[Haskell-cafe] Reading/writing packed bytes from file

Donald Bruce Stewart dons at cse.unsw.edu.au
Tue Jun 19 22:02:38 EDT 2007

> I've read the documentation for some of the marshalling packages out
> there for Haskell, and I'm left confused as to which one I should be
> using and how to actually do what I want to do.   I have a file, a
> little over 2gb, of packed data in the format
> (recordcount) records of:
> 4-byte int (count),
> (count) 2-byte unsigned shorts,
> (count) 4-byte floats
> all in little-endian order.  What I want to do is read each record
> (lazily), and unpack it into Data.IntMap.IntMap Float where the unsigned
> shorts become the keys and the 4-byte floats become the values.
> Then I want to do a lot of interesting processing which we'll skip here,
> and write back out packed data to a file in the format of
> 4-byte float,
> 4-byte float,
> 4-byte float
> for each record. I need these output records to be four-byte C floats.
> I've gotten as far as datatypes and a couple of signatures, but I can't
> figure out the functions themselves that go with the signatures, and
> then again, maybe I have the signatures wrong.  
> -- 
> import qualified Data.IntMap as M
> import qualified Data.ByteString.Lazy.Char8 as B
> data InputRecord = M.IntMap Float
> data OutputRecord = (Float, Float, Float)
> -- open a file as a lazy ByteString and break up the individual records
> -- by reading the count variable, reading that many bytes times 
> -- sizeof short + sizeof float into a lazy ByteString.
> readRawRecordsFromFile :: String -> IO [B.ByteString] 
> -- take a bytestring as returned by readRawRecordsFromFile and turn it
> -- into a map.
> decodeRawRecord :: B.ByteString -> M.IntMap Float
> --
> Can anyone help with how to construct these functions?  I'm going to
> have to make a few passes over this file, so I'd like the IO to be as
> fast as Haskelly possible.
> -- Jeff

Data.ByteString.Lazy.Char8.readFile should suffice for the IO.
then use drop/take to split up the file in pieces if you know the length
of each field.

For converting ByteString chunks to Floats, I'd probably call C for that.

-- Don

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