Does Kotlin optimize sum() for ranges and progressions?

Question

What happens under the hood, when you call IntProgression.sum()?

Sample code:

val n: Int
val sum = (0..n).sum()

1. does Kotlin iterate all elements from 0 to n?

   // O(n)
   var sum = 0
   for (i in 0..n) {
       sum += i
   }
   

2. or does it use a formula for calculating the sum of arithmetic sequence? which I assume is more performant for large values of n.

   // O(1)
   var sum = (0 + n) * (n + 1) / 2
   

Answer 1

No, Kotlin does not optimize the sum().

The function is an extension function on Iterable (IntProgression implements Iterable<Int>). That function could implement different sum algorithms depending on the actual type, but it just loops over all elements of the Iterable:

var sum: Int = 0
for (element in this) {
    sum += element
}
return sum