Data to Buffers - ios

Question

If you had a file stored as data which is much larger than your buffer and you wanted to iterate over this data in pieces of a buffer size, what is a good way to do this? If you could provide some context, that would be great.
I was thinking,
let bufferSize: Int = 20000
let myData: Data = Data(..)
var buffer: ??? = ???
var theOffset: ??? = ???
func runWhenReady() {
buffer = &myData
let amount = sendingData(&buffer[bufferOffset] maxLength: bufferSize - bufferOffset)
bufferOffset += amount
}
// pseudocode
// from foundation, but changed a bit (taken from Obj-C foundations just for types)
// writes the bytes from the specified buffer to the stream up to len bytes. Returns the number of bytes actually written.
func sendingData(_ buffer: const uint8_t *, maxLength len: NSUInteger) -> Int {
...
}

Answer 1

If you want to iterate you need a loop.
This is an example to slice the data in chunks of bufferSize with stride.
let bufferSize = 20000
var buffer = [UInt8]()
let myData = Data(..)
let dataCount = myData.count
for currentIndex in stride(from: 0, to: dataCount, by: bufferSize) {
let length = min(bufferSize, dataCount - currentIndex) // ensures that the last chunk is the remainder of the data
let endIndex = myData.index(currentIndex, offsetBy: length)
buffer = [UInt8](myData[currentIndex..<endIndex])
// do something with buffer
}

Answer 2

Open the file
let fileUrl: URL = ...
let fileHandle = try! FileHandle(forReadingFrom: fileUrl)
defer {
fileHandle.closeFile()
}
Create buffer:
let bufferSize = 20_000
let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: bufferSize)
defer {
buffer.deallocate()
}
Read until end of file is reached:
while true {
let bytesRead = read(fileHandle.fileDescriptor, buffer, bufferSize)
if bytesRead < 0 {
// handle error
break
}
if bytesRead == 0 {
// end of file
break
}
// do something with data in buffer
}