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98 lines
6.2 KiB
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>diffmark</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article" title="diffmark"><div class="titlepage"><div><div><h1 class="title"><a name="idp1469496"></a>
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<a class="ulink" href="http://www.mangrove.cz/diffmark" target="_top">diffmark</a>
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</h1></div><div><div class="author"><h3 class="author"><span class="firstname">Václav</span> <span class="surname">Bárta</span></h3><div class="affiliation"><div class="address"><p><code class="email"><<a class="email" href="mailto:vbar@comp.cz">vbar@comp.cz</a>></code></p></div></div></div></div></div><hr></div><p><span class="application">diffmark</span> is an XML diff and merge
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package. It consists of a shared C++ library,
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<code class="filename">libdiffmark</code>, plus two programs wrapping the
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library into a command-line interface: <span class="command"><strong>dm</strong></span> and
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<span class="command"><strong>dm-merge</strong></span>. <span class="command"><strong>dm</strong></span> takes 2 XML files
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and prints their diff (also an XML document) on its standard
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output. <span class="command"><strong>dm-merge</strong></span> takes the first document passed
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to dm and its output and produces the second document.</p><p><span class="application">diffmark</span> has a close (not to say
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convoluted) relationship
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with the Perl module <span class="application">XML::DifferenceMarkup</span>
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(available on
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<a class="ulink" href="http://www.cpan.org" target="_top">CPAN</a>). Current versions of
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<span class="application">XML::DifferenceMarkup</span> are built on top of
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<code class="filename">libdiffmark</code>, making the packages compatible.
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<code class="filename">libdiffmark</code> depends on
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<span class="application">libxml2</span> (available from
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<a class="ulink" href="http://xmlsoft.org" target="_top">http://xmlsoft.org</a>).</p><p>The diff format is meant to be human-readable
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(i.e. simple, as opposed to short) - basically the diff is a subset
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of the input trees, annotated with instruction element nodes
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specifying how to convert the source tree to the target by inserting
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and deleting nodes. To prevent name colisions with input trees, all
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added elements are in a namespace
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<code class="systemitem">http://www.locus.cz/diffmark</code>
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(the diff will fail on input trees which already use that
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namespace).</p><p>The top-level node of the diff is always
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<span class="markup"><diff></span> (or rather <span class="markup"><dm:diff
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xmlns:dm="http://www.locus.cz/diffmark">
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... </dm:diff></span> - this description omits the namespace
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specification from now on); under it are fragments of the input
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trees and instruction nodes: <span class="markup"><insert/></span>,
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<span class="markup"><delete/></span> and
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<span class="markup"><copy/></span>. <span class="markup"><copy/></span> is
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used in places where the input subtrees are the same - in the limit,
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the diff of 2 identical documents is
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</p><pre class="programlisting"><?xml version="1.0"?>
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<dm:diff xmlns:dm="http://www.locus.cz/XML/diffmark">
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<dm:copy count="1"/>
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</dm:diff></pre><p>
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(<span class="markup"><copy/></span> always has the count attribute and
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no other content).</p><p><span class="markup"><insert/></span> and
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<span class="markup"><delete/></span> have the obvious meaning - in the
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limit a diff of 2 documents which have nothing in common is
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something like
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</p><pre class="programlisting"><?xml version="1.0"?>
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<dm:diff xmlns:dm="http://www.locus.cz/XML/diffmark">
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<dm:delete>
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<old/>
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</dm:delete>
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<dm:insert>
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<new>
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<tree>with the whole subtree, of course</tree>
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</new>
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</dm:insert>
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</dm:diff></pre><p>
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</p><p>A combination of <span class="markup"><insert/></span>,
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<span class="markup"><delete/></span> and <span class="markup"><copy/></span>
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can capture any difference, but it's sub-optimal for the case where
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(for example) the top-level elements in the two input documents
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differ while their subtrees are exactly the same. dm handles this
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case by putting the element from the second document into the diff,
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adding to it a special attribute <span class="markup">dm:update</span> (whose
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value is the element name from the first document) marking the
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element change:
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</p><pre class="programlisting"><?xml version="1.0"?>
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<dm:diff xmlns:dm="http://www.locus.cz/XML/diffmark">
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<top-of-second dm:update="top-of-first">
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<dm:copy count="42"/>
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</top-of-second>
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</dm:diff></pre><p>
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</p><p><span class="markup"><delete/></span> contains just one level of
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nested nodes - their subtrees are not included in the diff (but the
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element nodes which are included always come with all their
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attributes). <span class="markup"><insert/></span> and
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<span class="markup"><delete/></span> don't have any attributes and
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always contain some subtree.</p><p>Instruction nodes are never nested; all nodes above an
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instruction node (except the top-level
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<span class="markup"><diff/></span>) come from the input trees. A node
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from the second input tree might be included in the output diff to
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provide context for instruction nodes when it's an element node
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whose subtree is not the same in the two input documents. When such
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an element has the same name, attributes (names and values) and
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namespace declarations in both input documents, it's always included
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in the diff (its different output trees guarantee that it will have
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some chindren there). If the corresponding elements are different,
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the one from the second document might still be included, with an
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added <span class="markup">dm:update</span> attribute, provided that both
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corresponding elements have non-empty subtrees, and these subtrees
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are so similar that deleting the first corresponding element and
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inserting the second would lead to a larger diff. And if this
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paragraph seems too complicated, don't despair - just ignore it and
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look at some examples.</p></div></body></html>
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