bytetwerking.swift

func toInt24(value: Int) -> NSData {
    var result = [UInt8]()
    result.append(UInt8(value >> 16))
    result.append(UInt8(value >> 8))
    result.append(UInt8(value))
    return NSData(bytes: result, length: result.count)
}

func int24FromData(data: NSData, inout range: NSRange) -> Int {
    let offset = sizeof(UInt8)
    var read = -1
    data.getBytes(&read, range: range)
    var result = (read & 0xff) << 16
    range.location += offset
    data.getBytes(&read, range: range)
    result |= (read & 0xff) << 8
    range.location += offset
    data.getBytes(&read, range: range)
    result |= read & 0xff
    range.location += offset
    return result
}

let data = NSMutableData(data: toInt24(100))
data.appendData(toInt24(50))

var range = NSRange(location: 0, length: sizeof(UInt8))

let first = int24FromData(data, range: &range)
let second = int24FromData(data, range: &range)

Didn't think about it much, but maybe it will give you some insight to start

import Foundation

func toInt24(value: Int) -> NSData {
    var result = [UInt8]()
    result.append(UInt8(value >> 16))
    result.append(UInt8(value >> 8))
    result.append(UInt8(value))
    return NSData(bytes: result, length: result.count)
}

func int24FromData(data: NSData) -> [Int] {
    let dataLength = data.length
    let biteSize = sizeof(UInt8)
    let bitesCount = dataLength / biteSize

    var numbers = [UInt8](count: bitesCount, repeatedValue: 0)

    data.getBytes(&numbers, length: dataLength)

    return numbers.enumerate().filter({ idx, el in (idx + 1) % 3 == 0}).map { Int($1) }
}

let data = NSMutableData(data: toInt24(100))
data.appendData(toInt24(50))
data.appendData(toInt24(201))
data.appendData(toInt24(202))
data.appendData(toInt24(203))
data.appendData(toInt24(204))

print(int24FromData(data))
// return: [100, 50, 201, 202, 203, 204]

It is much appreciated 🙇 The stream comes with mixed types (24 bit ones, 32 bit ones as well as strings) in it. But could extract sub ranges for expected areas and work on those in the simplified fashion