Swift code to calculate the Jaro-Winkler edit distance between two strings

JaroWinkler.swift

/// The Jaro-Winkler edit distance between two strings (0 - 1)
func editDistance(_ lhs: String, _ rhs: String) -> Double {
    return 1 - jaroWinklerSimilarity(Array(lhs), Array(rhs))
}

/// Jaro-Winkler similarity between two strings (0 - 1)
/// https://www.geeksforgeeks.org/jaro-and-jaro-winkler-similarity/
private func jaroWinklerSimilarity(_ s1: [Character], _ s2: [Character]) -> Double {
    var jaro = jaroSimilarity(s1, s2)

    // If the jaro Similarity is above a threshold
    if jaro > 0.7 {
        // Find the length of common prefix
        var prefix = 0.0

        for i in 0 ..< min(s1.count, s2.count) {
            // If the characters match
            if s1[i] == s2[i] {
                prefix += 1
            } else {
                break
            }
        }

        // Maximum of 4 characters are allowed in prefix
        prefix = min(4, prefix)

        // Calculate jaro winkler Similarity
        jaro += 0.1 * prefix * (1 - jaro)
    }
    return jaro
}

/// The Jaro similarity between two strings (0 - 1)
/// https://www.geeksforgeeks.org/jaro-and-jaro-winkler-similarity/
private func jaroSimilarity(_ s1: [Character], _ s2: [Character]) -> Double {
    // If the Strings are equal
    if s1 == s2 {
        return 1
    }

    // Lengths
    let len1 = s1.count
    let len2 = s2.count

    // Maximum distance up to which matching is allowed
    let maxDist = Int(Double(max(len1, len2) / 2) - 1)

    // Number of matches
    var matches = 0.0

    // Hash for matches
    var hash1 = [Int](repeating: 0, count: len1)
    var hash2 = [Int](repeating: 0, count: len2)

    // Traverse through the first String
    for i in 0 ..< len1 {
        // Check for matches
        let start = max(0, i - maxDist)
        for j in start ..< max(start, min(len2, i + maxDist + 1)) {
            // If there is a match
            if s1[i] == s2[j], hash2[j] == 0 {
                hash1[i] = 1
                hash2[j] = 1
                matches += 1
                break
            }
        }
    }

    // If there is no match
    if matches == 0 {
        return 0
    }

    // Number of transpositions
    var t = 0.0

    // Count number of occurances where two characters match but
    // there is a third matched character in between the indices
    var j = 0
    for i in 0 ..< len1 where hash1[i] == 1 {
        // Find the next matched character
        // in second String
        while hash2[j] == 0 {
            j += 1
        }

        if s1[i] != s2[j] {
            j += 1
            t += 1
        }
    }

    // Return the Jaro Similarity
    return (matches / Double(len1) + matches / Double(len2) + (matches - t / 2) / matches) / 3
}