/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @format
 * @flow strict-local
 * @emails oncall+draft_js
 */
'use strict';

import type EditorState from "./EditorState";
import type SelectionState from "./SelectionState";

const warning = require("fbjs/lib/warning");
/**
 * Given a collapsed selection, move the focus `maxDistance` backward within
 * the selected block. If the selection will go beyond the start of the block,
 * move focus to the end of the previous block, but no further.
 *
 * This function is not Unicode-aware, so surrogate pairs will be treated
 * as having length 2.
 */


function moveSelectionBackward(editorState: EditorState, maxDistance: number): SelectionState {
  const selection = editorState.getSelection(); // Should eventually make this an invariant

  warning(selection.isCollapsed(), 'moveSelectionBackward should only be called with a collapsed SelectionState');
  const content = editorState.getCurrentContent();
  const key = selection.getStartKey();
  const offset = selection.getStartOffset();
  let focusKey = key;
  let focusOffset = 0;

  if (maxDistance > offset) {
    const keyBefore = content.getKeyBefore(key);

    if (keyBefore == null) {
      focusKey = key;
    } else {
      focusKey = keyBefore;
      const blockBefore = content.getBlockForKey(keyBefore);
      focusOffset = blockBefore.getText().length;
    }
  } else {
    focusOffset = offset - maxDistance;
  }

  return selection.merge({
    focusKey,
    focusOffset,
    isBackward: true
  });
}

module.exports = moveSelectionBackward;