Get page objects to implement pagination

pagination.test.ts

import { assertEquals } from "jsr:@std/assert";

import { paginate } from "./pagination.ts";

Deno.test(function paginateTest() {
  assertEquals(paginate(1, 1), [1]);

  assertEquals(paginate(1, 2), [1, 2]);
  assertEquals(paginate(2, 2), [1, 2]);

  assertEquals(paginate(1, 3), [1, 2, 3]);
  assertEquals(paginate(2, 3), [1, 2, 3]);
  assertEquals(paginate(3, 3), [1, 2, 3]);

  assertEquals(paginate(1, 4), [1, 2, 3, 4]);
  assertEquals(paginate(2, 4), [1, 2, 3, 4]);
  assertEquals(paginate(3, 4), [1, 2, 3, 4]);
  assertEquals(paginate(4, 4), [1, 2, 3, 4]);

  assertEquals(paginate(1, 5), [1, 2, 3, 4, 5]);
  assertEquals(paginate(2, 5), [1, 2, 3, 4, 5]);
  assertEquals(paginate(3, 5), [1, 2, 3, 4, 5]);
  assertEquals(paginate(4, 5), [1, 2, 3, 4, 5]);
  assertEquals(paginate(5, 5), [1, 2, 3, 4, 5]);

  assertEquals(paginate(1, 6), [1, 2, 3, 4, 5]);
  assertEquals(paginate(2, 6), [1, 2, 3, 4, 5]);
  assertEquals(paginate(3, 6), [1, 2, 3, 4, 5]);
  assertEquals(paginate(4, 6), [2, 3, 4, 5, 6]);
  assertEquals(paginate(5, 6), [2, 3, 4, 5, 6]);
  assertEquals(paginate(6, 6), [2, 3, 4, 5, 6]);

  assertEquals(paginate(1, 7), [1, 2, 3, 4, 5]);
  assertEquals(paginate(2, 7), [1, 2, 3, 4, 5]);
  assertEquals(paginate(3, 7), [1, 2, 3, 4, 5]);
  assertEquals(paginate(4, 7), [2, 3, 4, 5, 6]);
  assertEquals(paginate(5, 7), [3, 4, 5, 6, 7]);
  assertEquals(paginate(6, 7), [3, 4, 5, 6, 7]);
  assertEquals(paginate(7, 7), [3, 4, 5, 6, 7]);

  assertEquals(paginate(1, 10), [1, 2, 3, 4, 5]);
  assertEquals(paginate(2, 10), [1, 2, 3, 4, 5]);
  assertEquals(paginate(3, 10), [1, 2, 3, 4, 5]);
  assertEquals(paginate(4, 10), [2, 3, 4, 5, 6]);
  assertEquals(paginate(5, 10), [3, 4, 5, 6, 7]);
  assertEquals(paginate(6, 10), [4, 5, 6, 7, 8]);
  assertEquals(paginate(8, 10), [6, 7, 8, 9, 10]);
  assertEquals(paginate(9, 10), [6, 7, 8, 9, 10]);
  assertEquals(paginate(10, 10), [6, 7, 8, 9, 10]);
});

pagination.ts

const LIMIT = 5;
const PREV_SIZE = 2;

/**
 * When you want to render page objects like following:
 * - [1] [2] [3] [4] [5]
 * - [...] [2] [3] [4] [5] [6] [...]
 * - [1] [2] [3]
 *
 * @example
 * ```ts
 * const page = 1;
 * const limit = 20;
 * const { items, total } = await fetchData({ offset: page - 1, limit });
 * const maxPage = Math.ceil(total / limit);
 * const pages = paginate(page, maxPage);
 * ```
 */
export function paginate(current: number, max: number): number[] {
  if (max <= LIMIT) {
    return [...Array(max).keys()].map((n) => n + 1);
  }

  const pages: number[] = [];

  let prev = current - 1;

  const prevSize = current + PREV_SIZE <= max
    ? PREV_SIZE
    : (PREV_SIZE * 2) - (max - current);

  while (prev > 0 && pages.length < prevSize) {
    pages.unshift(prev);
    prev--;
  }

  pages.push(current);

  let next = current + 1;
  while (pages.length < LIMIT && next <= max) {
    pages.push(next);
    next++;
  }

  return pages;
}