clean up tiff_ifd, use islice instead of complicated math..

ui/medashare/metadata/crw.py

@@ -7,6 +7,7 @@ from io import BytesIO
 
 import enum
 import io
+import itertools
 import pathlib
 import string
 import struct
@@ -941,14 +942,24 @@ TIFF_IFD_ENTRY_CNT = 4	# number of items returned by TIFF_IFD_ENTRY
 TIFF_IFD_OFFSET = 'I'
 
 def tiff_ifd(fh, endian, off):
+	'''Iterated over the TIFF IFD that starts as off.
+
+	yields the tag, data and the offset of the value if it
+	did not fit in 4 bytes, otherwise None.
+
+	When done, the tag will be None, and data will have the
+	offset of the next IFD.
+	'''
+
 	fh.seek(off)
 	cnt = readstruct(fh, endian + TIFF_IFD_CNT)[0]
 	entries = readstruct(fh, endian + TIFF_IFD_ENTRY * cnt)
 	nextifd = readstruct(fh, endian + TIFF_IFD_OFFSET)[0]
 
+	entryiter = iter(entries)
+
 	for i in range(cnt):
-		tag, ttype, length, valoff = entries[i *
-		    TIFF_IFD_ENTRY_CNT:(i + 1) * TIFF_IFD_ENTRY_CNT]
+		tag, ttype, length, valoff = itertools.islice(entryiter, TIFF_IFD_ENTRY_CNT)
 		#print('t:', repr(tag), repr(ttype), repr(length), repr(valoff))
 		typefun, typequantum = tifftypes[ttype]
 		blength = length * typequantum
@@ -1071,7 +1082,7 @@ def idcrw(fh):
 	hlen = readstruct(fh, endian + "H")[0]
 
 	if hlen == 0x2a:
-		#Tiff
+		#TIFF/CR2
 		hoff, idstr, ver, hlen = readstruct(fh, endian + "I2sHI")
 		if not isjpeg and (hoff != 0x10 or idstr != b'CR' or ver != 2):
 			raise ValueError('normal TIFF, not a CR2')