[Grouple] Selecting the fastest pages server

src/main/kotlin/org/koitharu/kotatsu/parsers/site/grouple/GroupleParser.kt

@@ -1,5 +1,7 @@
 package org.koitharu.kotatsu.parsers.site.grouple
 
+import kotlinx.coroutines.async
+import kotlinx.coroutines.coroutineScope
 import okhttp3.Headers
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.Response
@@ -27,6 +29,9 @@ internal abstract class GroupleParser(
 	private val siteId: Int,
 ) : MangaParser(context, source), MangaParserAuthProvider {
 
+	@Volatile
+	private var cachedPagesServer: String? = null
+
 	override val headers = Headers.Builder()
 		.add("User-Agent", userAgent)
 		.build()
@@ -226,14 +231,19 @@ internal abstract class GroupleParser(
 		val parts = page.url.split('|')
 		val path = parts.last()
 		val servers = parts.dropLast(1).toSet()
-        for (server in servers) {
-            val url = server + path
-            if (tryHead(url)) {
-                return url
+		val cachedServer = cachedPagesServer
+		if (cachedServer != null && cachedServer in servers && tryHead(cachedServer + path)) {
+			return cachedServer + path
 		}
+		val server = coroutineScope {
+			servers.map { server ->
+				async {
+					if (tryHead(server + path)) server else null
 				}
-        val fallbackServer = servers.firstOrNull() ?: throw ParseException("Cannot find any page url", page.url)
-        return fallbackServer + path
+			}.awaitFirst { it != null }
+		}
+		cachedPagesServer = server
+		return checkNotNull(server + path)
 	}
 
 	override suspend fun getTags(): Set<MangaTag> {

src/main/kotlin/org/koitharu/kotatsu/parsers/util/Coroutines.kt

@@ -0,0 +1,40 @@
+package org.koitharu.kotatsu.parsers.util
+
+import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.selects.select
+import kotlin.coroutines.cancellation.CancellationException
+
+fun Iterable<Job>.cancelAll(cause: CancellationException? = null) {
+	forEach { it.cancel(cause) }
+}
+
+suspend fun <T> Iterable<Deferred<T>>.awaitFirst(): T = select<T> {
+	for (async in this@awaitFirst) {
+		async.onAwait { it }
+	}
+}.also { this@awaitFirst.cancelAll() }
+
+suspend fun <T> Collection<Deferred<T>>.awaitFirst(condition: (T) -> Boolean): T {
+	var result: Any? = NULL
+	var counter = size
+	while (result === NULL && counter > 0) {
+		val candidate = select<T> {
+			for (async in this@awaitFirst) {
+				async.onAwait { it }
+			}
+		}
+		if (condition(candidate)) {
+			result = candidate
+		}
+		counter--
+	}
+	cancelAll()
+	if (result === NULL) {
+		throw NoSuchElementException()
+	}
+	@Suppress("UNCHECKED_CAST")
+	return result as T
+}
+
+private val NULL = Any()