CodeWars/7kyu/sum_of_odd_numbers/solution.kt

// How to get k-th odd number: p = 2k + 1 (zero indexed)
// Decomposition:
// 1.  Find out how many odd numbers we have in front
// 2.  Figure out next index
// 3.  Sum those numbers

private fun kthOdd(k: Int): Int = 2*k + 1
private fun oddNumbersBefore(n: Int) = n.times(n + 1).div(2)

fun rowSumOddNumbers(n: Int): Int =
    (oddNumbersBefore(n - 1)..oddNumbersBefore(n) - 1)
    .sumOf { kthOdd(it) }