A version string comparator that adheres to the SemVer standard (https://semver.org)

SemanticVersionComparator.kt

class SemanticVersionComparator: Comparator<String> {

    private val nonDigits: Regex = Regex("[^\\d.]")
    private val onPeriods: Regex = Regex("\\.")

    override fun compare(currentVersion: String, otherVersion: String): Int {

        // remove everything after the "+" character, sanitize non-digits, then split on the periods
        val semVerCurrent: MutableList<String> = currentVersion
                .substringBefore("+")
                .replace(nonDigits, "")
                .split(onPeriods)
                .toMutableList()

        val semVerOther: MutableList<String> = otherVersion
                .substringBefore("+")
                .replace(nonDigits, "")
                .split(onPeriods)
                .toMutableList()

        // version numbers not in the form of MAJOR.MINOR.PATCH violate the SemVer standard, so
        // add padding zeros to account for legacy conditions (ex: 6.23 -> 6.23.0)
        for (i in semVerCurrent.size until 3) {
            semVerCurrent.add("0")
        }
        for (i in semVerOther.size until 3) {
            semVerOther.add("0")
        }

        // make a non-mutable defensive copy of the first 3 digits from the list
        val current: List<String> = semVerCurrent.subList(0, 3).toList()
        val other: List<String> = semVerOther.subList(0, 3).toList()

        // iterate through the semantic version digits,
        // these lists should be the same length, but use 'minOf' for safety
        for(i in 0 until minOf(current.size, other.size)) {
            val currentDigit = current[i].toInt()
            val otherDigit = other[i].toInt()
            // if the two digits aren't equal, compare them and return the comparison result
            if(currentDigit != otherDigit) {
                return currentDigit.compareTo(otherDigit)
            }
        }

        // the two versions are equal
        return 0
    }

}