Sum of of all the multiples of 4 or 7 under 2500

SumOfMultiples.kt

fun main() {
    println("Calculating using naive approach (in a loop).")
    execute(::calculateSum)

    println("Calculating using formula for AP series.")
    execute(::calculateSumAP)
}

fun execute(calculateSum: (Long, Long, Long) -> Long) {
    val testResult = calculateSum(4, 7, 10)
    if (testResult != 19L) {
        println("FAILED basic test: Sum of multiples of 4 or 7 under 10 is 19, but was $testResult")
        return
    }
    val start = System.currentTimeMillis()

    val first = 4L
    val second = 7L
    val limit = 2500L
    val sum = calculateSum(first, second, limit)
    println("Sum of all the multiples of $first or $second under $limit is $sum")
    println("Time: ${System.currentTimeMillis() - start}")
}

fun calculateSum(first: Long, second: Long, limit: Long): Long {
    val result = mutableSetOf(first, second)
    for (number in 1 until limit) {
        if (number % first == 0L || number % second == 0L) {
            result += number
        }
    }
    return result.sum()
}

fun calculateSumAP(first: Long, second: Long, limit: Long): Long {
    return calculateSumAP(first, limit) + calculateSumAP(second, limit) - calculateSumAP(first * second, limit)
}

fun calculateSumAP(first: Long, limit: Long): Long {
    // Number of multiples
    val m = (limit - 1) / first

    // sum of first m natural numbers
    val sum = m * (m + 1) / 2

    // sum of multiples
    return first * sum
}