Allow negative offsets to peek near the end of the file

warc-peek

@@ -16,6 +16,7 @@
 #   not attempt decompression when `1F 8B` is found in the last 512 bytes of the window. You can increase `LENGTH` to compensate for this if necessary.
 
 import argparse
+import io
 import logging
 import zlib
 
@@ -35,7 +36,14 @@ def finditer(b, sub):
 
 def find_offsets(warcfile, offset, length):
 	with open(warcfile, 'rb') as fp:
-		fp.seek(offset)
+		if offset >= 0:
+			fp.seek(offset)
+		else:
+			# Negative offset: go back from EOF and fix offset for correct output
+			fp.seek(0, io.SEEK_END)
+			size = fp.tell()
+			fp.seek(offset, io.SEEK_END)
+			offset = size + offset
 		buffer = fp.read(length)
 
 	logger.debug('Buffer length: {:d}'.format(len(buffer)))