react-native-full-calendars/src/calendar/utils/computed-array-merge.ts

React Native Full Calendar (RNFC) is an intuitive and powerful calendar component library designed for React Native.

import { CalendarDataWithMetadata } from '../type';

export const alignDataList = (
  prevList: CalendarDataWithMetadata[],
  currentList: CalendarDataWithMetadata[]
): CalendarDataWithMetadata[] => {
  // Create a map from currentList for quick lookup
  const currentMap = currentList.reduce((map, item) => {
    map[item.id] = item;
    return map;
  }, {});

  // Create the result array based on prevList order
  const alignedList = prevList.map((prevItem) => {
    if (!prevItem) {
      return null;
    }
    if (!prevItem.metadata?.isEnd && currentMap[prevItem.id]) {
      return currentMap[prevItem.id];
    }
    return null;
  });

  const cleanedArray =
    removeTrailingNulls<CalendarDataWithMetadata>(alignedList);

  // Find items in currentList that are not in prevList
  const prevIds = new Set(
    prevList.filter((item) => item).map((item) => item.id)
  );
  const additionalItems = currentList.filter((item) => !prevIds.has(item.id));

  return fillNullsWithItems(cleanedArray, additionalItems);
};

const fillNullsWithItems = (
  array: CalendarDataWithMetadata[],
  items: CalendarDataWithMetadata[]
): CalendarDataWithMetadata[] => {
  let itemIndex = 0; // To keep track of items to insert

  const filledArray = array.map((element) => {
    if (element === null && itemIndex < items.length) {
      const itemToInsert = items[itemIndex];
      itemIndex += 1;
      return itemToInsert;
    }
    return element;
  });
  // Append any remaining items
  const remainingItems = items.slice(itemIndex);
  return filledArray.concat(remainingItems);
};

const removeTrailingNulls = <T>(array: Array<T>): Array<T> => {
  let i = array.length - 1;
  while (i >= 0 && array[i] === null) {
    i--;
  }
  return array.slice(0, i + 1);
};