<button data-step="-1">left</button>
<button data-step="+1">right</button>
let pos = 0;
const
$slider = $('.slider'),
$blocks = $slider.find('.block'),
len = $blocks.length;
$('[data-step]').click(function() {
$blocks.filter('.active').removeClass('active');
pos = pos + +this.dataset.step;
$slider.css('transform', 'rotate(' + pos * 360 / len + 'deg)');
$blocks.eq(((-pos % len) + len) % len).addClass('active');
});