InMemory.swift

import JPEG
import JPEGSystem

extension System
{
    struct Blob
    {
        private(set)
        var data:[UInt8],
            position:Int
    }
}
extension System.Blob:JPEG.Bytestream.Source, JPEG.Bytestream.Destination
{
    init(_ data:[UInt8])
    {
        self.data       = data
        self.position   = data.startIndex
    }

    mutating
    func read(count:Int) -> [UInt8]?
    {
        guard self.position + count <= data.endIndex
        else
        {
            return nil
        }

        defer
        {
            self.position += count
        }

        return .init(self.data[self.position ..< self.position + count])
    }

    mutating
    func write(_ bytes:[UInt8]) -> Void?
    {
        self.data.append(contentsOf: bytes)
        return ()
    }
}

let path:String         = "examples/in-memory/karlie-2011.jpg"
guard let data:[UInt8]  = (System.File.Source.open(path: path)
{
    (source:inout System.File.Source) -> [UInt8]? in

    guard let count:Int = source.count
    else
    {
        return nil
    }
    return source.read(count: count)
} ?? nil)
else
{
    fatalError("failed to open or read file '\(path)'")
}

var blob:System.Blob = .init(data)
// read from blob
let spectral:JPEG.Data.Spectral<JPEG.Common>    = try .decompress(stream: &blob)
let image:JPEG.Data.Rectangular<JPEG.Common>    = spectral.idct().interleaved()
let rgb:[JPEG.RGB]                              = image.unpack(as: JPEG.RGB.self)
guard let _:Void = (System.File.Destination.open(path: "\(path).rgb")
{
    guard let _:Void = $0.write(rgb.flatMap{ [$0.r, $0.g, $0.b] })
    else
    {
        fatalError("failed to write to file '\(path).rgb'")
    }
})
else
{
    fatalError("failed to open file '\(path).rgb'")
}

// write to blob
blob = .init([])
try spectral.compress(stream: &blob)
guard let _:Void = (System.File.Destination.open(path: "\(path).jpg")
{
    guard let _:Void = $0.write(blob.data)
    else
    {
        fatalError("failed to write to file '\(path).jpg'")
    }
})
else
{
    fatalError("failed to open file '\(path).jpg'")
}