German TIN validation

GermanTin.swift

func isGermanTin(tin: String) -> Bool {
    let endIndex = tin.index(tin.startIndex, offsetBy: 10)
    let tinNumbers = tin[..<endIndex].compactMap { Int(String($0)) }
    // First number can't be 0
    guard tinNumbers.first != 0 else { return false }
    // There should be exactly 8 or 9 unique numbers and exactly one that has 2 occurrances
    let occurances = tinNumbers.reduce(into: [:]) { $0[$1, default: 0] += 1 }
    guard 8...9 ~= occurances.keys.count else { return false }
    let occurranceCount = occurances.keys.count == 9 ? 2 : 3
    guard occurances.values.contains(where: { $0 == occurranceCount }) else { return false }

    // Checksum should match the 11th character
    var sum = 0
    var product = 10
    for num in tinNumbers {
        sum = (num + product) % 10
        if sum == 0 { sum = 10 }
        product = (sum * 2) % 11
    }
    var check = 11 - product
    if check == 10 { check = 0 }
    guard let originalCheckNum = Int(String(tin[endIndex])) else { return false }
    return check == originalCheckNum
}