from PIL import Image

def flat( *nums ):
	'Build a tuple of ints from float or integer arguments. Useful because PIL crop and resize require integer points.'
	
	return tuple( int(round(n)) for n in nums )

class Size(object):
	def __init__(self, pair):
		self.width = float(pair[0])
		self.height = float(pair[1])

	@property
	def aspect_ratio(self):
		return self.width / self.height

	@property
	def size(self):
		return flat(self.width, self.height)

def cropped_thumbnail(img, size):
	'''
	Builds a thumbnail by cropping out a maximal region from the center of the original with
	the same aspect ratio as the target size, and then resizing. The result is a thumbnail which is
	always EXACTLY the requested size and with no aspect ratio distortion (although two edges, either
	top/bottom or left/right depending whether the image is too tall or too wide, may be trimmed off.)
	'''
	
	original = Size(img.size)
	target = Size(size)

	if target.aspect_ratio > original.aspect_ratio:
		# image is too tall: take some off the top and bottom
		scale_factor = target.width / original.width
		crop_size = Size( (original.width, target.height / scale_factor) )
		top_cut_line = (original.height - crop_size.height) / 2
		img = img.crop( flat(0, top_cut_line, crop_size.width, top_cut_line + crop_size.height) )
	elif target.aspect_ratio < original.aspect_ratio:
		# image is too wide: take some off the sides
		scale_factor = target.height / original.height
		crop_size = Size( (target.width/scale_factor, original.height) )
		side_cut_line = (original.width - crop_size.width) / 2
		img = img.crop( flat(side_cut_line, 0,  side_cut_line + crop_size.width, crop_size.height) )
		
	return img.resize(target.size, Image.ANTIALIAS)