import re
import os.path
import hashlib

from collections import namedtuple


Digest = namedtuple('Digest', ['is_tarsum', 'tarsum_version', 'hash_alg', 'hash_bytes'])


DIGEST_PATTERN = r'(tarsum\.(v[\w]+)\+)?([\w]+):([0-9a-f]+)'
DIGEST_REGEX = re.compile(DIGEST_PATTERN)


class InvalidDigestException(RuntimeError):
  pass


def parse_digest(digest):
  """ Returns the digest parsed out to its components. """
  match = DIGEST_REGEX.match(digest)
  if match is None or match.end() != len(digest):
    raise InvalidDigestException('Not a valid digest: %s', digest)

  is_tarsum = match.group(1) is not None
  return Digest(is_tarsum, match.group(2), match.group(3), match.group(4))


def content_path(digest):
  """ Returns a relative path to the parsed digest. """
  parsed = parse_digest(digest)
  components = []

  if parsed.is_tarsum:
    components.extend(['tarsum', parsed.tarsum_version])

  # Generate a prefix which is always two characters, and which will be filled with leading zeros
  # if the input does not contain at least two characters. e.g. ABC -> AB, A -> 0A
  prefix = parsed.hash_bytes[0:2].zfill(2)
  components.extend([parsed.hash_alg, prefix, parsed.hash_bytes])

  return os.path.join(*components)


def sha256_digest(content):
  """ Returns a sha256 hash of the content bytes in digest form. """
  def single_chunk_generator():
    yield content
  return sha256_digest_from_generator(single_chunk_generator())


def sha256_digest_from_generator(content_generator):
  """ Reads all of the data from the iterator and creates a sha256 digest from the content
  """
  digest = hashlib.sha256()
  for chunk in content_generator:
    digest.update(chunk)
  return 'sha256:{0}'.format(digest.hexdigest())


def digests_equal(lhs_digest_string, rhs_digest_string):
  """ Parse and compare the two digests, returns True if the digests are equal, False otherwise.
  """
  return parse_digest(lhs_digest_string) == parse_digest(rhs_digest_string)