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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2016, VU University Amsterdam
    7    All rights reserved.
    8
    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
   20
   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   25    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   26    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   27    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   28    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   29    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   31    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   32    POSSIBILITY OF SUCH DAMAGE.
   33*/
   34
   35:- module(hash_stream,
   36          [ open_hash_stream/3,         % +OrgStream, -HashStream, +Options
   37            stream_hash/2               % +HashStream, -Hash
   38          ]).   39:- use_foreign_library(foreign(hashstream)).   40:- predicate_options(open_hash_stream/3, 3,
   41                     [ close_parent(boolean),
   42                       algorithm(oneof([md5,sha1,sha224,sha256,sha384,sha512]))
   43                     ]).

Maintain a hash on a stream

This library defines a filter stream that maintains a hash of the data that passes through the stream. It can be used to compute the hash of input data while it is being processed. This is notably interesting if data is processed from a socket as it avoids the need for collecting the data first in a temporary file.

A typical processing sequence is illustrated below, where process/2 somehow processed the data and save_result/3 records the result as obtained from URL with content digest SHA256 its Result.

    ...,
    http_open(URL, In0, []),
    open_hash_stream(In0, In, [algorithm(sha256)]),
    process(In, Result),
    stream_hash(In, SHA256),
    close(In),
    save_result(URL, SHA256, Result)

This library can also be used to compute the hash for the content of a file. The advantage is that this code doesn't rely on external tools. It is considerably faster for short files, but considerably slower on large files because Prolog I/O is based on character streams rather than blocks.

file_hash(Algorithm, File, Hash) :-
    setup_call_cleanup(
        open(File, read, In0, [type(binary)]),
        setup_call_cleanup(
            open_hash_stream(In0, In,
                             [ algorithm(Algorithm),
                               close_parent(false)
                             ]),
            ( setup_call_cleanup(
                  open_null_stream(Null),
                  copy_stream_data(In, Null),
                  close(Null)),
              stream_hash(In, Hash)
            ),
            close(In)),
        close(In0)).
See also
- In addition to this hash library, SWI-Prolog provides library(md5), library(sha) and hash functions through library(crypto), part of the ssl package. */
 open_hash_stream(+OrgStream, -HashStream, +Options) is det
Open a filter stream on OrgStream that maintains a hash. The hash can be retrieved at any time using stream_hash/2. Provided options:
algorithm(+Algorithm)
One of md5, sha1, sha224, sha256, sha384 or sha512. Default is sha1.
close_parent(+Bool)
If true (default), closing the filter stream also closes the original (parent) stream.
 stream_hash(+HashStream, -Digest:atom) is det
Unify Digest with a hash for the bytes send to or read from HashStream. Note that the hash is computed on the stream buffers. If the stream is an output stream, it is first flushed and the Digest represents the hash at the current location. If the stream is an input stream the Digest represents the hash of the processed input including the already buffered data.