How to make reading an array from disk more efficient

[andrew cooke]

Hi,

I have some code (http://www.acooke.org/andrew/ReadTest.hs) that reads
data from a file (an image in ppm format; example data (256*256 pixels) at
http://www.acooke.org/andrew/test.ppm) and stores it in an array of Word8
values.  The aim is to read a file that contains 5000 * 5000 * 3 Word8
values.  I think this should take under 100Mb of memory, if the Array
implementation is efficient.  However, when I run the code on a file of
that size it looks like it will need several days to complete.  This seems
rather slow - the GIMP can read the same file maybe 30 seconds).

How do I make the Haskell code faster while keeping it flexible?  My
requirements are:

- machine limited to 1Gb memory
- display "status bar"
- the possibility to filter the pixel stream so that the image is
subsampled (see "everyN" in the code)
- the possibility to filter the pixel strean so that a subsection of the
image is selected.

All that is possible in the code I have now, but it's slow.

Thanks,
Andrew

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