Subject: Re: [xsl] Beware the count method with Muenchian grouping (was: Testing by counting or positional predicate) From: "Daniel Bowen" <dbowen2@xxxxxxxxx> Date: Fri, 12 Jan 2001 10:32:29 -0700 |
Francis Norton wrote: > Daniel Bowen wrote: > > > > Just for an opposing view point on this, on one stylesheet that I use > > Muenchian grouping, I have a filter on the key "match". When I use the > > "count" method, I have to use the filter again when selecting the nodes, but > > when I use the "generate-id" method, I don't have to use the filter again. > > In my case, the "generate-id" method is 2-3 times faster! > > > > Example: > > > > <xsl:key > > name="key-texture" > > match="Texture[0=count(preceding-sibling::Texture[1])]" > > use="concat(@texture, ':', @u, ':', @v)" /> > > <xsl:variable > > name="primary-textures" > > select="//Texture[generate-id(.) = generate-id(key('key-texture', > > concat(@texture, ':', @u, ':', @v))[1])]" /> > > > > is faster for me than: > > > > <xsl:key > > name="key-texture" > > match="Texture[0=count(preceding-sibling::Texture[1])]" > > use="concat(@texture, ':', @u, ':', @v)" /> > > <xsl:variable > > name="primary-textures" > > > > select="//Texture[0=count(preceding-sibling::Texture[1])][count(.|key('key-t > > exture', concat(@texture, ':', @u, ':', @v))[1])]" /> > > > > Did you try > > <xsl:variable > name="primary-textures" > select="//Texture[count(. | (key('key-texture', concat(@texture, ':', > @u, ':', @v))[1]) = 1]" > /> > > which seems to me to be the equivalent to your generate-id()version? At first, I thought it was too. However I learned an interesting leason when trying this way. Consider the XML: <Areals> <Areal ... > <Texture texture="farm.rgb" u="1" v="1" /> <Texture texture="secondary.rgb" u="2" v="2" /> </Areal> <Areal ... > <Texture texture="farm.rgb" u="1" v="1" /> <Texture texture="secondary.rgb" u="2" v="2" /> </Areal> <Areal ... > <Texture texture="farm.rgb" u="3" v="3" /> <Texture texture="secondary2.rgb" u="4" v="4" /> </Areal> <Areal ... > <Texture texture="farm.rgb" u="1" v="1" /> <!-- No secondary texture --> </Areal> <Areal ... > <Texture texture="lake.rgb" u="5" v="5" /> </Areal> </Areals> The "distinct primary textures" are: <Texture texture="farm.rgb" u="1" v="1" /> <Texture texture="farm.rgb" u="3" v="3" /> <Texture texture="lake.rgb" u="5" v="5" /> which is what the generate-id way will result in, and is what is desired in this case. Because of of the filter in the match: [0=count(preceding-sibling::Texture[1])] "key-texture" will not have keys for "secondary" textures. If we just did: <xsl:variable name="primary-textures-using-count-method" select="//Texture[count(. | (key('key-texture', concat(@texture, ':', @u, ':', @v))[1]) = 1]" /> The when the filter is applied to the "secondary texture" nodes (nodes excluded by the filter in the key's match), the union of "." and "(key('key-texture', concat(@texture, ':', @u, ':', @v))[1])" is 1! This is because no key was built for the secondary texture nodes, so the "." gives the node set with the secondary texture, and the "key..." gives an empty node set. So $primary-textures-using-count-method would have: <Texture texture="farm.rgb" u="1" v="1" ... /> <Texture texture="secondary.rgb" u="2" v="2" /> <Texture texture="secondary.rgb" u="2" v="2" /> <Texture texture="farm.rgb" u="3" v="3" /> <Texture texture="secondary2.rgb" u="4" v="4" /> <Texture texture="lake.rgb" u="5" v="5" /> So that's why you need the same filter as used in the "match" on the key to be there if you do it the count way: <xsl:variable name="primary-textures" select="//Texture [0=count(preceding-sibling::Texture[1])] [1=count(.|key('key-texture', concat(@texture, ':', @u, ':', @v))[1])]" /> Without the filter used in the "match" on the key, the count method will give you the nodes that are part of a key (the first one of each unique combination), plus all the other nodes matched by the XPath that aren't part of the key. -Daniel XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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