zip with count

object Hamming {

    fun compute(leftStrand: String, rightStrand: String) =
        if (leftStrand.length != rightStrand.length)
            throw IllegalArgumentException("left and right strands must be of equal length")
        else
            (leftStrand zip rightStrand).count { it.first != it.second }
}

An object declaration is used to define Hamming as essentially a singleton object instantiation of the class. This is sufficient, since there is no object state that needs to change with each call of the compute method.

The compute method is implemented by an if/else expression. Although the function has multiple lines, it consists only of the one if/else expression, so it is defined using single-expression function syntax, with the curly braces omitted from the function call and the return type inferred.

An IllegalArgumentException is thrown if the two input Strings are not the same length.

Otherwise, the zip function is used to make a sequence of Pair values of the characters at the same index in both input Strings. Since zip is marked as an infix function, it can be called between the two strands (i.e. between the left receiver and the right parameter), as well as called leftStrand.zip(rightStrand). Which to use is a matter of stylistic preference.

The count function iterates the Pair values and uses the it keyword to count the Pair values that meet the Boolean condition of the first value in the current Pair not being equal to the second value in the Pair.

Finally, the compute function returns the result of calling count on the zipped Pair values.