Game_of_life.js

const directions = [[-1,-1],[-1,0],[-1,1],[0,-1],[0,1],[1,-1],[1,0],[1,1]];

function evolve(currentCells) {
  const equalTo = (a) => (b) => a[0] === b[0] && a[1] === b[1];
  const isCurrentlyAlive = (cell) => currentCells.some(equalTo(cell));
  const add = (a) => (b) => [a[0] + b[0], a[1] + b[1]]
  const neighboursOf = (cell) => directions.map(add(cell));
  const notIn = (list) => (item) => !list.some(equalTo(item));
  const countNeighbours = (cell) => neighboursOf(cell).filter(isCurrentlyAlive).length;
  
  const evolves = (cell) => 
    countNeighbours(cell) === 3 ||
    isCurrentlyAlive(cell) && countNeighbours(cell) === 2;
  
  return currentCells.reduce((evolvedCells, cell) => 
    evolvedCells.concat(
      neighboursOf(cell)
        .filter(evolves)
        .filter(notIn(evolvedCells))
    ), []);
}

describe ('gol', () => {
  it('should kill lonely', () => assert.deepEqual(evolve([[0,0]]), []));
  it('should kill 2', () => assert.deepEqual(evolve([[0,0], [1,0]]), []));
  it('should retain with 2 neighbours', 
    () => assert.deepEqual(evolve([[-1,-1], [0,0], [1, 1]]), [[0, 0]]));
  it('should retain with 3 neighbours', 
    () => assert.deepEqual(evolve([[-1,-1], [0,0], [1, 1], [1, -1]]), [[0,-1],[0, 0],[1,0]]));
  it('should create 1 when 3 parents', 
    () => assert.deepEqual(evolve([[0,0], [1,0], [2, 1]]), [[1, 0], [1, 1]]));
  it('should kill overcrowded', 
    () => assert.deepEqual(evolve([[-1,1], [1,1], [1, -1], [-1,-1], [0, 0]]), [[-1,0],[0,1],[1,0],[0,-1]]));
})