function pageExists(pagename, callback) {
    $.ajax(mw.util.getUrl(pagename), {
        method: 'HEAD',
        success: function() { callback(true); },
        error: function(xhr) { callback(xhr.status === 404 ? false : null); }
    });
}

function userExists(username, callback) {
    pageExists('Special:Contributions/' + username, callback);
}

function findUser(username, callback) {
    /* returns an object with the following properties:
        name -- username as a string, or null if not found
        user -- title of the user page as a string, or null if not found
        talk -- title of the user talk page as a string, or null if not found
    */

    /* example: 
        findUser('Surjection', function(result) {
            console.log(result);
            if (result.name) {
                console.log('User exists with the name ' + result.name);

                if (result.user)
                    console.log('User page exists: [[' + result.user + ']]');
                else
                    console.log('User does not have a user page');

                if (result.talk)
                    console.log('Talk page exists: [[' + result.talk + ']]');
                else
                    console.log('User does not have a talk page');
            } else {
                console.log('User not found');
            }
        });
    */

    function findUserFound(username) {
        var userpage = 'User:' + username;
        var talkpage = 'User talk:' + username;
        pageExists(userpage, function(userexists) {
            pageExists(talkpage, function(talkexists) {
                var result = { name: username };
                result.user = userexists ? userpage : null;
                result.talk = talkexists ? talkpage : null;
                callback(result);
            });
        });
    }

    userExists(username, function(found) {
        if (found) findUserFound(username);
        else {
            var commons = username + '~commonswiki';
            userExists(commons, function(found) {
                if (found) findUserFound(commons);
                else callback({ name: null, user: null, talk: null });
            });
        }
    });
}