text: # This file is dual licensed under the terms of the Apache License, Version
  # 2.0, and the BSD License. See the LICENSE file in the root of this repository
  # for complete details.
  
  from __future__ import absolute_import, division, print_function
  
  
  from cryptography import utils
  from cryptography.exceptions import UnsupportedAlgorithm, _Reasons
  from cryptography.hazmat.primitives import hashes
  
  
  @utils.register_interface(hashes.HashContext)
  class _HashContext(object):
      def __init__(self, backend, algorithm, ctx=None):
          self._algorithm = algorithm
  
          self._backend = backend
  
          if ctx is None:
              ctx = self._backend._lib.EVP_MD_CTX_create()
              ctx = self._backend._ffi.gc(ctx,
                                          self._backend._lib.EVP_MD_CTX_destroy)
              evp_md = self._backend._lib.EVP_get_digestbyname(
                  algorithm.name.encode("ascii"))
              if evp_md == self._backend._ffi.NULL:
                  raise UnsupportedAlgorithm(
                      "{0} is not a supported hash on this backend.".format(
                          algorithm.name),
                      _Reasons.UNSUPPORTED_HASH
                  )
              res = self._backend._lib.EVP_DigestInit_ex(ctx, evp_md,
                                                         self._backend._ffi.NULL)
              assert res != 0
  
          self._ctx = ctx
  
      algorithm = utils.read_only_property("_algorithm")
  
      def copy(self):
          copied_ctx = self._backend._lib.EVP_MD_CTX_create()
          copied_ctx = self._backend._ffi.gc(
              copied_ctx, self._backend._lib.EVP_MD_CTX_destroy
          )
          res = self._backend._lib.EVP_MD_CTX_copy_ex(copied_ctx, self._ctx)
          assert res != 0
          return _HashContext(self._backend, self.algorithm, ctx=copied_ctx)
  
      def update(self, data):
          res = self._backend._lib.EVP_DigestUpdate(self._ctx, data, len(data))
          assert res != 0
  
      def finalize(self):
          buf = self._backend._ffi.new("unsigned char[]",
                                       self._backend._lib.EVP_MAX_MD_SIZE)
          outlen = self._backend._ffi.new("unsigned int *")
          res = self._backend._lib.EVP_DigestFinal_ex(self._ctx, buf, outlen)
          assert res != 0
          assert outlen[0] == self.algorithm.digest_size
          res = self._backend._lib.EVP_MD_CTX_cleanup(self._ctx)
          assert res == 1
          return self._backend._ffi.buffer(buf)[:outlen[0]]
  