import * as Y from 'yjs';
import { TextChange, minimalChange } from '../util/textDiff';

const isHigh = (code: number) => code >= 0xd800 && code <= 0xdbff;
const isLow = (code: number) => code >= 0xdc00 && code <= 0xdfff;

/**
 * `minimalChange`, widened so it never splits a surrogate pair: Yjs would
 * replace half a pair with U+FFFD, and every copy would get the broken text.
 */
export function safeChange(before: string, after: string): TextChange | undefined {
  const change = minimalChange(before, after);
  if (!change) return undefined;
  let { from, to } = change;
  let endAfter = after.length - (before.length - to);
  // A common start ending in a high surrogate, or a common end starting with
  // a low one, would split a pair: take the whole character into the change.
  if (from > 0 && isHigh(before.charCodeAt(from - 1))) from--;
  if (to < before.length && isLow(before.charCodeAt(to))) {
    to++;
    endAfter++;
  }
  return { from, to, insert: after.slice(from, endAfter) };
}

/** Makes the Y.Text read `after` with one minimal edit; false when it already does. */
export function applyToYText(text: Y.Text, after: string): boolean {
  const change = safeChange(text.toString(), after);
  if (!change) return false;
  if (change.to > change.from) text.delete(change.from, change.to - change.from);
  if (change.insert) text.insert(change.from, change.insert);
  return true;
}