jeff: > 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 _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
