KDS: Data Structures

Kds is a Data Structure library for Multiplatform Kotlin 1.3.



Build Status Maven Version

Table of contents:

Using with gradle


Requires Gradle 6.8.3 (JVM 8~13) for building and Kotlin >=1.4.31 for running:

val kdsVersion = "2.0.8"

repositories {
    maven { url("https://dl.bintray.com/korlibs/korlibs") }

// For multiplatform projects
kotlin {
    sourceSets {
        commonMain {
            dependencies {

dependencies {
    // For JVM only
    // For Android only
    // For JS only



ArrayList: IntArrayList, FloatArrayList and DoubleArrayList

Kds provides specialized equivalents of ArrayList so it doesn’t involve object allocation through boxing. It uses typed arrays internally to store the elements of the ArrayList so it just requires one additional object allocation per list (the Array). It will just allocate a new object when the capacity of the list is exhausted.


You can construct literals using the *arrayListOf constructors:

val ilist = intArrayListOf(10, 20)
val flist = floatArrayListOf(10f, 20f)
val dlist = doubleArrayListOf(10.0, 20.0)

Expected behaviour

IntArrayList, FloatArrayList and DoubleArrayList work like a normal ArrayList but without incurring into boxing.

val list = IntArrayList()
list += 10
list += 20
list[0] = 15
println(list.toList().map { it * 20 })

Optimized collection transformations

mapInt, mapFloat and mapDouble generate optimized *ArrayList. And *ArrayList have an specialized filter function too.

val filter = (0 until 10).mapInt { it * 3 }.filter { it % 2 == 0 }

Array2: Array2, IntArray2, FloatArray2, DoubleArray2

Array2 is a bidimensional version of Array variants. It includes a width and a height instead of size (length) measuing its dimensions.

It provides bidimensional indexers and some convenience methods.

val biarray = IntArray2(64, 64) { 0 }
val biarray = IntArray2(64, 64, 0)
biarray[0, 0] = 1
biarray.width == 64
biarray.height == 64

Internally it is represented as a single 1D Array and actual indices are computed using simple arithmetic.


BitSet structure that works like a BoolArray but it is more efficient in terms of memory usage.

val array = BitSet(100) // Stores 100 bits
array[99] = true
val bool: Boolean = array[99]

It packs bits in an IntArray internally so it requires up to eight times less space than a BoolArray that potentially uses internally a ByteArray.


Works like a LinkedHashMap with a limited amount of elements. When inserting new elements after reaching the maximum amount of elements, the oldest element inserted is deleted.

val cache = CacheMap<String, Int>(maxSize = 2)
cache["a"] = 1
cache["b"] = 2
cache["c"] = 3
assertEquals("{b=2, c=3}", cache.toString())


Map with String keys considered case insensitive. Case of the original keys is preserved, but keys can be accessed with any case.

val map = CaseInsensitiveStringMap("hELLo" to 1, "World" to 2)
assertEquals(2, map.size)
assertEquals(1, map["hello"])
assertEquals(2, map["world"])

It is possible to convert a normal Map<String, *> to a CaseInsensitive one with the toCaseInsensitiveMap extension:

val map = mapOf("hELLo" to 1, "World" to 2).toCaseInsensitiveMap()

Deque/CircularList: Deque, ByteDeque, IntDeque, FloatDeque, DoubleDeque

Deque variants (and its CircularList typealias) allows to insert and delete elements to/from the start or the end of the deque in constant time except when growing the collection. It can be used to implement queues or produce/consumers in an efficient way. The typed variants allow to reduce memory and allocation usage.

val l = IntDeque()
for (n in 0 until 1000) l.addFirst(n)
for (n in 0 until 1000) l.removeFirst()
for (n in 0 until 1000) l.addLast(n)

FastMap: FastIntMap, FastStringMap

Simpler Map-like structures that uses native specific implementations to improve performance and reduce allocations.

val map = FastIntMap<String>()
assertEquals(0, map.size)
map[1] = "a"
map[2] = "b"
assertEquals(listOf(1, 2), map.keys.sorted())
assertEquals(2, map.size)
assertEquals("a", map[1])
assertEquals("b", map[2])
assertEquals(null, map[3])

IntMap: IntMap, IntIntMap, IntFloatMap

Variants of a hashmap implementation using int as keys without boxing (and Object, int or float for values). The implementation requires just a couple of arrays for working (no nodes at all). It uses a multihash approach for filling as much as possible with a logarithmic stash. Just allocates when growing.

val m = IntIntMap()
m[0] = 98
assertEquals(1, m.size)
assertEquals(98, m[0])
assertEquals(0, m[1])


A set working with integers without boxing.

val set = intSetOf(1, 2, 4)
assertEquals(3, set.size)

assertEquals(true, 1 in set)
assertEquals(true, 2 in set)
assertEquals(false, 3 in set)
assertEquals(true, 4 in set)

assertEquals(2, set.size)
assertEquals(true, 1 in set)
assertEquals(false, 2 in set)
assertEquals(true, 4 in set)


A reader for lists. It can peek, read or expect a specific value in order.

val reader = listOf(1, 2, 3).reader()
assertEquals(true, reader.hasMore)
assertEquals(1, reader.peek())
assertEquals(1, reader.peek())
assertEquals(1, reader.read())
assertEquals(2, reader.read())
assertEquals(3, reader.expect(3))
assertEquals(false, reader.hasMore)


A simple pool implementation allowing to preallocate, to reset objects and to temporally allocate (freeing automatically) using an inline function. It accepts an instance allocator, and an optional function to reset instances.

val pool = Pool { Demo() }
pool.alloc { demo ->
    println("Temporarilly allocated $demo")
val pool = Pool(reset = {
    it.x = 0
    it.y = 0
},  gen = {
val a = pool.alloc()
val b = pool.alloc()

assertEquals(0, pool.itemsInPool)
assertEquals(1, pool.itemsInPool)

pool.alloc {
    assertEquals(1, pool.itemsInPool)
assertEquals(2, pool.itemsInPool)

assertEquals(5, totalResetCount) // Number of resets
assertEquals(3, totalAllocCount) // Number of allocs

PriorityQueue: PriorityQueue, IntPriorityQueue, FloatPriorityQueue, DoublePriorityQueue

Provides a PriorityQueue that allows to insert items in a Queue by priority. It allows reordering specific items after modification.

val pq = IntPriorityQueue()
assertEquals(5, pq.removeHead())
assertEquals(10, pq.removeHead())
assertEquals(15, pq.removeHead())
assertEquals(0, pq.size)

Allows to provide a custom Comparator:

val pq = IntPriorityQueue { a, b -> (-a).compareTo(-b) }
pq.addAll(listOf(1, 2, 3, 4))
assertEquals(listOf(4, 3, 2, 1), pq.toList())

And to repriorize objects after modification:

val item = Item(10)
item.value = 20

It is implemented using a Min Heap so addition, removing and updating happens in O(log(n)).

Queue: Queue, IntQueue, FloatQueue, DoubleQueue

A FIFO (First In First Out) collection.

val queue = IntQueue()
assertEquals(1, queue.dequeue())

Internally implemented using a Deque.

Stack: Stack, IntStack, FloatStack, DoubleStack

A LIFO (Last In First Out) collection.

val queue = IntStack()
assertEquals(2, queue.pop())

Internally implemented using an ArrayList.


Provides a WeakMap data structure that internally uses JS’s WeakMap, JVM’s WeakHashMap and Native’s WeakReference. WeakProperty allow to define external/extrinsic properties to objects that are collected once the object is not referenced anymore.

val map = WeakMap<Demo, String>()
val demo1 = Demo()
map[demo1] = "hello"

assertEquals("hello", map[demo1])

Note that using this primitive on JavaScript requires ES6 support (and it doesn’t work on IE10 or lower). Check the JS’s WeakMap compatibility table for more information.

MapList extensions

Instead of providing a MutableMap<K, MutableList<V>> implementation. Kds provides a set of methods and extension methods to easily work with those kind of maps.

val map = linkedHashMapListOf("a" to 10, "a" to 20, "b" to 30)

assertEquals(10, map.getFirst("a"))
assertEquals(20, map.getLast("a"))

assertEquals(30, map.getFirst("b"))
assertEquals(30, map.getLast("b"))

assertEquals(null, map.getLast("c"))

assertEquals(listOf("a" to 10, "a" to 20, "b" to 30), map.flatten())

binarySearch: genericBinarySearch, binarySearch

Kds provides binarySearch for its collections limiting the indices used. Also provides a genericBinarySearch to execute the algorithm in any possible kind of collection. It allows to get exact possitions or nearest positionss when no value is found:

val v = intArrayOf(10, 20, 30, 40, 50)
assertEquals(0, v.binarySearch(10).index)
assertEquals(1, v.binarySearch(20).index)
assertEquals(2, v.binarySearch(30).index)
assertEquals(3, v.binarySearch(40).index)
assertEquals(4, v.binarySearch(50).index)

assertEquals(true, v.binarySearch(10).found)
assertEquals(false, v.binarySearch(11).found)

assertEquals(2, v.binarySearch(21).nearIndex)


genericSort allows to sort any array-like structure fully or partially without allocating and without having to reimplementing any sort algorithm again. You just have to implement a compare and swap methods that receive indices in the array to compare and optionally a shiftLeft method (that fallbacks to use the swap one). The SortOps implementation is usually an object to prevent allocating.

fun <T> genericSort(subject: T, left: Int, right: Int, ops: SortOps<T>): T
abstract class SortOps<T> {
    abstract fun compare(subject: T, l: Int, r: Int): Int
    abstract fun swap(subject: T, indexL: Int, indexR: Int)
    open fun shiftLeft(subject: T, indexL: Int, indexR: Int)

So a simple implementation that would sort any MutableList could be:

val result = genericSort(arrayListOf(10, 30, 20, 10, 5, 3, 40, 7), 0, 7, ArrayListSortOps)
assertEquals(listOf(3, 5, 7, 10, 10, 20, 30, 40), result)

object ArrayListSortOps : SortOps<ArrayList<Int>>() {
    override fun compare(subject: ArrayList<Int>, l: Int, r: Int): Int {
        return subject[l].compareTo(subject[r])

    override fun swap(subject: ArrayList<Int>, indexL: Int, indexR: Int) {
        val tmp = subject[indexA]
        subject[indexA] = subject[indexB]
        subject[indexB] = tmp

mapWhile: mapWhile, mapWhileArray, mapWhileInt, mapWhileFloat, mapWhileDouble

This method allows to generate a collection by providing a condition and a generator:

val iterator = listOf(1, 2, 3).iterator()
assertEquals(listOf(1, 2, 3), mapWhile({ iterator.hasNext() }) { iterator.next()})

getCyclic: List.getCyclic, Array.getCyclic

For lists and arrays Kds defines a getCyclic extension method to get an element wrapping its bounds. So list.getCylic(-1) would return the last element of the List, and list.getCyclic(size) would return the element at 0:

assertEquals("a", arrayOf("a", "b").getCyclic(2))
assertEquals("b", arrayOf("a", "b").getCyclic(-1))

Property Delegates: Extra.Property, Computed, WeakProperty


Provides a Extra funtionality to define extrinsic properties to an object that has been decorated with Extra interface implemented by Extra.Mixin. It just adds a extra hashmap to the object, so it can be used to externally define properties. The idea is similar to WeakProperty but doesn’t require weak references at all. But just works with objects that implements Extra interface.

class Demo : Extra by Extra.Mixin() {
    val default = 9

// Externally defined for classes implementing Extra
var Demo.demo by Extra.Property { 0 }
var Demo.demo2 by Extra.PropertyThis<Demo, Int> { default }

val demo = Demo()
assertEquals(0, demo.demo)
assertEquals(9, demo.demo2)
demo.demo = 7
assertEquals(7, demo.demo)
assertEquals("{demo=7, demo2=9}", demo.extra.toString())


Allows to create nullable properties with a parent object that tries to get its value from the parent or from a default when it is not defined locally:

class Format(override var parent: Format? = null) : Computed.WithParent<Format> {
    var size: Int? = null

    val computedSize by Computed(Format::size) { 10 }

val f2 = Format()
val f1 = Format(f2)
assertEquals(10, f1.computedSize)
f2.size = 12
assertEquals(12, f1.computedSize)
f1.size = 15
assertEquals(15, f1.computedSize)


Similar to Extra, to extend objects, but do not require the objects to implement the Extra interface. Each externally defined property creates a WeakMap whose keys are the objects that are going to contain the extra properties. But those properties do not retain the object themselves, so they can be collected when not referenced anywhere else.

class C {
    val value = 1

var C.prop by WeakProperty { 0 }
var C.prop2 by WeakPropertyThis<C, String> { "${value * 2}" }

val c1 = C()
val c2 = C()
assertEquals(0, c1.prop)
assertEquals(0, c2.prop)
c1.prop = 1
c2.prop = 2
assertEquals(1, c1.prop)
assertEquals(2, c2.prop)

assertEquals("2", c2.prop2)
c2.prop2 = "3"
assertEquals("3", c2.prop2)