https://stackoverflow.com/a/56070306/1271826

Not32BitAligned.swift

class WithUnsafeBytesDemo {
    func unalignedExample() {
        // deliberately throw in extra byte at the start
        let originalData = Data([0xff, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x04])
        print("original base address:", originalData.withUnsafeBytes { $0.baseAddress } ?? "Unknown address")
        print("original start index: ", originalData.startIndex) // is 0
        print("original binary data: ", originalData as NSData)
        print("")

        // get the data after that extra byte
        let index = originalData.startIndex.advanced(by: 1)
        let data = originalData[index...]
        print("subdata base address: ", data.withUnsafeBytes { $0.baseAddress } ?? "Unknown address")
        print("subdata start index:  ", data.startIndex) // is 1
        print("subdata binary data:  ", data as NSData)

        print("result (should be 7): ", extractPacketSizeFromIV(iv: data)!)
    }

    func alignedExample() {
        let data = [Int32(1).bigEndian, Int32(1<<1).bigEndian, Int32(1<<2).bigEndian]
            .withUnsafeBytes { Data($0) }

        print("aligned binary data:  ", data as NSData)
        print("result (should be 7): ", extractPacketSizeFromIV(iv: data)!)
        print("")
    }

    func extractPacketSizeFromIV(iv: Data) -> Int32? {
        return iv.withUnsafeBytes { rawBuffer -> Int32 in
            let buffer = rawBuffer.bindMemory(to: Int32.self)
            let r0 = buffer[0].bigEndian
            let r1 = buffer[1].bigEndian
            let r2 = buffer[2].bigEndian

            return r2 ^ r1 ^ r0
        }
    }
}