val result = resolve(url("/shows/one"))
+        assertEquals(FeedResolver.Resolution.Feed(url("/rss")), result)
+    }
+
+    /**
+     * An HTML page with no feed link is the case that produced a raw parser exception. The
+     * message has to say what happened in terms the person can act on.
+     */
+    @Test
+    fun `a page with no feed link fails understandably`() = runTest {
+        server.enqueue(
+            MockResponse().setHeader("Content-Type", "text/html")
+                .setBody("<!DOCTYPE html><html><body>Just a page</body></html>")
+        )
+
+        val result = resolve(url("/page"))
+        val failed = result as FeedResolver.Resolution.Failed
+        assertTrue("should mention it is a web page: '${failed.message}'",
+            failed.message.contains("web page", ignoreCase = true))
+        assertTrue("must not leak parser internals: '${failed.message}'",
+            !failed.message.contains("START_DOCUMENT") && !failed.message.contains("InputStreamReader"))
+    }
+
+    @Test
+    fun `a 404 reports the status rather than a parse error`() = runTest {
+        server.enqueue(MockResponse().setResponseCode(404))
+        val failed = resolve(url("/missing")) as FeedResolver.Resolution.Failed
+        assertTrue(failed.message, failed.message.contains("404"))
+    }
+
+    @Test
+    fun `shared text with surrounding words still resolves`() = runTest {
+        server.enqueue(MockResponse().setBody(feedXml))
+        val result = resolve("Check this out ${url("/feed.xml")} it's good")
+        assertEquals(FeedResolver.Resolution.Feed(url("/feed.xml")), result)
+    }
+
+    @Test
+    fun `empty input fails without a request`() = runTest {
+        val failed = resolve("no link here") as FeedResolver.Resolution.Failed
+        assertEquals(0, server.requestCount)
+        assertTrue(failed.message, failed.message.contains("link", ignoreCase = true))
+    }
+
+    // --- YouTube ---------------------------------------------------------------------------
+
+    @Test
+    fun `a youtube channel resolves without a fetch`() = runTest {
+        val result = resolve("https://www.youtube.com/channel/UCabc123")
+        assertEquals(
+            FeedResolver.Resolution.Feed("https://www.youtube.com/feeds/videos.xml?channel_id=UCabc123"),
+            result,
+        )
+        assertEquals("should not need the network", 0, server.requestCount)
+    }
+
+    /** A handle URL carries no channel id, so say what to do instead of failing opaquely. */
+    @Test
+    fun `a youtube handle explains what is needed`() = runTest {
+        val failed = resolve("https://www.youtube.com/@SomeHandle") as FeedResolver.Resolution.Failed
+        assertTrue("should mention the channel URL: '${failed.message}'",
+            failed.message.contains("channel", ignoreCase = true))
+    }
+
+    // --- Apple Podcasts --------------------------------------------------------------------
+
+    /** Apple Podcasts links are a very common share and are not feeds. */
+    @Test
+    fun `an apple podcasts link resolves through the lookup api`() = runTest {
+        server.enqueue(
+            MockResponse().setBody(
+                """{"resultCount":1,"results":[{"feedUrl":"https://feeds.example.com/show"}]}"""
+            )
+        )
+        val resolverWithLookup = FeedResolver(OkHttpClient(), lookupBase = url("/lookup"))
+
+        val result = resolverWithLookup.resolve("https://podcasts.apple.com/gb/podcast/x/id1537788786")
+        assertEquals(FeedResolver.Resolution.Feed("https://feeds.example.com/show"), result)
+    }
+
+    @Test
+    fun `an apple link with no result fails understandably`() = runTest {
+        server.enqueue(MockResponse().setBody("""{"resultCount":0,"results":[]}"""))
+        val resolverWithLookup = FeedResolver(OkHttpClient(), lookupBase = url("/lookup"))
+
+        val failed = resolverWithLookup.resolve("https://podcasts.apple.com/gb/podcast/x/id999")
+            as FeedResolver.Resolution.Failed
+        assertTrue(failed.message, failed.message.contains("Apple", ignoreCase = true))
+    }
+
+    @Test
+    fun `the apple lookup sends the podcast id`() = runTest {
+        server.enqueue(
+            MockResponse().setBody("""{"resultCount":1,"results":[{"feedUrl":"https://f/x"}]}""")
+        )
+        val resolverWithLookup = FeedResolver(OkHttpClient(), lookupBase = url("/lookup"))
+        resolverWithLookup.resolve("https://podcasts.apple.com/us/podcast/id1537788786")
+
+        val request = server.takeRequest()
+        assertTrue(request.path!!, request.path!!.contains("1537788786"))
+    }
+}
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