PNG_dimension_test.fsx

#r "nuget: ply"
open System
open FSharp.Control.Tasks
open System.Threading.Tasks

let bigEndianUInt32 (data : Span<byte>) =
    let mutable total = 0u
    if data.Length = 4 then
        for i = 0 to 3 do
            let value = data.[i]
            let shiftNum = 3 - i
            total <- total +
                ((uint (value / 16uy)) <<< (shiftNum * 8 + 4)) +
                ((uint (value % 16uy)) <<< (shiftNum * 8))
        Some total
    else
        None

let dataArray = [|
    0x89uy; 0x50uy; 0x4euy; 0x47uy; 0x0duy; 0x0auy; 0x1auy; 0x0auy;
    0x00uy; 0x00uy; 0x00uy; 0x0duy; 0x49uy; 0x48uy; 0x44uy; 0x52uy;
    0x00uy; 0x00uy; 0x02uy; 0x00uy; 0x00uy; 0x00uy; 0x02uy; 0x00uy;
    0x08uy; 0x06uy; 0x00uy; 0x00uy; 0x00uy; 0xf4uy; 0x78uy; 0xd4uy;
    0xfauy
|]

let sign = 
    use data = new IO.MemoryStream(dataArray)
    task {
        let c = Threading.CancellationToken()
        let signMemory = Memory<byte>(Array.zeroCreate 8)
        let chunkLength = Memory<byte>(Array.zeroCreate 4)
        let chunkType = Memory<byte>(Array.zeroCreate 4)
        let! _ = data.ReadAsync(signMemory, c)
        let mutable position = ValueTask<int>(0)
        if (signMemory.ToArray() = 
                [|137uy; 80uy; 78uy; 71uy; 13uy; 10uy; 26uy; 10uy|]) then

            // Which one?
            let! _ = data.ReadAsync(chunkLength, c)
            do position <- data.ReadAsync(chunkType, c)
            
            let chunkLength = chunkLength.Span
            let ihdrLength = bigEndianUInt32 chunkLength
            match ihdrLength, chunkType.ToArray() with
            | Some 13u, [| 73uy; 72uy; 68uy; 82uy|] ->
                let chunkData = Memory<byte>(Array.zeroCreate (int 13))
                let! ihdrData = data.ReadAsync(chunkData, c)
                let ihdrSpan =
                    chunkData.Span
                let width = bigEndianUInt32 (ihdrSpan.Slice(0,4))
                let height = bigEndianUInt32 (ihdrSpan.Slice(4,4))
                return width, height
            | _ ->
                return None, None
        else
            return None, None
    }
    |> Async.AwaitTask
    |> Async.RunSynchronously