[YouTube] Add JavaScript lexer to parse completely throttling decryption function (#905)

Author: Theta-Dev

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

@@ -3,7 +3,7 @@
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.utils.JavaScript;
 import org.schabi.newpipe.extractor.utils.Parser;
-import org.schabi.newpipe.extractor.utils.StringUtils;
+import org.schabi.newpipe.extractor.utils.jsextractor.JavaScriptExtractor;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -119,21 +119,12 @@ private static String parseDecodeFunctionName(final String playerJsCode)
     private static String parseDecodeFunction(final String playerJsCode, final String functionName)
             throws Parser.RegexException {
         try {
-            return parseWithParenthesisMatching(playerJsCode, functionName);
+            return parseWithLexer(playerJsCode, functionName);
         } catch (final Exception e) {
             return parseWithRegex(playerJsCode, functionName);
         }
     }
 
-    @Nonnull
-    private static String parseWithParenthesisMatching(final String playerJsCode,
-                                                       final String functionName) {
-        final String functionBase = functionName + "=function";
-        return validateFunction(functionBase
-                + StringUtils.matchToClosingParenthesis(playerJsCode, functionBase)
-                + ";");
-    }
-
     @Nonnull
     private static String parseWithRegex(final String playerJsCode, final String functionName)
             throws Parser.RegexException {
@@ -153,6 +144,14 @@ private static String validateFunction(@Nonnull final String function) {
         return function;
     }
 
+    @Nonnull
+    private static String parseWithLexer(final String playerJsCode, final String functionName)
+            throws ParsingException {
+        final String functionBase = functionName + "=function";
+        return functionBase + JavaScriptExtractor.matchToClosingBrace(playerJsCode, functionBase)
+                + ";";
+    }
+
     private static boolean containsNParam(final String url) {
         return Parser.isMatch(N_PARAM_PATTERN, url);
     }

extractor/src/main/java/org/schabi/newpipe/extractor/utils/StringUtils.java

@@ -0,0 +1,50 @@
+package org.schabi.newpipe.extractor.utils.jsextractor;
+
+import org.schabi.newpipe.extractor.exceptions.ParsingException;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Utility class for extracting functions from JavaScript code.
+ */
+public final class JavaScriptExtractor {
+    private JavaScriptExtractor() {
+
+    }
+
+    /**
+     * Searches the given JavaScript code for the identifier of a function
+     * and returns its body.
+     *
+     * @param jsCode JavaScript code
+     * @param start start of the function (without the opening brace)
+     * @return extracted code (opening brace + function + closing brace)
+     * @throws ParsingException
+     */
+    @Nonnull
+    public static String matchToClosingBrace(final String jsCode, final String start)
+            throws ParsingException {
+        int startIndex = jsCode.indexOf(start);
+        if (startIndex < 0) {
+            throw new ParsingException("Start not found");
+        }
+        startIndex += start.length();
+        final String js = jsCode.substring(startIndex);
+
+        final Lexer lexer = new Lexer(js);
+        boolean visitedOpenBrace = false;
+
+        while (true) {
+            final Lexer.ParsedToken parsedToken = lexer.getNextToken();
+            final Token t = parsedToken.token;
+
+            if (t == Token.LC) {
+                visitedOpenBrace = true;
+            } else if (visitedOpenBrace && lexer.isBalanced()) {
+                return js.substring(0, parsedToken.end);
+            } else if (t == Token.EOF) {
+                throw new ParsingException("Could not find matching braces");
+            }
+        }
+    }
+}