module ChunkyPNG::Canvas::Adam7Interlacing

def adam7_extract_pass(pass, canvas)
  m_o = adam7_multiplier_offset(pass)
  sm_pixels = []
  
  m_o[:y_offset].step(canvas.height - 1, 1 << m_o[:y_shift]) do |y|
    m_o[:x_offset].step(canvas.width - 1, 1 << m_o[: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)
  m_o = adam7_multiplier_offset(pass)
  for y in 0...subcanvas.height do
    for x in 0...subcanvas.width do
      new_x = (x << m_o[:x_shift]) | m_o[:x_offset]
      new_y = (y << m_o[:y_shift]) | m_o[:y_offset]
      canvas[new_x, new_y] = subcanvas[x, y]
    end
  end
  canvas
end

def adam7_multiplier_offset(pass)
  {
    :x_shift      => 3 - (pass >> 1), 
    :x_offset     => (pass & 1 == 0) ? 0 : 8 >> ((pass + 1) >> 1),
    :y_shift      => pass == 0 ? 3 : 3 - ((pass - 1) >> 1),
    :y_offset     => (pass == 0 || pass & 1 == 1) ? 0 : 8 >> (pass >> 1)
  }
end

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

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