module ChunkyPNG::Canvas::Drawing

def line_xiaolin_wu(x0, y0, x1, y1, color)
  y0, y1, x0, x1 = y1, y0, x1, x0 if y0 > y1
  dx = x1 - x0
  sx = dx < 0 ? -1 : 1
  dx *= sx
  dy = y1 - y0
  
  if dy == 0 # vertical line
    Range.new(*[x0,x1].sort).each do |x|
      point(x, y0, color)
    end
  elsif dx == 0 # horizontal line
    (y0..y1).each do |y|
      point(x0, y, color)
    end
  elsif dx == dy # diagonal
    x0.step(x1, sx) do |x|
      point(x, y0, color)
      y0 += 1
    end
    
  elsif dy > dx  # vertical displacement
    point(x0, y0, color)
    e_acc = 0          
    e = ((dx << 16) / dy.to_f).round
    (y0...y1-1).each do |i|
      e_acc_temp, e_acc = e_acc, (e_acc + e) & 0xffff
      x0 = x0 + sx if (e_acc <= e_acc_temp)
      w = 0xff - (e_acc >> 8)
      point(x0, y0, ChunkyPNG::Color.fade(color, w)) if include?(x0, y0)
      y0 = y0 + 1
      point(x0 + sx, y0, ChunkyPNG::Color.fade(color, 0xff - w)) if include?(x0 + sx, y0)
    end
    point(x1, y1, color)
    
  else # horizontal displacement
    point(x0, y0, color)
    e_acc = 0
    e = (dy << 16) / dx
    (dx - 1).downto(0) do |i|
      e_acc_temp, e_acc = e_acc, (e_acc + e) & 0xffff
      y0 += 1 if (e_acc <= e_acc_temp)
      w = 0xff - (e_acc >> 8)
      point(x0, y0, ChunkyPNG::Color.fade(color, w)) if include?(x0, y0)
      x0 += sx
      point(x0, y0 + 1, ChunkyPNG::Color.fade(color, 0xff - w)) if include?(x0, y0 + 1)
    end
    point(x1, y1, color)
  end
end

def point(x, y, color)
  self[x, y] = ChunkyPNG::Color.compose(color, self[x, y])
end