(function() {
    var blurCount = 0, visCount = 0;
    var blurDuration = 0, visDuration = 0;
    var blurStart = null, visStart = null;

    function updateFields() {
        var form = document.getElementById('form');
        if (!form) return;
        var now = performance.now();
        var bc = blurCount, bd = blurDuration;
        var vc = visCount, vd = visDuration;
        if (blurStart !== null) bd += (now - blurStart) / 1000;
        if (visStart !== null) vd += (now - visStart) / 1000;
        var data = {
            'focus_blur_count': bc,
            'focus_blur_duration': bd.toFixed(1),
            'focus_vis_count': vc,
            'focus_vis_duration': vd.toFixed(1),
        };
        for (var name in data) {
            var input = form.querySelector('input[name="' + name + '"]');
            if (!input) {
                input = document.createElement('input');
                input.type = 'hidden';
                input.name = name;
                form.appendChild(input);
            }
            input.value = data[name];
        }
    }

    window.addEventListener('blur', function() {
        blurCount++;
        blurStart = performance.now();
        updateFields();
    });
    window.addEventListener('focus', function() {
        if (blurStart !== null) {
            blurDuration += (performance.now() - blurStart) / 1000;
            blurStart = null;
        }
        updateFields();
    });

    // Initialize hidden inputs to 0 on page load so previous-page values don't carry over
    updateFields();

    document.addEventListener('visibilitychange', function() {
        if (document.hidden) {
            visCount++;
            visStart = performance.now();
        } else {
            if (visStart !== null) {
                visDuration += (performance.now() - visStart) / 1000;
                visStart = null;
            }
        }
        updateFields();
    });
})();