# Identifies APNGs
# Written by Foone/Popcorn Mariachi#!9i78bPeIxI
# This code is in the public domain
# identify_png returns:
# '?'   for non-png or corrupt PNG
# 'png'  for a standard png 
# 'apng' for an APNG
# takes one argument, a file handle. When the function reutrns it'll be positioned at the start of the file
# usage:
#  fop=open('tempfile.dat','rb')
#  type=identify_png(fop)
import struct
PNG_SIGNATURE='\x89\x50\x4e\x47\x0d\x0a\x1a\x0a'
def identify_png(file):
	data=file.read(8)
	if data!=PNG_SIGNATURE:
		return '?' # not a PNG/APNG
	try:
		while 1:
			buffer=file.read(8)
			if len(buffer)!=8:
				return '?' # Early EOF
			length,type=struct.unpack('!L4s',buffer)
			if type in ('IDAT','IEND'):
				# acTL must come before IDAT, so if we see an IDAT this is plain PNG
				# IEND is end of file.
				return 'png'
			if type=='acTL':
				return 'apng'
			file.seek(length+4,1) # +4 because of the CRC (not checked)
	finally:
		file.seek(0)