stuff I should have remembered from school

arraynotes

The below is ruby(ish) - but is really about matrix math

array.transpose flips array around diagonal axis (UL to LR):

a            = [[1, 2, 3],
                [4, 5, 6],
                [7, 8, 9]]

a.transpose => [[1, 4, 7],
                [2, 5, 8],
                [3, 6, 9]]

if you transpose then reverse - you've rotated the array CCW 90 degrees:

a.t.reverse => [[3, 6, 9],
                [2, 5, 8],
                [1, 4, 7]]

Repeat that 4 times, you're back to where you started.