Iterating through a directory with Ant - ant

Question

Let's say I have a collection of PDF files with the following paths:
/some/path/pdfs/birds/duck.pdf
/some/path/pdfs/birds/goose.pdf
/some/path/pdfs/insects/fly.pdf
/some/path/pdfs/insects/mosquito.pdf
What I'd like to do is generate thumbnails for each PDF that respect the relative path structure, and output to another location, i.e.:
/another/path/thumbnails/birds/duck.png
/another/path/thumbnails/birds/goose.png
/another/path/thumbnails/insects/fly.png
/another/path/thumbnails/insects/mosquito.png
I'd like this to be done in Ant. Assume I'm going to use Ghostscript on the command line and I've already worked out the call to GS:
<exec executable="${ghostscript.executable.name}">
<arg value="-q"/>
<arg value="-r72"/>
<arg value="-sDEVICE=png16m"/>
<arg value="-sOutputFile=${thumbnail.image.path}"/>
<arg value="${input.pdf.path}"/>
</exec>
So what I need to do is work out the correct values for ${thumbnail.image.path} and ${input.pdf.path} while traversing the PDF input directory.
I have access to ant-contrib (just installed the "latest", which is 1.0b3) and I'm using Ant 1.8.0. I think I can make something work using the <for> task, <fileset>s and <mapper>s, but I am having trouble putting it all together.
I tried something like:
<for param="file">
<path>
<fileset dir="${some.dir.path}/pdfs">
<include name="**/*.pdf"/>
</fileset>
</path>
<sequential>
<echo message="#{file}"/>
</sequential>
</for>
But unfortunately the #{file} property is an absolute path, and I can't find any simple way of decomposing it into the relative components.
If I can only do this using a custom task, I guess I could write one, but I'm hoping I can just plug together existing components.

Answer 1

In the sequential task you could may be able to use the ant-contrib propertyregex task to map the input paths to output. For example:
<propertyregex override="yes" property="outfile" input="#{file}"
regexp="/some/path/pdfs/(.*).pdf"
replace="/another/path/\1.png" />
Which maps, for example /some/path/pdfs/birds/duck.pdf to /another/path/birds/duck.png.

Answer 2

For the sake of completeness, here's what I came up with for a target based on martin clayton's answer. It only works on Windows for now but that's because I haven't gotten around to installing Ghostscript in a non-proxy way yet on Mac OS X. Note that to be a cross-platform solution I had to "scrub" the file separators to be consistently forward-slash only.
<target name="make-thumbnails" depends="">
<taskdef resource="net/sf/antcontrib/antlib.xml">
<classpath>
<pathelement location="/path/to/ant-contrib-1.0b3.jar"/>
</classpath>
</taskdef>
<condition property="ghostscript.executable.name" value="/path/to/gswin32c.exe">
<os family="windows"/>
</condition>
<condition property="ghostscript.executable.name" value="">
<os family="mac"/>
</condition>
<for param="file">
<path>
<fileset dir="/path/to/pdfs">
<include name="**/*.pdf"/>
</fileset>
</path>
<sequential>
<propertyregex override="yes" property="file-scrubbed" input="#{file}"
regexp="\\"
replace="/" />
<propertyregex override="yes" property="output-path-directory-fragment" input="${file-scrubbed}"
regexp=".*/pdfs/(.*)/.+\.pdf"
replace="\1" />
<propertyregex override="yes" property="output-path-file-fragment" input="${file-scrubbed}"
regexp=".*/pdfs.*/(.+)\.pdf"
replace="\1.png" />
<mkdir dir="${thumbnails.base.dir}/${output-path-directory-fragment}"/>
<exec executable="${ghostscript.executable.name}">
<arg value="-q"/>
<arg value="-dLastPage=1"/>
<arg value="-dNOPAUSE"/>
<arg value="-dBATCH"/>
<arg value="-dSAFER"/>
<arg value="-r72"/>
<arg value="-sDEVICE=png16m"/>
<arg value="-sOutputFile=${thumbnails.base.dir}/${output-path-directory-fragment}/${output-path-file-fragment}"/>
<arg value="${file-scrubbed}"/>
</exec>
</sequential>
</for>
</target>