import hashing

import entry

class FileFormatError(Exception):
	def __init__(self, string):
		self.string = string
		self.line = None

	def __str__(self):
		if self.line == None:
			return self.string
		else:
			return 'Line %i: %s' % (self.line, self.string)

class VersionMismatch(Exception):
	def __init__(self, string):
		self.string = string
		self.line = None

	def __str__(self):
		if self.line == None:
			return self.string
		else:
			return 'Line %i: %s' % (self.line, self.string)

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

	# Check that it ends in a newline
	if len(header) == 0 or header[-1] != 0x0a:
		raise FileFormatError('No newline after header')

	# Split it into fields and make sure we have at least the magic and
	# the version
	fields = header[:-1].split(b' ', 3)
	if len(fields) < 2:
		raise FileFormatError('Too few fields in the header, expected at least magic and version')

	# Check the magic
	if fields[0] != b'SSHWOT':
		raise FileFormatError('Invalid magic')

	# See if we have a comment
	if len(fields) == 3:
		# It says we have
		if len(fields[2]) == 0:
			# 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. Extract it
			try:
				file_comment = fields[2].decode('utf-8')
			except UnicodeDecodeError:
				raise FileFormatError('Comment is not valid utf-8')

	else:
		file_comment = ''

	return file_comment

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

	def decode_b64_field(b64):
		try:
			return hashing.base64dec(b64)
		except (ValueError, base64.binascii.Error) as err:
			raise FileFormatError('Malformed base64 string: %s' % b64.decode('utf-8')) from err

	# Check that it ends in a newline
	if len(line) == 0 or line[-1] != 0x0a:
		raise FileFormatError('No newline after entry')

	# Split the line into fields and make sure we have at least the
	# salt, the hashed host, and the fingerprint
	fields = line[:-1].split(b' ', 3)
	if len(fields) < 3:
		raise FileFormatError('Too few fields in the entry, expected in the very least salt, hashed host, and fingerprint')

	salt = decode_b64_field(fields[0])
	hashed_host = decode_b64_field(fields[1])
	fingerprint = decode_b64_field(fields[2])

	# See if we have a comment
	if len(fields) == 4:
		# It says we have
		if len(fields[3]) == 0:
			# 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 entry')
		else:
			# Yes, we do. Extract it
			try:
				comment = fields[3].decode('utf-8')
			except UnicodeDecodeError:
				raise FileFormatError('Comment is not valid utf-8')

	else:
		comment = ''

	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')

	try:
		file_comment = parse_header(lines[0])
	except (FileFormatError, VersionMismatch) as err:
		err.line = 1
		raise err

	entries = []
	# Since line numbers are 1-indexed while lists in python are
	# 0-indexed and we handle the first one separately, first one in the
	# list is line 2
	for linenum_minus_2, line in enumerate(lines[1:]):
		try:
			entries.append(parse_entry(line))
		except FileFormatError as err:
			err.line = linenum_minus_2 + 2
			raise err

	return entries, file_comment