Levenshtein distance in swift 4

Levenshtein.swift

class Levenshtein {
    private(set) var cache = [Set<String.SubSequence>: Int]()
    
    func calculateDistance(a: String.SubSequence, b: String.SubSequence) -> Int {
        let key = Set([a, b])
        if let distance = cache[key] {
            return distance
        } else {
            let distance: Int = {
                if a.count == 0 || b.count == 0 {
                    return abs(a.count - b.count)
                } else if a.first == b.first {
                    return calculateDistance(a: a[a.index(after: a.startIndex)...], b: b[b.index(after: b.startIndex)...])
                } else {
                    let add = calculateDistance(a: a, b: b[b.index(after: b.startIndex)...])
                    let replace = calculateDistance(a: a[a.index(after: a.startIndex)...], b: b[b.index(after: b.startIndex)...])
                    let delete = calculateDistance(a: a[a.index(after: a.startIndex)...], b: b)
                    return min(add, replace, delete) + 1
                }
            }()
            cache[key] = distance
            return distance
        }
    }
}