- From: John Boyer <boyerj@ca.ibm.com>
- Date: Wed, 28 Sep 2011 14:51:18 -0700
- To: public-forms@w3.org
- Message-ID: <OF7447F17C.1E810108-ON88257919.00664824-88257919.00780DE7@ca.ibm.com>

Dear XForms working group, In the past, it has been claimed that the 'iterate' attribute on an XForms action is desirable because it is simpler and easier to understand than using 'while' on for-each style iteration. This is true, but it is not the full extent of its value to XForms actions. XPath predicates are able to look and behave like array indexes, but they are not as performant. If there are N elements e, then E[1] is an O(N) expression. As a result, a simple forward iteration through a nodeset using 'while' is an O(N^2) operation, whereas it is only O(N) using iterate. This is helpful, but it quickly becomes pointless when trying even a relatively trivial mutation algorithm, like a simple selection sort (or the far less efficient and much reviled bubble sort). The insert action has an important special behavior if you provide a context attribute and no nodeset (or bind) attribute. It will insert the origin node as the first child of the context node. This behavior was deliberately designed to maximize the likelihood that an implementation could perform the insertion in constant time. RECOMMENDATION: Add a note indicating that implementations SHOULD achieve O(1) performance on this operation. Given this recommendation, it is possible to sort in O(N^2) time rather than a dismal O(N^3) time. Currently, the delete action can directly delete an indicated node, but there is no way to indicate the first child of a node in constant time. Also, there is no way to efficiently indicate in constant time the first child node in order to use it as the origin of an insert or to simply obtain a value from it. A stack push operation correspond to the insert action above, and a stack pop operation requires an insert/delete to move the first element to a new location outside the stack, where it can be efficiently queried. RECOMMENDATION: Add a function with signature along the lines of "nodeset first-child(nodeset?, boolean?)" that returns the first child node of a given node, or the context node if no first parameter. The function SHOULD achieve O(1) on the operation. By default ignore whitespace text nodes, but if second parameter given and true, then include them in result. With this function implemented efficiently (and the efficient insert above), N stack operations will take O(N) processing time, and without it, the result is O(N^2). Efficiently implementing queue-based algorithms requires the ability to push on one side of a list and pop from the other. Given the ability described above to efficiently insert at the beginning of the list, it seems we only need a way to efficiently identify the last child of a node in constant time. RECOMMENDATION: Add a function with signature along the lines of "nodeset last-child(nodeset?, boolean?)" that returns the last child node of a given node, or the context node if no first parameter. The function SHOULD achieve O(1) on the operation. By default ignore whitespace text nodes, but if second parameter given and true, then include them in result. Incidentally, the last-child() function also allows efficient insertion at the end of a list of children of a node by using it in the insert nodeset. This is helpful in creating an efficient sort that constructs its result in least to greatest order. The efficient first-child() and last-child() functions also appear to enable optimal O(N log N) sorting via XForms action loops, when used in combination with the iterate attribute. Without having at least first-child(), the best sort performance appears to be O(N log N log log N). A number of more useful data structures become possible to implement if it is also possible to obtain the parent, next sibling, and previous sibling of a node in constant time. These functions would prove convenient in implementing some sorting methods in the most natural ways, but efficient versions of several comparison-based data structures (e.g. various priority queues, trees and graphs) appear to become possible as well. RECOMMENDATION: Add functions with signatures along the lines of "nodeset parent(nodeset?)" returns the parent of a given node, or the context node if no first parameter. "nodeset next-sibling(nodeset?, boolean?)" returns the next sibling of a given node, or the context node if no first parameter. "nodeset prev-sibling(nodeset?, boolean?)" returns the previous sibling of a given node, or the context node if no first parameter. These functions SHOULD achieve O(1) on the operation. For latter two functions, ignore whitespace text nodes by default, but if second parameter given and true, then include them in result. For all of the functions, clearly if the first, last, next, previous or parent node doesn't exist, then an empty nodeset is returned. Finally, the eval() function is already on the list for XForms 2.0 with the use case from Michael Sperberg-McQueen of being able to use a repeat to drill into and edit arbitrarily structured XML. A second use case for the function pertains to algorithms like sorting as well as comparison-based data structure management. Given an eval() function, it would be possible to allow a form author to provide the comparison expression as a string so that the sort or comparison data structure could compare strings, numbers, dates or even secondary keys in case of primary key equality. This would help separate the algorithm or data structure from the application details. RECOMMENDATION: Add function with signature along lines of "object eval(string, nodeset?, number?)" that evaluates an xpath given in the string using the evaluation context of the eval() function, or in the context of a nodeset and a node at a given position, if those params are specified. These functions look very easy to implement at the optimal level, and add a lot of power to XForms actions, so I'm looking at them as extension functions for now, and I'd be happy to do the spec work for XForms 2.0 if these functions are acceptable to the group. Thanks, John M. Boyer, Ph.D. Distinguished Engineer, IBM Forms and Smarter Web Applications IBM Canada Software Lab, Victoria E-Mail: boyerj@ca.ibm.com Blog: http://www.ibm.com/developerworks/blogs/page/JohnBoyer Blog RSS feed: http://www.ibm.com/developerworks/blogs/rss/JohnBoyer?flavor=rssdw

Received on Wednesday, 28 September 2011 21:51:52 UTC