Alternate 2mg implementation

Rwrite of the 2mg parsing code which saves the comment and creator blocks in
memory by putting the code into the Disk class.

Doesn't yet attempt to parse out 2mg image format, and the question is open as
to whether or not I'll try to follow cppo's historical method of handling this
(chop the header and go), AppleCommander's method (consider 2mg its own unique
image format which happens to contain one of the others) or CiderPress's (use
one of several possible wrappers independent of the image orders.  CP's is
probably the most flexible.

So basically this code is probably far from final, but it works for what it
does so far.

cppo

@@ -96,34 +96,6 @@ def unpack_u24le(buf: bytes, offset: int = 0) -> int:
 	lo16, hi8 = struct.unpack_from('<HB', buf, offset)
 	return lo16 | (hi8 << 16)
 
-# FIXME: I know -> namedtuple below is _wrong_.  It's a hint, and I am doing
-# things wrong here.  I'm not 100% sure how to do this right.  If you are,
-# please submit a more pythonic fix.
-def unpack_2mg(buf: bytes, offset: int = 0) -> namedtuple:
-	try:
-		# 2017-07-01: Version 1 data is the only kind so far...
-		a2mg = A2mg1(*struct.unpack_from(A2MG1_UNPACK, buf, offset))
-		if a2mg.magic == b'2IMG':
-			if a2mg.version == 1:
-				vol = A2mg1Vol(
-					locked=bool(a2mg.vol & (1<<31)),
-					dosvol=a2mg.vol & 0xff if a2mg.vol & 0x100 else None
-					)
-				a2mg = a2mg._replace(vol=vol)
-			else:
-				log.warn(
-						"Unrecognized 2mg version {}: '{}'".format(
-							a2mg.version, name
-							)
-						)
-				a2mg = A2mgUnk(*a2mg[0:len(A2MG_UNK_ATTRS)])
-		else:
-			a2mg = None
-	except ValueError:
-		a2mg = None
-	print(a2mg)
-	return a2mg
-
 
 def date_prodos_to_unix(prodos_date: bytes) -> int:
 	"""Returns a UNIX timestamp given a raw ProDOS date"""
@@ -907,7 +879,7 @@ def isnumber(number):
 
 ### NEW DISK CLASSES
 
-A2MG1_UNPACK = (
+TWOIMG_V1_UNPACK = (
 		'<'              # use little-endian numbers
 		'4s'             # magic string '2IMG'
 		'4s'             # creator string
@@ -924,21 +896,15 @@ A2MG1_UNPACK = (
 		'L'              # creator private use length
 		'16x'            # reserved for future use
 		)
-A2MG1_ATTRS = (
+TWOIMG_V1_ATTRS = (
 		'magic', 'creator', 'hdr_len', 'version',
-		'img_fmt', 'vol', 'num_blocks',
+		'img_fmt', 'flags', 'num_blocks',
 		'data_offset', 'data_len',
-		'cmnt_offset', 'cmnt_len',
+		'comment_offset', 'comment_len',
 		'creator_offset', 'creator_len'
 		)
-A2MG1_VOL_ATTRS = ('locked', 'dosvol')
 
-# We assume 2mg files with unknown version have these fields
-A2MG_UNK_ATTRS = ('magic', 'creator', 'hdr_len', 'version')
-
-A2mg1 = namedtuple('A2mg1', A2MG1_ATTRS)
-A2mg1Vol = namedtuple('A2mg1Vol', A2MG1_VOL_ATTRS)
-A2mgUnk = namedtuple('A2mgUnk', A2MG_UNK_ATTRS)
+TwoImgV1 = namedtuple('TwoImgV1', TWOIMG_V1_ATTRS)
 
 class Disk:
 	def __init__(self, name=None):
@@ -951,7 +917,38 @@ class Disk:
 			with open(to_sys_name(name), "rb") as f:
 				self.image = f.read()
 
-			self.a2mg = unpack_2mg(self.image)
+			if self.ext in ('.2mg', '.2img'):
+				self._parse_2mg()
+
+	def _parse_2mg(self):
+		self.twoimg = None
+		self.twoimg_comment = None
+		self.twoimg_creator = None
+		self.twoimg_locked = None
+		hdr = TwoImgV1(*struct.unpack_from(TWOIMG_V1_UNPACK, self.image))
+		if hdr.magic == b'2IMG':
+			self._raw_twoimg = self.image[:hdr.hdr_len]
+			if hdr.version == 1:
+				if hdr.hdr_len == 64:
+					if hdr.comment_offset and hdr.comment_len:
+						self.twoimg_comment = self.image[
+								hdr.comment_offset
+								: hdr.comment_offset + hdr.comment_len]
+					if hdr.creator_offset and hdr.creator_len:
+						self.twoimg_creator = self.image[
+								hdr.creator_offset
+								: hdr.creator_offset + hdr.creator_len]
+					self.twoimg_locked = bool(hdr.flags & 0x80000000)
+					self.twoimg = hdr
+				else:
+					log.warn('2mg header length: {} (expected 64 '
+							'for version 1)'.format(hdr.hdr_len))
+			else:
+				log.warn('2mg version unsupported: {} (only support '
+						'version 1)'.format(hdr.version))
+		else:
+			log.warn('2mg header not found: magic is {}'.format(hdr.magic))
+			self._raw_twoimg = None
 
 
 ### UTIL