Utilities for fetching an item's index in an array being treated as a table.

array-as-table-utils.ts

export const
  getRowIndex = (itemIdx: number, itemsPerRow: number): number =>
    Math.round((itemIdx - (itemIdx % itemsPerRow)) / itemsPerRow),

  getColumnIndex = (itemIdx: number, itemsPerRow: number): number =>
    itemIdx % itemsPerRow
;

array-as-table-utils_test.ts

import {getColumnIndex, getRowIndex} from "./array-as-table-utils";

type Index = number;
type ItemsPerRow = number;

describe ('#getRowIndex', () => {
  (<[Index, ItemsPerRow, Index][]>[
    [0, 5, 0],
    [24, 5, 4],
    [1, 5, 0],
    [23, 5, 4],
    [12, 5, 2],
  ]).forEach(([itemIdx, itemsPerRow, expectedRowIndex]) => {
    it(`getRowIndex(${itemIdx}, ${itemsPerRow}) === ${expectedRowIndex}`, function () {
      const rslt = getRowIndex(itemIdx, itemsPerRow);
      expect(rslt).toEqual(expectedRowIndex);
    });
  });
});

describe('#getColumnIndex', () => {
  (<[Index, ItemsPerRow, Index][]>[
    [0, 5, 0],
    [24, 5, 4],
    [1, 5, 1],
    [23, 5, 3],
    [12, 5, 2],
  ]).forEach(([itemIdx, itemsPerRow, expectedRowIndex]) => {
    it(`getColumnIndex(${itemIdx}, ${itemsPerRow}) === ${expectedRowIndex}`, function () {
      const rslt = getColumnIndex(itemIdx, itemsPerRow);
      expect(rslt).toEqual(expectedRowIndex);
    });
  });
});