From a n x n array print out all the diagonals

diagonal.rb

def diagonal(a)
  x = y = 0
  out = []
  limit = a.size - 1
  (0..limit * 2).each do
    out << down(x,y,a)
    x += 1 if y == limit
    y += 1 if y < limit
  end
  out
end

def down(x,y,a)
  out = []
  while y >= 0 && x < a.size
    out << a[x][y]
    x += 1
    y -= 1
  end
  out
end

describe '#down' do
  it 'return the correct diagonal from 0,3' do
    expect(down(0,3,[[1,2,3,4],[1,2,3,4],[1,2,3,4],[1,2,3,4]])).to eq [4,3,2,1]
  end

  it 'return the correct diagonal from 1,3' do
    expect(down(1,3,[[1,2,3,4],[1,2,3,4],[1,2,3,4],[1,2,3,4]])).to eq [4,3,2]
  end

  it 'return the correct diagonal from 0,0' do
    expect(down(0,0,[[1,2,3,4],[1,2,3,4],[1,2,3,4],[1,2,3,4]])).to eq [1]
  end
end

describe '#diagonal' do
  it 'return the correct diagonals' do
    expect(diagonal [[1,2,3,4],[1,2,3,4],[1,2,3,4],[1,2,3,4]]).to eq [[1],[2,1],[3,2,1],[4,3,2,1],[4,3,2],[4,3],[4]]
  end
end