CBOR decoding crashes when Data startIndex is non-zero

[Jerry-Carter]

Data operations often do not copy the underlying bytes but simple adjust the startIndex and endIndex. The CBORDecoder expects a startIndex of zero. The workaround is easy, but should be unnecessary.

let cborPlusTwoHeaderBytes = Data("F00DA3010203262001".hex)  // two extra bytes before the CBOR
let cborData = cborPlusTwoHeaderBytes[2...]  // trim off the two extra bytes; this fails
let cborSimple = Data(cborData)  // copy operation resets startIndex; this works

Here is a simple test case.

// swift-tools-version: 6.0
import PackageDescription

let package = Package(
    name: "FidoTest",
    platforms: [.macOS(.v15)],
    products: [ .executable(name: "SimpleTest", targets: ["Exercise1"]) ],
    dependencies: [ .package(url: "https://github.com/outfoxx/PotentCodables.git", from: "3.5.0") ],
    targets: [
        .executableTarget(name: "Exercise1", dependencies: [ .product(name: "PotentCodables", package: "PotentCodables")]),
    ]
)

import Foundation
import PotentCBOR

extension String {
    var hex: some Sequence<UInt8> {
        self[...].hex
    }

    var hexData: Data {
        return Data(hex)
    }
}

extension Substring {
    var hex: some Sequence<UInt8> {
        sequence(state: self, next: { remainder in
            guard remainder.count >= 2 else { return nil }
            let nextTwo = remainder.prefix(2)
            remainder.removeFirst(2)
            return UInt8(nextTwo, radix: 16)
        })
    }
}

let cborPlusTwoHeaderBytes = Data("F00DA3010203262001".hex)
let cborData = cborPlusTwoHeaderBytes[2...]

struct DecodedCBOR: Decodable {
    let x:Int
    let y:Int
    let ref:Int
    
    enum CodingKeys: String, CodingKey {
        case x = "1", y = "3", ref = "-1"
    }
}

let coordinates:DecodedCBOR
do {
    coordinates = try CBORDecoder().decode(DecodedCBOR.self, from: cborData)
} catch {
    print("EXCEPTION: \(error.localizedDescription)")
    throw error
}