code.swift

import Foundation

func test(count: Int) -> UInt8 {
    precondition(count >= 1)

    let ptr = UnsafeMutableRawPointer.allocate(byteCount: count, alignment: 1)
    ptr.storeBytes(of: 0, toByteOffset: 0, as: UInt8.self)

    let data = Data(bytesNoCopy: ptr, count: count, deallocator: .none)
    ptr.storeBytes(of: 13, toByteOffset: 0, as: UInt8.self)
    return data[0]
}

for i in 1...24 {
    print("\(i): \(test(count: i))")
}

output.txt

1: 0
2: 0
3: 0
4: 0
5: 0
6: 0
7: 0
8: 0
9: 0
10: 0
11: 0
12: 0
13: 0
14: 0
15: 13
16: 13
17: 13
18: 13
19: 13
20: 13
21: 13
22: 13
23: 13
24: 13