module ChunkyPNG::Canvas::Adam7Interlacing

def adam7_extract_pass(pass, canvas)
  x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
  sm_pixels = []
  y_offset.step(canvas.height - 1, 1 << y_shift) do |y|
    x_offset.step(canvas.width - 1, 1 << x_shift) do |x|
      sm_pixels << canvas[x, y]
    end
  end
  new_canvas_args = adam7_pass_size(pass, canvas.width, canvas.height) + [sm_pixels]
  ChunkyPNG::Canvas.new(*new_canvas_args)
end

def adam7_merge_pass(pass, canvas, subcanvas)
  x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
  for y in 0...subcanvas.height do
    for x in 0...subcanvas.width do
      new_x = (x << x_shift) | x_offset
      new_y = (y << y_shift) | y_offset
      canvas[new_x, new_y] = subcanvas[x, y]
    end
  end
end

def adam7_multiplier_offset(pass)
  [
    3 - (pass >> 1),
    pass & 1 == 0 ? 0 : 8 >> ((pass + 1) >> 1),
    pass == 0 ? 3 : 3 - ((pass - 1) >> 1),
    pass == 0 || pass & 1 == 1 ? 0 : 8 >> (pass >> 1),
  ]
end

def adam7_pass_size(pass, original_width, original_height)
  x_shift, x_offset, y_shift, y_offset = adam7_multiplier_offset(pass)
  [
    (original_width  - x_offset + (1 << x_shift) - 1) >> x_shift,
    (original_height - y_offset + (1 << y_shift) - 1) >> y_shift,
  ]
end

def adam7_pass_sizes(original_width, original_height)
  (0...7).map { |pass| adam7_pass_size(pass, original_width, original_height) }
end