import Foundation

func vanEckSequence(start: [Int], maxLength: Int) -> [Int] {
    let toCompute = maxLength - start.count
    var seq = start + Array(repeating: 0, count: toCompute)
    // print(seq.count)
    
    for i in start.count..<maxLength {
        // print("i: \(i)")
        if let prevIndex = seq[0..<(i - 1)].lastIndex(of: seq[i - 1]) {
            seq[i] = (i - 1) - prevIndex
        } else {
            seq[i] = 0
        }
    }
    
    return seq
}

let seq = vanEckSequence(start: [9,12,1,4,17,0,18], maxLength: 30000000)
// print(seq)
print(seq.last!)