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10 </style><title>Library internals</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="GNOME2 Logo" /></a><a href="http://www.w3.org/Status"><img src="w3c.png" alt="W3C logo" /></a><a href="http://www.redhat.com"><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href="http://xmlsoft.org/XSLT/"><img src="Libxslt-Logo-180x168.gif" alt="Made with Libxslt Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XSLT C library for GNOME</h1><h2>Library internals</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html">Home</a></li><li><a href="intro.html">Introduction</a></li><li><a href="docs.html">Documentation</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="help.html">How to help</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="news.html">News</a></li><li><a href="xsltproc2.html">The xsltproc tool</a></li><li><a href="docbook.html">DocBook</a></li><li><a href="API.html">The programming API</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="internals.html">Library internals</a></li><li><a href="extensions.html">Writing extensions</a></li><li><a href="contribs.html">Contributions</a></li><li><a href="EXSLT/index.html" style="font-weight:bold">libexslt</a></li><li><a href="xslt.html">flat page</a>, <a href="site.xsl">stylesheet</a></li><li><a href="html/index.html" style="font-weight:bold">API Menu</a></li><li><a href="ChangeLog.html">ChangeLog</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="tutorial/libxslttutorial.html">Tutorial</a>,
11 <a href="tutorial2/libxslt_pipes.html">Tutorial2</a></li><li><a href="xsltproc.html">Man page for xsltproc</a></li><li><a href="http://mail.gnome.org/archives/xslt/">Mail archive</a></li><li><a href="http://xmlsoft.org/">XML libxml2</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxslt">Bug Tracker</a></li><li><a href="http://codespeak.net/lxml/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXSLT">Perl XSLT bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading17">XSLT with PHP</a></li><li><a href="http://www.mod-xslt2.com/">Apache module</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://xsldbg.sourceforge.net/">Xsldbg Debugger</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><h3>Table of contents</h3><ul><li><a href="internals.html#Introducti">Introduction</a></li>
12 <li><a href="internals.html#Basics">Basics</a></li>
13 <li><a href="internals.html#Keep">Keep it simple stupid</a></li>
14 <li><a href="internals.html#libxml">The libxml nodes</a></li>
15 <li><a href="internals.html#XSLT">The XSLT processing steps</a></li>
16 <li><a href="internals.html#XSLT1">The XSLT stylesheet compilation</a></li>
17 <li><a href="internals.html#XSLT2">The XSLT template compilation</a></li>
18 <li><a href="internals.html#processing">The processing itself</a></li>
19 <li><a href="internals.html#XPath">XPath expressions compilation</a></li>
20 <li><a href="internals.html#XPath1">XPath interpretation</a></li>
21 <li><a href="internals.html#Descriptio">Description of XPath
23 <li><a href="internals.html#XPath3">XPath functions</a></li>
24 <li><a href="internals.html#stack">The variables stack frame</a></li>
25 <li><a href="internals.html#Extension">Extension support</a></li>
26 <li><a href="internals.html#Futher">Further reading</a></li>
27 <li><a href="internals.html#TODOs">TODOs</a></li>
28 </ul><h3><a name="Introducti2" id="Introducti2">Introduction</a></h3><p>This document describes the processing of <a href="http://xmlsoft.org/XSLT/">libxslt</a>, the <a href="http://www.w3.org/TR/xslt">XSLT</a> C library developed for the <a href="http://www.gnome.org/">GNOME</a> project.</p><p>Note: this documentation is by definition incomplete and I am not good at
29 spelling, grammar, so patches and suggestions are <a href="mailto:veillard@redhat.com">really welcome</a>.</p><h3><a name="Basics1" id="Basics1">Basics</a></h3><p>XSLT is a transformation language. It takes an input document and a
30 stylesheet document and generates an output document:</p><p align="center"><img src="processing.gif" alt="the XSLT processing model" /></p><p>Libxslt is written in C. It relies on <a href="http://www.xmlsoft.org/">libxml</a>, the XML C library for GNOME, for
31 the following operations:</p><ul><li>parsing files</li>
32 <li>building the in-memory DOM structure associated with the documents
34 <li>the XPath implementation</li>
35 <li>serializing back the result document to XML and HTML. (Text is handled
37 </ul><h3><a name="Keep1" id="Keep1">Keep it simple stupid</a></h3><p>Libxslt is not very specialized. It is built under the assumption that all
38 nodes from the source and output document can fit in the virtual memory of
39 the system. There is a big trade-off there. It is fine for reasonably sized
40 documents but may not be suitable for large sets of data. The gain is that it
41 can be used in a relatively versatile way. The input or output may never be
42 serialized, but the size of documents it can handle are limited by the size
43 of the memory available.</p><p>More specialized memory handling approaches are possible, like building
44 the input tree from a serialization progressively as it is consumed,
45 factoring repetitive patterns, or even on-the-fly generation of the output as
46 the input is parsed but it is possible only for a limited subset of the
47 stylesheets. In general the implementation of libxslt follows the following
48 pattern:</p><ul><li>KISS (keep it simple stupid)</li>
49 <li>when there is a clear bottleneck optimize on top of this simple
50 framework and refine only as much as is needed to reach the expected
52 </ul><p>The result is not that bad, clearly one can do a better job but more
53 specialized too. Most optimization like building the tree on-demand would
54 need serious changes to the libxml XPath framework. An easy step would be to
55 serialize the output directly (or call a set of SAX-like output handler to
56 keep this a flexible interface) and hence avoid the memory consumption of the
57 result.</p><h3><a name="libxml" id="libxml">The libxml nodes</a></h3><p>DOM-like trees, as used and generated by libxml and libxslt, are
58 relatively complex. Most node types follow the given structure except a few
59 variations depending on the node type:</p><p align="center"><img src="node.gif" alt="description of a libxml node" /></p><p>Nodes carry a <strong>name</strong> and the node <strong>type</strong>
60 indicates the kind of node it represents, the most common ones are:</p><ul><li>document nodes</li>
61 <li>element nodes</li>
63 </ul><p>For the XSLT processing, entity nodes should not be generated (i.e. they
64 should be replaced by their content). Most nodes also contains the following
65 "navigation" information:</p><ul><li>the containing <strong>doc</strong>ument</li>
66 <li>the <strong>parent</strong> node</li>
67 <li>the first <strong>children</strong> node</li>
68 <li>the <strong>last</strong> children node</li>
69 <li>the <strong>prev</strong>ious sibling</li>
70 <li>the following sibling (<strong>next</strong>)</li>
71 </ul><p>Elements nodes carries the list of attributes in the properties, an
72 attribute itself holds the navigation pointers and the children list (the
73 attribute value is not represented as a simple string to allow usage of
74 entities references).</p><p>The <strong>ns</strong> points to the namespace declaration for the
75 namespace associated to the node, <strong>nsDef</strong> is the linked list
76 of namespace declaration present on element nodes.</p><p>Most nodes also carry an <strong>_private</strong> pointer which can be
77 used by the application to hold specific data on this node.</p><h3><a name="XSLT" id="XSLT">The XSLT processing steps</a></h3><p>There are a few steps which are clearly decoupled at the interface
78 level:</p><ol><li>parse the stylesheet and generate a DOM tree</li>
79 <li>take the stylesheet tree and build a compiled version of it (the
80 compilation phase)</li>
81 <li>take the input and generate a DOM tree</li>
82 <li>process the stylesheet against the input tree and generate an output
84 <li>serialize the output tree</li>
85 </ol><p>A few things should be noted here:</p><ul><li>the steps 1/ 3/ and 5/ are optional</li>
86 <li>the stylesheet obtained at 2/ can be reused by multiple processing 4/
87 (and this should also work in threaded programs)</li>
88 <li>the tree provided in 2/ should never be freed using xmlFreeDoc, but by
89 freeing the stylesheet.</li>
90 <li>the input tree 4/ is not modified except the _private field which may
91 be used for labelling keys if used by the stylesheet</li>
92 </ul><h3><a name="XSLT1" id="XSLT1">The XSLT stylesheet compilation</a></h3><p>This is the second step described. It takes a stylesheet tree, and
93 "compiles" it. This associates to each node a structure stored in the
94 _private field and containing information computed in the stylesheet:</p><p align="center"><img src="stylesheet.gif" alt="a compiled XSLT stylesheet" /></p><p>One xsltStylesheet structure is generated per document parsed for the
95 stylesheet. XSLT documents allow includes and imports of other documents,
96 imports are stored in the <strong>imports</strong> list (hence keeping the
97 tree hierarchy of includes which is very important for a proper XSLT
98 processing model) and includes are stored in the <strong>doclist</strong>
99 list. An imported stylesheet has a parent link to allow browsing of the
100 tree.</p><p>The DOM tree associated to the document is stored in <strong>doc</strong>.
101 It is preprocessed to remove ignorable empty nodes and all the nodes in the
102 XSLT namespace are subject to precomputing. This usually consist of
103 extracting all the context information from the context tree (attributes,
104 namespaces, XPath expressions), and storing them in an xsltStylePreComp
105 structure associated to the <strong>_private</strong> field of the node.</p><p>A couple of notable exceptions to this are XSLT template nodes (more on
106 this later) and attribute value templates. If they are actually templates,
107 the value cannot be computed at compilation time. (Some preprocessing could
108 be done like isolation and preparsing of the XPath subexpressions but it's
109 not done, yet.)</p><p>The xsltStylePreComp structure also allows storing of the precompiled form
110 of an XPath expression that can be associated to an XSLT element (more on
111 this later).</p><h3><a name="XSLT2" id="XSLT2">The XSLT template compilation</a></h3><p>A proper handling of templates lookup is one of the keys of fast XSLT
112 processing. (Given a node in the source document this is the process of
113 finding which templates should be applied to this node.) Libxslt follows the
114 hint suggested in the <a href="http://www.w3.org/TR/xslt#patterns">5.2
115 Patterns</a> section of the XSLT Recommendation, i.e. it doesn't evaluate it
116 as an XPath expression but tokenizes it and compiles it as a set of rules to
117 be evaluated on a candidate node. There usually is an indication of the node
118 name in the last step of this evaluation and this is used as a key check for
119 the match. As a result libxslt builds a relatively more complex set of
120 structures for the templates:</p><p align="center"><img src="templates.gif" alt="The templates related structure" /></p><p>Let's describe a bit more closely what is built. First the xsltStylesheet
121 structure holds a pointer to the template hash table. All the XSLT patterns
122 compiled in this stylesheet are indexed by the value of the the target
123 element (or attribute, pi ...) name, so when a element or an attribute "foo"
124 needs to be processed the lookup is done using the name as a key.</p><p>Each of the patterns is compiled into an xsltCompMatch structure. It holds
125 the set of rules based on the tokenization of the pattern stored in reverse
126 order (matching is easier this way). It also holds some information about the
127 previous matches used to speed up the process when one iterates over a set of
128 siblings. (This optimization may be defeated by trashing when running
129 threaded computation, it's unclear that this is a big deal in practice.)
130 Predicate expressions are not compiled at this stage, they may be at run-time
131 if needed, but in this case they are compiled as full XPath expressions (the
132 use of some fixed predicate can probably be optimized, they are not yet).</p><p>The xsltCompMatch are then stored in the hash table, the clash list is
133 itself sorted by priority of the template to implement "naturally" the XSLT
134 priority rules.</p><p>Associated to the compiled pattern is the xsltTemplate itself containing
135 the information required for the processing of the pattern including, of
136 course, a pointer to the list of elements used for building the pattern
137 result.</p><p>Last but not least a number of patterns do not fit in the hash table
138 because they are not associated to a name, this is the case for patterns
139 applying to the root, any element, any attributes, text nodes, pi nodes, keys
140 etc. Those are stored independently in the stylesheet structure as separate
141 linked lists of xsltCompMatch.</p><h3><a name="processing" id="processing">The processing itself</a></h3><p>The processing is defined by the XSLT specification (the basis of the
142 algorithm is explained in <a href="http://www.w3.org/TR/xslt#section-Introduction">the Introduction</a>
143 section). Basically it works by taking the root of the input document and
144 applying the following algorithm:</p><ol><li>Finding the template applying to it. This is a lookup in the template
145 hash table, walking the hash list until the node satisfies all the steps
146 of the pattern, then checking the appropriate(s) global templates to see
147 if there isn't a higher priority rule to apply</li>
148 <li>If there is no template, apply the default rule (recurse on the
150 <li>else walk the content list of the selected templates, for each of them:
151 <ul><li>if the node is in the XSLT namespace then the node has a _private
152 field pointing to the preprocessed values, jump to the specific
154 <li>if the node is in an extension namespace, look up the associated
156 <li>otherwise copy the node.</li>
157 </ul><p>The closure is usually done through the XSLT
158 <strong>apply-templates</strong> construct recursing by applying the
159 adequate template on the input node children or on the result of an
160 associated XPath selection lookup.</p>
162 </ol><p>Note that large parts of the input tree may not be processed by a given
163 stylesheet and that on the opposite some may be processed multiple times.
164 (This often is the case when a Table of Contents is built).</p><p>The module <code>transform.c</code> is the one implementing most of this
165 logic. <strong>xsltApplyStylesheet()</strong> is the entry point, it
166 allocates an xsltTransformContext containing the following:</p><ul><li>a pointer to the stylesheet being processed</li>
167 <li>a stack of templates</li>
168 <li>a stack of variables and parameters</li>
169 <li>an XPath context</li>
170 <li>the template mode</li>
171 <li>current document</li>
172 <li>current input node</li>
173 <li>current selected node list</li>
174 <li>the current insertion points in the output document</li>
175 <li>a couple of hash tables for extension elements and functions</li>
176 </ul><p>Then a new document gets allocated (HTML or XML depending on the type of
177 output), the user parameters and global variables and parameters are
178 evaluated. Then <strong>xsltProcessOneNode()</strong> which implements the
179 1-2-3 algorithm is called on the root element of the input. Step 1/ is
180 implemented by calling <strong>xsltGetTemplate()</strong>, step 2/ is
181 implemented by <strong>xsltDefaultProcessOneNode()</strong> and step 3/ is
182 implemented by <strong>xsltApplyOneTemplate()</strong>.</p><h3><a name="XPath" id="XPath">XPath expression compilation</a></h3><p>The XPath support is actually implemented in the libxml module (where it
183 is reused by the XPointer implementation). XPath is a relatively classic
184 expression language. The only uncommon feature is that it is working on XML
185 trees and hence has specific syntax and types to handle them.</p><p>XPath expressions are compiled using <strong>xmlXPathCompile()</strong>.
186 It will take an expression string in input and generate a structure
187 containing the parsed expression tree, for example the expression:</p><pre>/doc/chapter[title='Introduction']</pre><p>will be compiled as</p><pre>Compiled Expression : 10 elements
189 COLLECT 'child' 'name' 'node' chapter
190 COLLECT 'child' 'name' 'node' doc
195 COLLECT 'child' 'name' 'node' title
197 ELEM Object is a string : Introduction
198 COLLECT 'child' 'name' 'node' title
199 NODE</pre><p>This can be tested using the <code>testXPath</code> command (in the
200 libxml codebase) using the <code>--tree</code> option.</p><p>Again, the KISS approach is used. No optimization is done. This could be
201 an interesting thing to add. <a href="http://www-106.ibm.com/developerworks/library/x-xslt2/?dwzone=x?open&l=132%2ct=gr%2c+p=saxon">Michael
202 Kay describes</a> a lot of possible and interesting optimizations done in
203 Saxon which would be possible at this level. I'm unsure they would provide
204 much gain since the expressions tends to be relatively simple in general and
205 stylesheets are still hand generated. Optimizations at the interpretation
206 sounds likely to be more efficient.</p><h3><a name="XPath1" id="XPath1">XPath interpretation</a></h3><p>The interpreter is implemented by <strong>xmlXPathCompiledEval()</strong>
207 which is the front-end to <strong>xmlXPathCompOpEval()</strong> the function
208 implementing the evaluation of the expression tree. This evaluation follows
209 the KISS approach again. It's recursive and calls
210 <strong>xmlXPathNodeCollectAndTest()</strong> to collect nodes set when
211 evaluating a <code>COLLECT</code> node.</p><p>An evaluation is done within the framework of an XPath context stored in
212 an <strong>xmlXPathContext</strong> structure, in the framework of a
213 transformation the context is maintained within the XSLT context. Its content
214 follows the requirements from the XPath specification:</p><ul><li>the current document</li>
215 <li>the current node</li>
216 <li>a hash table of defined variables (but not used by XSLT)</li>
217 <li>a hash table of defined functions</li>
218 <li>the proximity position (the place of the node in the current node
220 <li>the context size (the size of the current node list)</li>
221 <li>the array of namespace declarations in scope (there also is a namespace
222 hash table but it is not used in the XSLT transformation).</li>
223 </ul><p>For the purpose of XSLT an <strong>extra</strong> pointer has been added
224 allowing to retrieve the XSLT transformation context. When an XPath
225 evaluation is about to be performed, an XPath parser context is allocated
226 containing and XPath object stack (this is actually an XPath evaluation
227 context, this is a remain of the time where there was no separate parsing and
228 evaluation phase in the XPath implementation). Here is an overview of the set
229 of contexts associated to an XPath evaluation within an XSLT
230 transformation:</p><p align="center"><img src="contexts.gif" alt="The set of contexts associated " /></p><p>Clearly this is a bit too complex and confusing and should be refactored
231 at the next set of binary incompatible releases of libxml. For example the
232 xmlXPathCtxt has a lot of unused parts and should probably be merged with
233 xmlXPathParserCtxt.</p><h3><a name="Descriptio" id="Descriptio">Description of XPath Objects</a></h3><p>An XPath expression manipulates XPath objects. XPath defines the default
234 types boolean, numbers, strings and node sets. XSLT adds the result tree
235 fragment type which is basically an unmodifiable node set.</p><p>Implementation-wise, libxml follows again a KISS approach, the
236 xmlXPathObject is a structure containing a type description and the various
237 possibilities. (Using an enum could have gained some bytes.) In the case of
238 node sets (or result tree fragments), it points to a separate xmlNodeSet
239 object which contains the list of pointers to the document nodes:</p><p align="center"><img src="object.gif" alt="An Node set object pointing to " /></p><p>The <a href="http://xmlsoft.org/html/libxml-xpath.html">XPath API</a> (and
240 its <a href="http://xmlsoft.org/html/libxml-xpathinternals.html">'internal'
241 part</a>) includes a number of functions to create, copy, compare, convert or
242 free XPath objects.</p><h3><a name="XPath3" id="XPath3">XPath functions</a></h3><p>All the XPath functions available to the interpreter are registered in the
243 function hash table linked from the XPath context. They all share the same
244 signature:</p><pre>void xmlXPathFunc (xmlXPathParserContextPtr ctxt, int nargs);</pre><p>The first argument is the XPath interpretation context, holding the
245 interpretation stack. The second argument defines the number of objects
246 passed on the stack for the function to consume (last argument is on top of
247 the stack).</p><p>Basically an XPath function does the following:</p><ul><li>check <code>nargs</code> for proper handling of errors or functions
248 with variable numbers of parameters</li>
249 <li>pop the parameters from the stack using <code>obj =
250 valuePop(ctxt);</code></li>
251 <li>do the function specific computation</li>
252 <li>push the result parameter on the stack using <code>valuePush(ctxt,
254 <li>free up the input parameters with
255 <code>xmlXPathFreeObject(obj);</code></li>
257 </ul><p>Sometime the work can be done directly by modifying in-situ the top object
258 on the stack <code>ctxt->value</code>.</p><h3><a name="stack" id="stack">The XSLT variables stack frame</a></h3><p>Not to be confused with XPath object stack, this stack holds the XSLT
259 variables and parameters as they are defined through the recursive calls of
260 call-template, apply-templates and default templates. This is used to define
261 the scope of variables being called.</p><p>This part seems to be the most urgent attention right now, first it is
262 done in a very inefficient way since the location of the variables and
263 parameters within the stylesheet tree is still done at run time (it really
264 should be done statically at compile time), and I am still unsure that my
265 understanding of the template variables and parameter scope is actually
266 right.</p><p>This part of the documentation is still to be written once this part of
267 the code will be stable. <span style="background-color: #FF0000">TODO</span></p><h3><a name="Extension" id="Extension">Extension support</a></h3><p>There is a separate document explaining <a href="extensions.html">how the
268 extension support works</a>.</p><h3><a name="Futher" id="Futher">Further reading</a></h3><p>Michael Kay wrote <a href="http://www-106.ibm.com/developerworks/library/x-xslt2/?dwzone=x?open&l=132%2ct=gr%2c+p=saxon">a
269 really interesting article on Saxon internals</a> and the work he did on
270 performance issues. I wishes I had read it before starting libxslt design (I
271 would probably have avoided a few mistakes and progressed faster). A lot of
272 the ideas in his papers should be implemented or at least tried in
273 libxslt.</p><p>The <a href="http://xmlsoft.org/">libxml documentation</a>, especially <a href="http://xmlsoft.org/xmlio.html">the I/O interfaces</a> and the <a href="http://xmlsoft.org/xmlmem.html">memory management</a>.</p><h3><a name="TODOs" id="TODOs">TODOs</a></h3><p>redesign the XSLT stack frame handling. Far too much work is done at
274 execution time. Similarly for the attribute value templates handling, at
275 least the embedded subexpressions ought to be precompiled.</p><p>Allow output to be saved to a SAX like output (this notion of SAX like API
276 for output should be added directly to libxml).</p><p>Implement and test some of the optimization explained by Michael Kay
277 especially:</p><ul><li>static slot allocation on the stack frame</li>
278 <li>specific boolean interpretation of an XPath expression</li>
279 <li>some of the sorting optimization</li>
280 <li>Lazy evaluation of location path. (this may require more changes but
281 sounds really interesting. XT does this too.)</li>
282 <li>Optimization of an expression tree (This could be done as a completely
283 independent module.)</li>
284 </ul><p></p><p>Error reporting, there is a lot of case where the XSLT specification
285 specify that a given construct is an error are not checked adequately by
286 libxslt. Basically one should do a complete pass on the XSLT spec again and
287 add all tests to the stylesheet compilation. Using the DTD provided in the
288 appendix and making direct checks using the libxml validation API sounds a
289 good idea too (though one should take care of not raising errors for
290 elements/attributes in different namespaces).</p><p>Double check all the places where the stylesheet compiled form might be
291 modified at run time (extra removal of blanks nodes, hint on the
292 xsltCompMatch).</p><p></p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>
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