Add rss discovery from home page

app/src/main/java/me/ash/reader/infrastructure/rss/RssHelper.kt

@@ -10,6 +10,7 @@ import com.rometools.rome.feed.synd.SyndFeed
 import com.rometools.rome.feed.synd.SyndImageImpl
 import com.rometools.rome.io.SyndFeedInput
 import com.rometools.rome.io.XmlReader
+import java.io.ByteArrayInputStream
 import dagger.hilt.android.qualifiers.ApplicationContext
 import java.nio.charset.Charset
 import java.util.*
@@ -45,28 +46,72 @@ constructor(
     private val okHttpClient: OkHttpClient,
 ) {
 
+    data class SearchFeedResult(
+        val feed: SyndFeed,
+        val feedLink: String,
+    )
+
     @Throws(Exception::class)
-    suspend fun searchFeed(feedLink: String): SyndFeed {
+    suspend fun searchFeed(feedLink: String): SearchFeedResult {
         return withContext(ioDispatcher) {
-            val response = response(okHttpClient, feedLink)
-            val contentType = response.header("Content-Type")
-            val httpContentType =
-                contentType?.let {
-                    if (it.contains("charset=", ignoreCase = true)) it
-                    else "$it; charset=UTF-8"
-                } ?: "text/xml; charset=UTF-8"
+            val directResponse = response(okHttpClient, feedLink)
+            if (!directResponse.commonIsSuccessful) throw IOException(directResponse.message)
+            val directBody = directResponse.body.bytes()
+            val directHttpContentType = toHttpContentType(directResponse.header("Content-Type"))
 
+            val parsedDirectFeed = runCatching { parseFeed(directBody, directHttpContentType) }.getOrNull()
 
-            response.body.byteStream().use { inputStream ->
-                    SyndFeedInput().build(XmlReader(inputStream, httpContentType)).also {
-                    it.icon = SyndImageImpl()
-                    it.icon.link = queryRssIconLink(feedLink)
-                    it.icon.url = it.icon.link
+            val resolvedFeedLink =
+                if (parsedDirectFeed != null) feedLink
+                else discoverFeedLink(feedLink, directBody)
+                    ?: throw IOException("Unable to detect RSS feed URL")
+
+
+            val feed = parsedDirectFeed ?: run {
+                val discoveredResponse = response(okHttpClient, resolvedFeedLink)
+                if (!discoveredResponse.commonIsSuccessful) {
+                    throw IOException(discoveredResponse.message)
                 }
+                parseFeed(
+                    discoveredResponse.body.bytes(),
+                    toHttpContentType(discoveredResponse.header("Content-Type")),
+                )
             }
+
+            feed.also {
+                it.icon = SyndImageImpl()
+                it.icon.link = queryRssIconLink(resolvedFeedLink)
+                it.icon.url = it.icon.link
+            }
+
+            SearchFeedResult(feed = feed, feedLink = resolvedFeedLink)
         }
     }
 
+    private fun toHttpContentType(contentType: String?): String =
+        contentType?.let {
+            if (it.contains("charset=", ignoreCase = true)) it else "$it; charset=UTF-8"
+        } ?: "text/xml; charset=UTF-8"
+
+    private fun parseFeed(body: ByteArray, httpContentType: String): SyndFeed =
+        ByteArrayInputStream(body).use { inputStream ->
+            SyndFeedInput().build(XmlReader(inputStream, httpContentType))
+        }
+
+    private fun discoverFeedLink(pageUrl: String, body: ByteArray): String? {
+        val document = Jsoup.parse(String(body, Charsets.UTF_8), pageUrl)
+        val links = document.select("head link[rel~=(?i)alternate][href]")
+        val preferred =
+            links.firstOrNull {
+                val type = it.attr("type").lowercase(Locale.ROOT)
+                type == "application/rss+xml" ||
+                    type == "application/atom+xml" ||
+                    type == "application/rdf+xml"
+            }
+        val fallback = links.firstOrNull()
+        return (preferred ?: fallback)?.absUrl("href")?.takeIf { it.isNotBlank() }
+    }
+
     @Throws(Exception::class)
     suspend fun parseFullContent(link: String, title: String): String {
         return withContext(ioDispatcher) {

app/src/main/java/me/ash/reader/ui/page/home/feeds/subscribe/SubscribeViewModel.kt

@@ -150,11 +150,21 @@ constructor(
                 viewModelScope.launch {
                     runCatching { rssHelper.searchFeed(feedLink) }
                         .onSuccess {
+                            if (rssService.get().isFeedExist(it.feedLink)) {
+                                _subscribeState.value =
+                                    currentState.copy(
+                                        errorMessage =
+                                            androidStringsHelper.getString(
+                                                R.string.already_subscribed
+                                            )
+                                    )
+                                return@onSuccess
+                            }
                             val groups = groupsFlow.value
                             _subscribeState.value =
                                 SubscribeState.Configure(
-                                    searchedFeed = it,
-                                    feedLink = feedLink,
+                                    searchedFeed = it.feed,
+                                    feedLink = it.feedLink,
                                     groups = groups,
                                     selectedGroupId = firstGroupId,
                                 )