[YouTube] Add utility class around signatures and fix signature deobfuscation function extraction

The goal of this class is to decouple the extraction of signature timestamp and
signature deobfuscation function from YoutubeStreamExtractor.

The extraction of the signature deobfuscation function has been also adapted to
support the latest YouTube player versions.

This new class, YoutubeSignatureUtils, doens't store anything temporary such as
a copy of the player code, which has to be passed where required. It is not
public, as it will be used by a JavaScript player manager class in the future,
in order to handle in a better way fetching, caching and resetting cache of the
player code.

Author: AudricV

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

@@ -0,0 +1,150 @@
+package org.schabi.newpipe.extractor.services.youtube;
+
+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.jsextractor.JavaScriptExtractor;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Utility class to get the signature timestamp of YouTube's base JavaScript player and deobfuscate
+ * signature of streaming URLs from HTML5 clients.
+ */
+final class YoutubeSignatureUtils {
+
+    /**
+     * The name of the deobfuscation function which needs to be called inside the deobfuscation
+     * code.
+     */
+    static final String DEOBFUSCATION_FUNCTION_NAME = "deobfuscate";
+
+    private static final String[] FUNCTION_REGEXES = {
+            "\\bm=([a-zA-Z0-9$]{2,})\\(decodeURIComponent\\(h\\.s\\)\\)",
+            "\\bc&&\\(c=([a-zA-Z0-9$]{2,})\\(decodeURIComponent\\(c\\)\\)",
+            // CHECKSTYLE:OFF
+            "(?:\\b|[^a-zA-Z0-9$])([a-zA-Z0-9$]{2,})\\s*=\\s*function\\(\\s*a\\s*\\)\\s*\\{\\s*a\\s*=\\s*a\\.split\\(\\s*\"\"\\s*\\)",
+            // CHECKSTYLE:ON
+            "([\\w$]+)\\s*=\\s*function\\((\\w+)\\)\\{\\s*\\2=\\s*\\2\\.split\\(\"\"\\)\\s*;"
+    };
+
+    private static final String STS_REGEX = "signatureTimestamp[=:](\\d+)";
+
+    private static final String DEOBF_FUNC_REGEX_START = "(";
+    private static final String DEOBF_FUNC_REGEX_END = "=function\\([a-zA-Z0-9_]+\\)\\{.+?\\})";
+
+    private static final String SIG_DEOBF_HELPER_OBJ_NAME_REGEX = ";([A-Za-z0-9_\\$]{2,})\\...\\(";
+    private static final String SIG_DEOBF_HELPER_OBJ_REGEX_START = "(var ";
+    private static final String SIG_DEOBF_HELPER_OBJ_REGEX_END = "=\\{(?>.|\\n)+?\\}\\};)";
+
+    private YoutubeSignatureUtils() {
+    }
+
+    /**
+     * Get the signature timestamp property of YouTube's base JavaScript file.
+     *
+     * @param javaScriptPlayerCode the complete JavaScript base player code
+     * @return the signature timestamp
+     * @throws ParsingException if the signature timestamp couldn't be extracted
+     */
+    @Nonnull
+    static String getSignatureTimestamp(@Nonnull final String javaScriptPlayerCode)
+            throws ParsingException {
+        try {
+            return Parser.matchGroup1(STS_REGEX, javaScriptPlayerCode);
+        } catch (final ParsingException e) {
+            throw new ParsingException(
+                    "Could not extract signature timestamp from JavaScript code", e);
+        }
+    }
+
+    /**
+     * Get the signature deobfuscation code of YouTube's base JavaScript file.
+     *
+     * @param javaScriptPlayerCode the complete JavaScript base player code
+     * @return the signature deobfuscation code
+     * @throws ParsingException if the signature deobfuscation code couldn't be extracted
+     */
+    @Nonnull
+    static String getDeobfuscationCode(@Nonnull final String javaScriptPlayerCode)
+            throws ParsingException {
+        try {
+            final String deobfuscationFunctionName = getDeobfuscationFunctionName(
+                    javaScriptPlayerCode);
+
+            String deobfuscationFunction;
+            try {
+                deobfuscationFunction = getDeobfuscateFunctionWithLexer(
+                        javaScriptPlayerCode, deobfuscationFunctionName);
+            } catch (final Exception e) {
+                deobfuscationFunction = getDeobfuscateFunctionWithRegex(
+                        javaScriptPlayerCode, deobfuscationFunctionName);
+            }
+
+            // Assert the extracted deobfuscation function is valid
+            JavaScript.compileOrThrow(deobfuscationFunction);
+
+            final String helperObjectName =
+                    Parser.matchGroup1(SIG_DEOBF_HELPER_OBJ_NAME_REGEX, deobfuscationFunction);
+
+            final String helperObject = getHelperObject(javaScriptPlayerCode, helperObjectName);
+
+            final String callerFunction = "function " + DEOBFUSCATION_FUNCTION_NAME
+                    + "(a){return "
+                    + deobfuscationFunctionName
+                    + "(a);}";
+
+            return helperObject + deobfuscationFunction + callerFunction;
+        } catch (final Exception e) {
+            throw new ParsingException("Could not parse deobfuscation function", e);
+        }
+    }
+
+    @Nonnull
+    private static String getDeobfuscationFunctionName(@Nonnull final String javaScriptPlayerCode)
+            throws ParsingException {
+        Parser.RegexException exception = null;
+        for (final String regex : FUNCTION_REGEXES) {
+            try {
+                return Parser.matchGroup1(regex, javaScriptPlayerCode);
+            } catch (final Parser.RegexException e) {
+                if (exception == null) {
+                    exception = e;
+                }
+            }
+        }
+
+        throw new ParsingException(
+                "Could not find deobfuscation function with any of the known patterns", exception);
+    }
+
+    @Nonnull
+    private static String getDeobfuscateFunctionWithLexer(
+            @Nonnull final String javaScriptPlayerCode,
+            @Nonnull final String deobfuscationFunctionName) throws ParsingException {
+        final String functionBase = deobfuscationFunctionName + "=function";
+        return functionBase + JavaScriptExtractor.matchToClosingBrace(
+                javaScriptPlayerCode, functionBase) + ";";
+    }
+
+    @Nonnull
+    private static String getDeobfuscateFunctionWithRegex(
+            @Nonnull final String javaScriptPlayerCode,
+            @Nonnull final String deobfuscationFunctionName) throws ParsingException {
+        final String functionPattern = DEOBF_FUNC_REGEX_START
+                + deobfuscationFunctionName.replace("$", "\\$")
+                + DEOBF_FUNC_REGEX_END;
+        return "var " + Parser.matchGroup1(functionPattern, javaScriptPlayerCode) + ";";
+    }
+
+    @Nonnull
+    private static String getHelperObject(@Nonnull final String javaScriptPlayerCode,
+                                          @Nonnull final String helperObjectName)
+            throws ParsingException {
+        final String helperPattern = SIG_DEOBF_HELPER_OBJ_REGEX_START
+                + helperObjectName.replace("$", "\\$")
+                + SIG_DEOBF_HELPER_OBJ_REGEX_END;
+        return Parser.matchGroup1(helperPattern, javaScriptPlayerCode)
+                .replace("\n", "");
+    }
+}