/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

/**
 * These variables store information about text content of a target node,
 * allowing comparison of content before and after a given event.
 *
 * Identify the node where selection currently begins, then observe
 * both its text content and its current position in the DOM. Since the
 * browser may natively replace the target node during composition, we can
 * use its position to find its replacement.
 *
 *
 */

let root = null;
let startText = null;
let fallbackText = null;

export function initialize(nativeEventTarget) {
  root = nativeEventTarget;
  startText = getText();
  return true;
}

export function reset() {
  root = null;
  startText = null;
  fallbackText = null;
}

export function getData() {
  if (fallbackText) {
    return fallbackText;
  }

  let start;
  const startValue = startText;
  const startLength = startValue.length;
  let end;
  const endValue = getText();
  const endLength = endValue.length;

  for (start = 0; start < startLength; start++) {
    if (startValue[start] !== endValue[start]) {
      break;
    }
  }

  const minEnd = startLength - start;
  for (end = 1; end <= minEnd; end++) {
    if (startValue[startLength - end] !== endValue[endLength - end]) {
      break;
    }
  }

  const sliceTail = end > 1 ? 1 - end : undefined;
  fallbackText = endValue.slice(start, sliceTail);
  return fallbackText;
}

export function getText() {
  if ('value' in root) {
    return root.value;
  }
  return root.textContent;
}
