Re: [xsl] Tree Comparing Algorithm

["Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

The only facility in the XSLT 3.0 to allow streaming of two input files "in
parallel" is xsl:merge, and as Martin points out, that's rather specialised
and not really suited to your requirements.

In Saxon, streaming is in most cases done in push mode (where the parser owns
the control loop, and sends events to the XSLT processor). You can't have two
parallel control loops except with multi-threading, so the opportunities for
streaming multiple files are limited (with xsl:merge, Saxon indeed uses
multi-threading).

At first sight, I don't see an XSLT-based answer to this one.

Except, perhaps: you could do a streamed transformation of each input
documents into an XML representation of an event stream, like

<startElement name="folder" path="" hash=""/>
<startElement name="folder" path="" hash=""/>
<endElement name="folder"/>

etc

and then attempt to do an xsl:merge of the two event streams.

Michael Kay
Saxonica

> On 3 Feb 2020, at 13:47, Vasu Chakkera vasucv@xxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hi All,
> I am planning to write a XML Tree comparing XSLT using streaming.
> The XML Trees look something like this
> <root path="" mhash =" ">
>   <folder path ="" mhash ="">
>     <folder path ="" mhash ="">
>        <leaf path ="" mhash ="">
>        </leaf>
>     </folder>
>   </folder>
> </root>
> There will be two such XML files to compare . These two XMLs are generated
before and after moving a folder from source to destination. Source and
destination could be two different OS.
> This is essentially the serialized Merkle Tree output of a folder structure.
The idea is to run a Merkle Tree comparator that will pick the nodes that did
not match. Rules are as follows.
> If the root node in both the tree matches, then there is not difference in
the entire tree(because of how the Merkle tree is generated)
> If root node hash does not match, we go to the child container and compare
the hash of the child container in both the XML files. ( the XML folders
structure will be identical with respect to the hash, but the folder  path may
be different because of the linux, windows path conventions. Otherwise the
folder structure is meant to be the same.)
> If the hash of a folder from both the trees are same, the entire tree under
the folder that matches the hash is ignored.
> if the hash of a folder from both the trees are not the same, then the tree
is further traversed and the step 3 is repeated.
> The XSLT keeps writing out the nodes that do not match the hashes in the
source and target xml files
>
> So at the end of the processing, A comparator tree should be serialized,
that has the nodes that have a non matching leaf node.
> Looking at the serialized tree, we can determine, which files got messed up
while doing a transfer from Source to target.
>
>
>
> I am able to do this using non streaming xslt, but with streaming, since we
need to stream two trees at a time and match compare the nodes,  i am not very
sure how to proceed.
> I am able to do manipulations on one XML with streaming. I tried a few
tricks, but did not get anywhere ( I am not very comfortable copying my code
scribbling here)
>
> I need streaming because the XML files may be big.
> If someone has done something similar, or point me to an  intelligent way to
do this, I will be thankful.
>
> Vasu
>
>
>
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