first cut at verification of bittorrent files...

This needs to be expanded a bit, and handle utf-8 encoded filenames..

__init__.py

@@ -0,0 +1,148 @@
+
+from . import bencode
+from hashlib import sha1
+import importlib.resources
+import itertools
+import os
+import pathlib
+import shutil
+import sys
+import tempfile
+import unittest
+
+class Storage:
+	def __init__(self, rootpath, files, piecelen):
+		self._rootpath = pathlib.Path(rootpath)
+		self._files = files
+		self._piecelen = piecelen
+
+		self._buildindex()
+
+	def _buildindex(self):
+		self._index = []
+		files = iter(self._files)
+		left = 0
+		curfile = None
+
+		while True:
+			if curfile is None or curfileoff == curfile['length']:
+				# next file
+				try:
+					curfile = next(files)
+					fname = pathlib.Path(
+					    *(x.decode('us-ascii') for x in
+					    curfile['path']))
+					curfilepath = self._rootpath / fname
+				except StopIteration:
+					break
+				curfileoff = 0
+
+			if left == 0:
+				current = []
+				self._index.append(current)
+				left = self._piecelen
+
+			sz = min(curfile['length'] - curfileoff, left)
+
+			current.append(dict(file=curfilepath, fname=fname,
+			    offset=curfileoff, size=sz))
+
+			curfileoff += sz
+			left -= sz
+
+	def apply_piece(self, idx, fun):
+		for i in self._index[idx]:
+			with open(i['file'], 'rb') as fp:
+				fp.seek(i['offset'])
+				fun(fp.read(i['size']))
+
+def validate(torrent, basedir):
+	info = torrent['info']
+
+	basedir = pathlib.Path(basedir)
+
+	print(repr(torrent))
+
+	torrentdir = basedir / info['name'].decode('us-ascii')
+
+	stor = Storage(torrentdir, info['files'], info['piece length'])
+
+	pieces = info['pieces']
+	for num, i in enumerate(pieces[x:x+20] for x in range(0, len(pieces),
+	    20)):
+		hash = sha1()
+
+		stor.apply_piece(num, hash.update)
+
+		if hash.digest() != i:
+			raise ValueError
+
+class _TestCases(unittest.TestCase):
+	dirname = 'somedir'
+	origfiledata = {
+		'filea.txt':	b'foo\n',
+		'fileb.txt':	b'bar\n',
+		'filec.txt':	b'bleha\n',
+		'filed.txt':	b'somehow\n',
+		'filee.txt':	b'nowab\n',
+		'filef/filef.txt':	b'\n',
+	}
+
+	def setUp(self):
+		d = pathlib.Path(tempfile.mkdtemp()).resolve()
+
+		tor = importlib.resources.files(__name__)
+		tor = tor / 'fixtures' / 'somedir.torrent'
+		with tor.open('rb') as fp:
+			self.torrent = bencode.bdecode(fp.read())
+
+		self.basetempdir = d
+
+		self.oldcwd = os.getcwd()
+
+		os.chdir(d)
+
+	def tearDown(self):
+		shutil.rmtree(self.basetempdir)
+
+		os.chdir(self.oldcwd)
+
+	@staticmethod
+	def make_files(dname, fdict):
+		dname = pathlib.Path(dname)
+		for k, v in fdict.items():
+			k = dname / pathlib.PurePosixPath(k)
+			k.parent.mkdir(parents=True, exist_ok=True)
+			with open(k, 'wb') as fp:
+				fp.write(v)
+
+	def test_completeverif(self):
+		sd = self.basetempdir / self.dirname
+		sd.mkdir()
+
+		self.make_files(sd, self.origfiledata)
+
+		validate(self.torrent, self.basetempdir)
+
+	def test_verification(self):
+		# Testing for "missing" files
+		# piece size 2 (aka 4 bytes)
+		# empty file of 4 bytes		'foo\n'
+		# complete file of 4 bytes	'bar\n'
+		# partial missing file, 6 bytes, last two correct	'bleha\n'
+		# complete file of 8 bytes (multiple pieces)		'somehow\n'
+		# partial missing file, starting w/ 2 bytes, length 6	'nowab\n'
+		# complete file (length 1)				'\n'
+
+		missingfiles = self.origfiledata.copy()
+
+		missingfiles['filea.txt'] = b''
+		missingfiles['filec.txt'] = b'\x00\x00\x00\x00a\n'
+		missingfiles['filee.txt'] = b'no'
+
+		sd = self.basetempdir / self.dirname
+		sd.mkdir()
+
+		self.make_files(sd, missingfiles)
+
+		self.assertRaises(ValueError, validate, self.torrent, self.basetempdir)