Limit sending timetable requests in parallel to 3

Otherwise, the requests may fail (from my experience). Also we don't want to look suspicious (flooding their server with tons of requests at the same time).

src/main/kotlin/org/mosad/thecitadelofricks/hsoparser/TimetableParser.kt

@@ -22,6 +22,8 @@
 
 package org.mosad.thecitadelofricks.hsoparser
 
+import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.sync.Semaphore
 import org.jsoup.Jsoup
 import org.jsoup.nodes.Document
 import org.mosad.thecitadelofricks.Lesson
@@ -36,16 +38,26 @@ class TimetableParser(timetableURL: String? = null, htmlDoc: Document? = null) {
     private var logger: org.slf4j.Logger = LoggerFactory.getLogger(TimetableParser::class.java)
     private val days = arrayOf("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
 
+    companion object {
+        val semaphore = Semaphore(3, 0)
+    }
+
     private val htmlDoc: Document? =
         htmlDoc
             ?: if (timetableURL == null) {
                 null
             } else {
-                try {
-                    Jsoup.connect(timetableURL).get()
-                } catch (gex: Exception) {
-                    logger.error("general TimetableParser error", gex)
-                    null
+                runBlocking {
+                    try {
+                        // Only allow sending a limited amount of requests at the same time
+                        semaphore.acquire()
+                        Jsoup.connect(timetableURL).get()
+                    } catch (gex: Exception) {
+                        logger.error("general TimetableParser error", gex)
+                        null
+                    } finally {
+                        semaphore.release()
+                    }
                 }
             }