Function getModernOffsetsFromPoints

code/composition/public/app.js:6513–6580

* Returns {start, end} where `start` is the character/codepoint index of * (anchorNode, anchorOffset) within the textContent of `outerNode`, and * `end` is the index of (focusNode, focusOffset). * * Returns null if you pass in garbage input but we should probably just crash. * * Exported only

(outerNode, anchorNode, anchorOffset, focusNode, focusOffset)

 * Exported only for testing.
 */
function getModernOffsetsFromPoints(outerNode, anchorNode, anchorOffset, focusNode, focusOffset) {
  var length = 0;
  var start = -1;
  var end = -1;
  var indexWithinAnchor = 0;
  var indexWithinFocus = 0;
  var node = outerNode;
  var parentNode = null;

  outer: while (true) {
    var next = null;

    while (true) {
      if (node === anchorNode && (anchorOffset === 0 || node.nodeType === TEXT_NODE)) {
        start = length + anchorOffset;
      }
      if (node === focusNode && (focusOffset === 0 || node.nodeType === TEXT_NODE)) {
        end = length + focusOffset;
      }

      if (node.nodeType === TEXT_NODE) {
        length += node.nodeValue.length;
      }

      if ((next = node.firstChild) === null) {
        break;
      }
      // Moving from `node` to its first child `next`.
      parentNode = node;
      node = next;
    }

    while (true) {
      if (node === outerNode) {
        // If `outerNode` has children, this is always the second time visiting
        // it. If it has no children, this is still the first loop, and the only
        // valid selection is anchorNode and focusNode both equal to this node
        // and both offsets 0, in which case we will have handled above.
        break outer;
      }
      if (parentNode === anchorNode && ++indexWithinAnchor === anchorOffset) {
        start = length;
      }
      if (parentNode === focusNode && ++indexWithinFocus === focusOffset) {
        end = length;
      }
      if ((next = node.nextSibling) !== null) {
        break;
      }
      node = parentNode;
      parentNode = node.parentNode;
    }

    // Moving from `node` to its next sibling `next`.
    node = next;
  }

  if (start === -1 || end === -1) {