Flattening a matrix stored as a list of lists, in clockwise spiral pattern. From a coding interview - sorry about the Ruby, recruiters (I'm not actually sorry.)

spiral.rb

def spiral_flatten(matrix_chunks)
  [].tap do |result|
    until matrix_chunks.empty?
      result.concat matrix_chunks.shift
      matrix_chunks[0..-2].to_a.each { |chunk| result << chunk.pop }
      break if matrix_chunks.empty?
      result.concat matrix_chunks.pop.reverse
      matrix_chunks[1..-1].to_a.reverse_each { |chunk| result << chunk.shift }
    end
  end
end

spiral_flatten [[1,2,3],[4,5,6],[7,8,9],[10,11,12]]
#=> [1, 2, 3, 6, 9, 12, 11, 10, 7, 4, 5, 8]

def spiral_flatten(matrix):
    out = []
    matrix = iter(matrix)

    try:
        while True:
            out.extend(next(iter(matrix)))
            matrix = reversed(zip(*matrix))
    except StopIteration:
        pass
    return out

:3

If I import izip as zip from itertools, the above code doesn't make copies.

If you're not worried about copies, well

def spiral_flatten(matrix):
    out = []
    while matrix:
        out.extend(matrix.pop(0))
        matrix = list(reversed(zip(*matrix)))
    return out

Aaand in ruby

def spiral_flatten(matrix)
    matrix = Array.new(matrix)
    out = []
    until matrix.empty? do
        out.concat(matrix.shift)
        matrix = matrix.transpose.reverse
    end
    out
end

Edit: Threw in a copy, because