feat: implement positions_downwards and positions_upwards

lib/position.rb

@@ -28,12 +28,22 @@ def positions_to_the_left
     ("A".."H").first(index).map { |l| Position.new(l, number) }.reverse
   end
 
-  # all positions to the left, sorted by closest
+  # all positions to the right, sorted by closest
   def positions_to_the_right
     index = ("A".."H").find_index(letter)
     ("A".."H").last(7 - index).map { |l| Position.new(l, number) }
   end
 
+  # all positions up, sorted by closest
+  def positions_upwards
+    ((number+1)..(8)).map { |n| Position.new(letter, n) }
+  end
+
+  # all positions down, sorted by closest
+  def positions_downwards
+    (1..(number-1)).map { |n| Position.new(letter, n) }.reverse
+  end
+
   def ==(other)
     letter == other.letter && number == other.number
   end

test/position_test.rb

@@ -27,4 +27,12 @@ def test_positions_to_the_left
   def test_positions_to_the_right
     assert_equal Position.parse(%w[G4 H4]), Position.parse("F4").positions_to_the_right
   end
+
+  def test_positions_upwards
+    assert_equal Position.parse(%w[G7 G8]), Position.parse("G6").positions_upwards
+  end
+
+  def test_positions_downwards
+    assert_equal Position.parse(%w[G2 G1]), Position.parse("G3").positions_downwards
+  end
 end