Cache nParams to prevent executing the JavaScript function for the same nParam multiple times.

Closes #689

Author: TobiGr

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeThrottlingDecrypter.java

@@ -4,6 +4,8 @@
 import org.schabi.newpipe.extractor.utils.JavaScript;
 import org.schabi.newpipe.extractor.utils.Parser;
 
+import java.util.HashMap;
+import java.util.Map;
 import java.util.regex.Pattern;
 
 /**
@@ -24,6 +26,7 @@
 public class YoutubeThrottlingDecrypter {
 
     private static final String N_PARAM_REGEX = "[&?]n=([^&]+)";
+    private static final Map<String, String> nParams = new HashMap<>();
 
     private final String functionName;
     private final String function;
@@ -78,7 +81,12 @@ private String parseNParam(String url) throws Parser.RegexException {
     }
 
     private String decryptNParam(String nParam) {
-        return JavaScript.run(function, functionName, nParam);
+        if (nParams.containsKey(nParam)) {
+            return nParams.get(nParam);
+        }
+        final String decryptedNParam = JavaScript.run(function, functionName, nParam);
+        nParams.put(nParam, decryptedNParam);
+        return decryptedNParam;
     }
 
     private String replaceNParam(String url, String oldValue, String newValue) {