firefox parser/html/java/README.txt

firefox parser/html/java/README.txt (via) TIL (or TIR - Today I was Reminded) that the HTML5 Parser used by Firefox is maintained as Java code (commit history here) and converted to C++ using a custom translation script.

You can see that in action by checking out the ~8GB Firefox repository and running:

cd parser/html/java
make sync
make translate

Here's a terminal session where I did that, including the output of git diff showing the updated C++ files.

I did some digging and found that the code that does the translation work lives, weirdly, in the Nu Html Checker repository on GitHub which powers the W3C's validator.w3.org/nu/ validation service!

Here's a snippet from htmlparser/cpptranslate/CppVisitor.java showing how a class declaration is converted into C++:

protected void startClassDeclaration() {
    printer.print("#define ");
    printer.print(className);
    printer.printLn("\_cpp\_\_");
    printer.printLn();

    for (int i = 0; i < Main.H\_LIST.length; i++) {
        String klazz = Main.H\_LIST\[i\];
        if (!klazz.equals(javaClassName)) {
            printer.print("#include \\"");
            printer.print(cppTypes.classPrefix());
            printer.print(klazz);
            printer.printLn(".h\\"");
        }
    }

    printer.printLn();
    printer.print("#include \\"");
    printer.print(className);
    printer.printLn(".h\\"");
    printer.printLn();
}

Here's a fascinating blog post from John Resig explaining how validator author Henri Sivonen introduced the new parser into Firefox in 2009.