cells.spec.ts

type Neighbors = {neighbors: number};
type State = Neighbors & {alive: boolean};

function deadCell({neighbors}: Neighbors): State {
  return {alive: false, neighbors};
}
function livingCell({neighbors}: Neighbors): State {
  return {alive: true, neighbors};
}

function next({alive, neighbors}: State): State {
  switch(true) {
    case (!alive && neighbors == 3):
    case (alive && (neighbors == 2 || neighbors == 3)):
      return livingCell({neighbors});

    default:
      return deadCell({neighbors});
  }
}

let expectLivingCellToDie = function (neighbors: { neighbors: number }) {
  expect(next(livingCell(neighbors))).toEqual(deadCell(neighbors));
};
describe('cell', () => {
  it('should die with zero neighbors', () => {
    let neighbors = {neighbors: 0};
    expectLivingCellToDie(neighbors);
  });

  it('should stay dead with two neighbors', () => {
    let neighbors = {neighbors: 2};
    expect(next(deadCell(neighbors))).toEqual(deadCell(neighbors));
  });
  it('should stay alive with two neighbors', () => {
    let neighbors = {neighbors: 2};
    expect(next(livingCell(neighbors))).toEqual(livingCell(neighbors));
  });
  it('should stay alive with three neighbors', () => {
    let neighbors = {neighbors: 3};
    expect(next(livingCell(neighbors))).toEqual(livingCell(neighbors));
  });
  it('should become alive with three neighbors', () => {
    let neighbors = {neighbors: 3};
    expect(next(deadCell(neighbors))).toEqual(livingCell(neighbors));
  });
  it('should die with four neighbors', () => {
    let neighbors = {neighbors: 4};
    expectLivingCellToDie(neighbors);
  });
});