Storing and fetching raster values in F#

In my quest for a fast system to fetch values from rasters at random cell locations I've been experimenting with a few different implementations in F#. More specifically I have now 4 methods for reading raster files. As usual all source code is available online: AsciiToBin.fsx and all comments on it are very welcome.

The first and simplest method I came up with was converting the ASCII raster files with integers to binary files where every integer is converted to its binary equivalent and written to disk. This saves about 50% of disk space compared to the very large ASCII files but also provides a fast and easy to program way of accessing the values. Note that there are cells with no data so these are stored as Int32.MinValue. As you can see the code is rather short.

module SimpleReadWrite = 

    let writeValue (writer:BinaryWriter) (value:int option) =
        match value with
        | Some(v) -> writer.Write(v)
        | None -> writer.Write(Int32.MinValue)

    let writeValues fileName (values:seq<int option>) =
        use writer = new BinaryWriter(File.Open(fileName, FileMode.OpenOrCreate))
        values
        |> Seq.iter (writeValue writer)
            
    let readValue (reader:BinaryReader) cellIndex = 
        // set stream to correct location
        reader.BaseStream.Position <- cellIndex*4L
        match reader.ReadInt32() with
        | Int32.MinValue -> None
        | v -> Some(v)
        
    let readValues fileName indices = 
        use reader = new BinaryReader(File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read))
        // Use list or array to force creation of values (otherwise reader gets disposed before the values are read)
        let values = List.map (readValue reader) (List.ofSeq indices)
        values

The second version I created uses a memory mapped file for reading the values from the same format as before. This is slightly faster (about 2 times) when we want to query lots of values from the same raster. But also 2 times slower when you query for example 10000 times 10 values from different rasters.

module MemoryMappedSimpleRead =

    open System.IO.MemoryMappedFiles

    let readValue (reader:MemoryMappedViewAccessor) offset cellIndex =
        let position = (cellIndex*4L) - offset
        match reader.ReadInt32(position) with
        | Int32.MinValue -> None
        | v -> Some(v)
        
    let readValues fileName indices =
        use mmf = MemoryMappedFile.CreateFromFile(fileName, FileMode.Open)
        let offset = (Seq.min indices ) * 4L
        let last = (Seq.max indices) * 4L
        let length = 4L+last-offset
        use reader = mmf.CreateViewAccessor(offset, length, MemoryMappedFileAccess.Read)
        let values = (List.ofSeq indices) |> List.map (readValue reader offset)
        values

The third version is similar to the simple reader but it fetches multiple bytes at once when two or more indexes are within a certain range. The performance is a bit worse then the simple reader so I'm not going into any further details. But if you want you can check the solution on github and any suggestions on easier ways of grouping the indexes by inter-distance are welcome.

The last version I created is more space efficient in my case. As I work with world oceanic date, about two thirds of my grids don't have any data (land). To avoid storing this data I separately store a file indicating which cells don't have data and skip those cells without data when writing the binary file. The disadvantage is that this makes everything a lot more complex because you have to map your cell indexes to the location of your binary value in your file in a space efficient way. To be able to store the bitmap I created some BitConverter extension methods to convert a boolean array to a byte array and back which I have also posted on fssnip. Then end result has a performance comparable to the one from the simple reader so if disk space is no problem then this solution isn't worth the additional complexity.

module BitConverter = 
    let pow2 y = 1 <<< y
    // convert booleans to bytes in a space efficient way
    let FromBooleans (bools:bool []) =
        seq {
            let b = ref 0uy
            for i=0 to bools.Length-1 do
                let rem = (i  % 8)
                if rem = 0 && i<> 0 then 
                    yield !b
                    b := 0uy
                if bools.[i] then
                    b := !b + (byte (pow2 rem))
            yield !b
        } |> Array.ofSeq
    // to booleans only works for bytes created with FromBooleans
    let ToBooleans (bytes:byte []) = 
        bytes
        |> Array.map (fun b -> Array.init 8 (fun i -> ((pow2 i) &&& int b) > 0))
        |> Array.concat

After lots of tweaking I managed to get a similar performance as the SimpleReadWrite but with 30% less diskspace needed and a more complex codebase.

Some performance related things I learned on the way are:

The Get method from System.Collections.BitArray is slow
You might want to convert lots of Seq chaining to one for loop
Some use of mutable values (within a function) might be necessary
Precompute as much as you can

And as I've tweeted before, I really like to evaluate my code in the REPL (same for my Python and R work).

Any thought on how to make this faster ? Do you know a fast library/system that achieve the same results ? Should I compress my rasters more to decrease disk access ? Any other suggestions ?

Other posts you might like:

Samuel Bosch

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Storing and fetching raster values in F#

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