import base64

import entry

# TODO: Include file number in the error info
class FileFormatError(Exception): pass

class VersionMismatch(Exception): pass

def parse_header(header):
	"""parse_header(bytes) → str
	Throw an error if the header isn't good and return the file comment
	(if any) if it is"""
	assert type(header) == bytes

	magic = header[0:6]
	if magic != b'SSHWOT':
		raise FileFormatError('Invalid magic')

	# Version 0 is the current one
	version = header[6:7]
	if version == b'':
		raise FileFormatError('No newline after header')
	if version != b'0':
		raise VersionMismatch('Version %i not supported' % version[0])

	# See if we have a comment
	if header[7:8] == b' ':
		# It says we have
		if header[8:9] == b'\n':
			# No, we don't, but we do have a space telling we
			# have. The header is malformed
			raise FileFormatError('Missing comment or spurious space in the header')
		else:
			# Yes, we do
			# Check it ends with a newline
			if header[-1] != 0x0a:
				raise FileFormatError('Missing newline at the end of the header')

			try:
				file_comment = header[8:-1].decode('utf-8')
			except UnicodeDecodeError:
				raise FileFormatError('Comment is not valid utf-8')

			return file_comment

	elif header[7:8] == b'\n':
		# No, we have newline
		return ''

	else:
		# No, we have something else
		raise FileFormatError("Expected a space or a newline but got '%s' instead" % header[7:].decode('utf-8'))

def parse_entry(line):
	"""parse_entry(bytes) → Entry"""
	assert type(line) == bytes

	def extract_b64_field(rest):
		"""extract_b64_field(bytes) → (bytes: decoded_field, bytes:rest)"""
		field_b64 = rest[0:44]
		if len(field_b64) != 44:
			raise FileFormatError('Unexpected end of line')
		try:
			field = base64.b64decode(field_b64, validate = True)
		except (ValueError, base64.binascii.Error) as err:
			raise FileFormatError('Malformed base64 string: %s' % field_b64.decode('utf-8')) from err

		return field, rest[44:]

	salt, rest = extract_b64_field(line)
	hashed_host, rest = extract_b64_field(rest)
	fingerprint, rest = extract_b64_field(rest)

	# What do we have after that?
	if rest[0:1] == b' ':
		# A comment?
		if rest[1:2] == b'\n':
			# No, but it says we have. It's malformed
			raise FileFormatError('Missing comment or spurious space in the entry')
		else:
			# Yes. Make sure it ends in a newline
			if rest[-1] != 0x0a:
				raise FileFormatError('No newline after entry')

			try:
				comment = rest[1:-1].decode('utf-8')
			except UnicodeDecodeError:
				raise FileFormatError('Comment is not valid utf-8')

	elif rest[0:1] == b'\n':
		# A newline
		comment = ''

	else:
		# Something else
		raise FileFormatError('Expected a space or a newline but got "%s" instead' % rest.decode('utf-8'))

	return entry.Entry(salt, hashed_host, fingerprint, comment)

def read(f):
	"""read(file(rb)) → ([Entry]: entries, str: file_comment)"""
	lines = [line for line in f]

	if len(lines) == 0:
		raise FileFormatError('Missing header')

	file_comment = parse_header(lines[0])

	entries = []
	for line in lines[1:]:
		entries.append(parse_entry(line))

	return entries, file_comment