Saxon » SaxonJS

JS unit tests added ('performance' dir).

Various code fixes:

• References to variables always return a copy, to prevent node destruction (re bug https://saxonica.plan.io/issues/2944) - i.e. for 'gVarRef', 'varRef', 'supplied' in Expr.js

• Nodes are labelled with the property _saxonIsTemporary when they are created by Saxon-JS when constructing a temporary tree e.g. in the Context.createElement method (used by DomUtils.shallowCopy; in 'compElem' & 'elem' in Expr.js); when copying text nodes in DomUtils.copyItem, or creating them in 'valueOf' in Expr.js

• Element copying in Expr.makeComplexContent will now only occur if it is really necessary, i.e. when the element is not already a copy, i.e. when the element does not have the property _saxonIsTemporary, e.g. when it is an element in the HTML page or a source document

• Ensure that context.resultDocument is set to window.document for the context supplied to Expr.makeComplexContent when:

  (a) transform option destination is appendToBody|prependToBody|replaceBody, or

  (b) the constructed content of an xsl:result-document will be added to the HTML page (i.e. href is '?.' or begins with #)

Then the constructed content in the relevant document fragment can just be moved across to the target, without the need for further copying, because uses of context.createElement method will have produced HTMLElements within makeComplexContent as required (re issue https://saxonica.plan.io/boards/5/topics/6589).

We found that not only are newly created nodes sometimes being copied unnecessarily, but the code for deep copy of an element takes exponential time (specifically O(2^n) where n is the depth of the subtree). The existing code invokes shallowCopy with the content being a sequence containing copies of the original element's children; the shallowCopy routine then copies these again. So each of the children is copied twice; the grandchildren are therefore copied 4 times, the greatgrandchildren 8 times, and so on.

The previous "fix" to always return copies for references to variables (i.e. for 'gVarRef', 'varRef', 'supplied') has been retracted. This was unnecessary (and slow) in various cases. New mechanisms are used to determine when to make copies of nodes.

Instead of labelling created nodes with _saxonIsTemporary (as in previous fix), we label with _saxonIsLocal, as outlined below.

Firstly, we now do more at compile time (Java code fixes on 9.7 and 9.8 branches). The ExpressionTool.isLocalConstructor() method is added to ask whether a supplied expression is a nested node constructor. This returns true if the child expression creates nodes that will only be used as children of some parent node (meaning that they do not need to be copied). The child expression (i.e. compElem, copy, doc, elem, comment, compAtt, att, namespace, procInst, valueOf) is then labelled with an "l" flag when isLocal() is true.

In Saxon-JS, when this flag is present, the created node is labelled with 'node._saxonIsLocal = true'.

We check for this label during the append method of Expr.makeComplexContent. If child._saxonIsLocal == true, then the child is "attachable", i.e. it may be possible to attach the child directly to the parent without needing a copy. A copy is made if the child is not attachable, or if the parent and child are not of the same type (i.e. both HTMLElements or both XML elements) - this ensures that constructed trees are homogeneous. Once attached to a parent, the child is no longer attachable, so we set child._saxonIsLocal = false.

Also made the fix for Mike's previous point: removed the use of DomUtils.shallowCopy when doing DomUtils.copyItem on an element.