Collections

Always prefer read-only view collections over mutable ones. Since read-only view collections can't be changed, you can prevent a lot of bugs that relate to multithreading and state inconsistency. If you need to iterate elements of an ArrayList, consider the while loop:

inline fun <reified T> List<T>.foreach(crossinline invoke: (T) -> Unit): Unit {
    val size = size
    var i = 0
    while (i < size) {
        invoke(get(i))
        i ++
    }
}

Sequences can improve performance significantly, especially if you use methods such as first:

@Benchmark
fun sequence() = (0..1_000_000)
        .asSequence()
        .filter { it % 2 == 0 }
        .map { it * it }
        .first()