function createPagination(paginationOptions) {
  return {
    data: typeof paginationOptions.data === 'string'
      ? paginationOptions.data
      : 'collections.all',

    size: 1,

    alias: paginationOptions.alias ?? 'paginationChunk',

    before: function(paginationData, data) {
      const chunks = [];
      const pageSize = paginationOptions.pageSize;

      const allItems = typeof paginationOptions.data === 'function'
        ? paginationOptions.data(data)
        : data[paginationOptions.data];

      const getKey = typeof paginationOptions.key === 'function'
        ? paginationOptions.key
        : (item) => item[paginationOptions.key];

      const itemsByKey = allItems.reduce((map, item) => {
        const key = getKey(item);
        map[key] = map[key] || [];
        map[key].push(item);
        return map;
      }, {});

      for (const [key, group] of Object.entries(itemsByKey)) {
        const pageNumbers = Math.ceil(group.length / pageSize);
        const groupChunks = [];
        for (let page = 0; page < pageNumbers; page++) {
          const items = group.slice(pageSize * page, pageSize * (page + 1));
          const item = {
            key,
            items,
            pageSize,
            page,
          };
          Object.defineProperty(item, 'next', {
            enumerable: false,
            get() {
              return groupChunks[page + 1]
            }
          });
          Object.defineProperty(item, 'previous', {
            enumerable: false,
            get() {
              return groupChunks[page - 1]
            }
          });
          groupChunks.push(item);
        }
        chunks.push(...groupChunks);
      }

      return chunks;
    }
  }
}

module.exports = {
  users: [
    {
      id: 1,
      name: 'User 1',
      role: 'developer'
    },
    {
      id: 2,
      name: 'User 2',
      role: 'manager'
    },
    {
      id: 3,
      name: 'User 3',
      role: 'designer'
    },
    {
      id: 4,
      name: 'User 4',
      role: 'developer'
    },
    {
      id: 5,
      name: 'User 5',
      role: 'manager'
    },
    {
      id: 6,
      name: 'User 6',
      role: 'designer'
    },
    {
      id: 7,
      name: 'User 7',
      role: 'developer'
    },
    {
      id: 8,
      name: 'User 8',
      role: 'manager'
    },
    {
      id: 9,
      name: 'User 9',
      role: 'designer'
    },
  ],

  pagination: createPagination({
    alias: 'paginationChunk',
    pageSize: 2,
    data: data => data.users,
    key: (item) => item.role,
  }),

  eleventyComputed: {
    permalink: function(data) {
      const { paginationChunk } = data;
      const pageNumber = paginationChunk.page === 0 ? '' : `${paginationChunk.page}/`;
      return `/${paginationChunk.key}/${pageNumber}`
    },
  }
}