/* Part of SWI-Prolog
Author: Jan Wielemaker
Copyright (c) 2016, VU University Amsterdam
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[ open_hash_stream/3, % +OrgStream, -HashStream, +Options
stream_hash/2 % +HashStream, -Hash
:- predicate_options(open_hash_stream/3, 3,
/** <module> 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)]),
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
file_hash(Algorithm, File, Hash) :-
open(File, read, In0, [type(binary)]),
@see 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
% - 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.