Hi! First of all, I would love to give my feedback and opinions about the language before: I did not believe in love at first sight but after a week and a half using Kotlin, here I am, completely in love. Heart_eyes:
That being said, my question:
I am a video game developer and I have to be very careful with the performance and the allocated memory. I’m slowly moving my code to Kotlin and it’s time to write my custom collections but I’m not sure what’s the best way to iterate over them.
I’m decompiling to Java so I can see what’s going on under the hood and figure out the best way to do it.
I started with a very basic wrapper class:
class IntList(ordered: Boolean = true, capacity: Int = 16) {
private var items = IntArray(capacity)
var size: Int = 0
private set
var ordered: Boolean = ordered
private set
fun add(value: Int) {
var items = this.items
if (size == items.size) items = resize(Math.max(8, (size * 1.75f).toInt()))
items[size++] = value
}
operator fun get(index: Int): Int {
if (index >= size) throw IndexOutOfBoundsException("index can't be >= size: $index >= $size")
return items[index]
}
operator fun set(index: Int, value: Int) {
if (index >= size) throw IndexOutOfBoundsException("index can't be >= size: $index >= $size")
items[index] = value
}
private fun resize(newSize: Int): IntArray {
val newItems = IntArray(newSize)
val items = this.items
System.arraycopy(items, 0, newItems, 0, Math.min(size, newItems.size))
this.items = newItems
return newItems
}
}
And at that point I was ready to start iterating over the array. By ‘best way’ I mean (in order of priority): the most performant for the CPU; the cheapest in allocated memory and for garbage collection; and the cleanest and clearest.
In Java I would do:
IntList list = new IntList();
for(int i = 0, n = list.size; i < n; i++) {
int value = list.get(i);
}
but I know Kotlin can do better than that (in terms of clarity and simplicity).
I’ve seen the source code for IntArray and List classes, but that wasn’t really helpful because they are basically their Java counterparts with some extension methods and properties and it’s clear that the compiler is doing some special dark magic (specially for the primitive arrays) and I was wondering if there’s a way to achieved that same super optimal results for my class.
When I code in Kotlin
val list = IntArray(0) // or intArrayOf(someValues)
for(i in list) {
val value = i
}
for(i in list.indices) {
val value = list[i]
}
for(i in 0..list.lastIndex) {
val value = list[i]
}
I get a super optimal Java
int[] list = new int[0];
int i;
int var3;
for(var3 = 0; var3 < list.length; ++var3) {
i = list[var3];
}
i = 0;
var3 = ArraysKt.getLastIndex(list);
int var10000;
if(i <= var3) {
while(true) {
var10000 = list[i];
if(i == var3) {
break;
}
++i;
}
}
i = 0;
var3 = list.length - 1;
if(i <= var3) {
while(true) {
var10000 = list[i];
if(i == var3) {
break;
}
++i;
}
}
But instead if I do
val list = IntList()
for(i in list) {
val value = i
}
for(i in list.indices) {
val value = list[i]
}
for(i in 0..list.lastIndex) {
val value = list[i]
}
I can’t achieve any similar Java output.
For the first for, I couldn’t implement the iterator in any way without getting created an Iterator object.
For the second one I tried adding two very basic extension properties (imitating the IntArray implementation):
val IntList.lastIndex: Int
get() = size - 1
val IntList.indices: IntRange
get() = IntRange(0, lastIndex) // or 0..lastIndex (it's the same)
but I ended up with an IntRange created every time:
IntRange var10000 = IntListKt.getIndices(list);
i = var10000.getFirst();
var3 = var10000.getLast();
if(i <= var3) {
while(true) {
list.get(i);
if(i == var3) {
break;
}
++i;
}
}
And only the third option gave me the same Java output as with the primitive array. But it’s basically the same boilerplate (not that bad) way as the one I would have written in Java.
Is there any way I can get more optimal compiled Java code using any of the first two options? Anything I can tell the compiler to achieve better results? Do you know any other performant and “pretty” (or elegant) way of iterating over a custom collection?
I hope I made my self clear (sorry if my English wasn’t so good) and thanks in advance!
