Levenshtein distance between two String for Swift 4.x

Levenshtein.swift

import Foundation

extension String {
    subscript(index: Int) -> Character {
        return self[self.index(self.startIndex, offsetBy: index)]
    }
}

extension String {
    public func levenshtein(_ other: String) -> Int {
        let sCount = self.count
        let oCount = other.count

        guard sCount != 0 else {
            return oCount
        }

        guard oCount != 0 else {
            return sCount
        }

        let line : [Int]  = Array(repeating: 0, count: oCount + 1)
        var mat : [[Int]] = Array(repeating: line, count: sCount + 1)

        for i in 0...sCount {
            mat[i][0] = i
        }

        for j in 0...oCount {
            mat[0][j] = j
        }

        for j in 1...oCount {
            for i in 1...sCount {
                if self[i - 1] == other[j - 1] {
                    mat[i][j] = mat[i - 1][j - 1]       // no operation
                }
                else {
                    let del = mat[i - 1][j] + 1         // deletion
                    let ins = mat[i][j - 1] + 1         // insertion
                    let sub = mat[i - 1][j - 1] + 1     // substitution
                    mat[i][j] = min(min(del, ins), sub)
                }
            }
        }

        return mat[sCount][oCount]
    }
}

// Access older versions (Swift 3.x) in history
// Usage:
// "abc".levenshtein("abd")