// ==UserScript==
// @namespace https://gist.github.com/amacfie/04b1da41137ed0bb20c554468f34b67c
// @description Add keybindings for focusing the next/previous form element
// @include http*
// @run-at document-start
// @noframes
// ==/UserScript==

var isGoodEl = function (node) {
  var formTags = ['INPUT', 'SELECT', 'BUTTON', 'TEXTAREA'];
  var isFormEl = 'tagName' in node && formTags.includes(node.tagName);
  if (!isFormEl) return false;
  if ('type' in node && node.type === 'hidden') return false;
  if ('readOnly' in node && node.readOnly) return false;
  if ('disabled' in node && node.disabled) return false;
  return true;
};

// will start from selected/focused element but not highlighted search match
// because the DOM has no access to that
var focusNext = function (forwards, nodePred) {
  var start;
  if (document.activeElement !== document.body) {
    start = document.activeElement;
  } else if (document.getSelection().focusNode) {
    start = document.getSelection().focusNode;
  } else {
    start = document.body;
  }

  var walker = document.createTreeWalker(
      document.body,
      5,  // NodeFilter.SHOW_TEXT | NodeFilter.SHOW_ELEMENT doesn't work in GM
      null,
      false
  );
  var node;
  var nodes = [document.body];
  while(node = walker.nextNode()) {
    nodes.push(node);
  }

  var startIndex = nodes.indexOf(start);
  for (var i=1; i <= nodes.length - 1; ++i) {
    var pos;
    if (forwards) {
      pos = (startIndex + i) % nodes.length;
    } else {
      pos = (startIndex - i + nodes.length) % nodes.length;
    }
    if (nodePred(nodes[pos])) {
      var prev = document.activeElement;
      nodes[pos].focus();
      if (prev !== document.activeElement) break;
    }
  }
};

// bind ctrl-` and ctrl-~
var handleKey = function (e) {
  if (e.ctrlKey && e.key === '`') {
    focusNext(true, isGoodEl);
  } else if (e.ctrlKey && e.key === '~') {
    focusNext(false, isGoodEl);
  }
};
document.addEventListener('keydown', handleKey, false);