input_field_id = "im_editable2000000120";
dialog_url = "https://vk.com/im?sel=c120";

// Checks if the message is the task.
function check(message) {
    return (message.indexOf(") Подсказка: ") != -1)
}

// From the bare HTML returns message text.
function get_message_text(msg) {
    while (msg.indexOf("<br>") != -1) {
        msg = msg.replace("<br>", " ");
    }
    // Because VK developers are shit.
    if (msg[0] == "<") {  // Old message type
        msg = msg.slice(1);
        return msg.slice(msg.indexOf(">") + 1, msg.indexOf("<"));
    } else {              // New message type
        return msg.slice(0, msg.indexOf("<"));
    }
}

// Returns 4 probable answers for the task.
function get_answers(message) {
    url = "http://baza-otvetov.ru/search/view/?q=";

    var xmlHttp = new XMLHttpRequest();
    xmlHttp.open("GET", url + encodeURIComponent(message), false);
    xmlHttp.send();

    res = xmlHttp.responseText;

    if (res.indexOf("По вашему запросу ничего не найдено") != -1) {
        return "Nothing found";
    }

    var answers = [];
    for (var i = 0; i < 4; i++) {
        res = res.slice(res.indexOf("Ответ: "));
        ans = res.slice(14, res.indexOf("</h"));
        answers.push(ans);
        res = res.slice(res.indexOf("</h"));
    }

    return answers;
}

// Checks if any answer fits the task.
function get_answer(message, answers) {
    var len = message.split("•").length;
    var fchar = message[message.indexOf("Подсказка: ") + 11];
    for (var i = 0; i < 4; i++) {
        if (answers[i][0] == fchar && answers[i].length == len) {
            return answers[i];
        }
    }
    return "";
}

function pull_messages() {
    var last_messages = document.getElementsByClassName("im-mess--text");
    var last_message = get_message_text(last_messages[last_messages.length - 1].innerHTML);

    if (last_message != prev_message) {
        prev_message = last_message;
        if (check(last_message)) {
            var answers = get_answers(last_message);
            console.log("New problem. Answers: " + answers);

            var ans = get_answer(last_message, answers);
            im = document.getElementById(input_field_id);
            im.innerHTML = ans;
        }
    }

    if (document.URL == dialog_url) {
        setTimeout(pull_messages, 200);
    }
}


prev_message = "";
if (document.URL == dialog_url) {
    pull_messages();
}