fourth.rb

def evolve_cell(cell, neighbours)
  if cell == 1
    if neighbours.inject(&:+) == 2 || neighbours.inject(&:+) == 3
      1
    else
      0
    end
  else
    if neighbours.inject(&:+) == 3
      1
    else
      0
    end
  end
end

def neighbours(grid, width, height, index)
  [
    # north
    index - width,
    # northeast
    # east
    # southeast
    # south
    # southwest
    # west
    # northwest
    # north
  ]
end

def evolve_grid(grid, width, height)
  [
    0, 0, 0, 0, 0,
    0, 0, 1, 0, 0,
    0, 0, 1, 0, 0,
    0, 0, 1, 0, 0,
    0, 0, 0, 0, 0
  ]
end

describe '#evolve_grid' do
  it 'evolves oscillators' do
    expect(evolve_grid([
      0, 0, 0, 0, 0,
      0, 0, 0, 0, 0,
      0, 1, 1, 1, 0,
      0, 0, 0, 0, 0,
      0, 0, 0, 0, 0
    ], 5, 5)).to eq([
      0, 0, 0, 0, 0,
      0, 0, 1, 0, 0,
      0, 0, 1, 0, 0,
      0, 0, 1, 0, 0,
      0, 0, 0, 0, 0
    ])
  end

  xit 'evolves still lifes' do
    expect(evolve_grid([
      0, 0, 0, 0,
      0, 1, 1, 0,
      0, 1, 1, 0,
      0, 0, 0, 0
    ], 4, 4)).to eq([
      0, 0, 0, 0,
      0, 1, 1, 0,
      0, 1, 1, 0,
      0, 0, 0, 0
    ])
  end
end

describe '#evolve_cell' do
  context 'when the cell is live' do
    it 'dies with less than two live nieghbours' do
      expect(evolve_cell(1, [1, 0, 0])).to eq 0
      expect(evolve_cell(1, [0, 0, 0])).to eq 0
    end

    it 'lives with two live neighbours' do
      expect(evolve_cell(1, [1, 1, 0])).to eq 1
    end

    it 'lives with three live neighbours' do
      expect(evolve_cell(1, [1, 1, 1])).to eq 1
    end

    it 'dies with more than three live neighbours' do
      expect(evolve_cell(1, [1, 1, 1, 1])).to eq 0
    end
  end

  context 'when the cell is dead' do
    it 'becomes live with exactly three live neighbours' do
      expect(evolve_cell(0, [1, 1, 1])).to eq 1
    end

    it 'stays dead without exactly three live neighbours' do
      expect(evolve_cell(0, [1, 1, 0])).to eq 0
    end
  end
end