document.getElementById("precode").addEventListener("keyup",code);

function code() {
  // lovingly stolen from https://github.com/kpym/autoradio-renault
	var precode = document.getElementById("precode").value.toUpperCase();

    if (!/^[A-Z]\d{3}$/.test(precode) || precode.startsWith("A0")) { return ''; }

    x = precode.charCodeAt(1) + precode.charCodeAt(0) * 10 - 698;
    y = precode.charCodeAt(3) + precode.charCodeAt(2) * 10 + x - 528;
    z = (y*7) % 100;
    code = Math.floor(z / 10) + (z % 10) * 10 + ((259 % x) % 100) * 100;

  document.getElementById("code").value = code.toString().padStart(4,'0');
  // document.getElementById('code').innerHTML = code.toString().padStart(4,'0');
}