Kotlin: Update Immutable List Element

Eric picture Eric · Oct 21, 2016 · Viewed 23.5k times · Source

Kotlin beginner here. How do I take a list and without mutating it, create a second (immutable) list with one updated element at a specific index?

I'm thinking of two ways, both of which seem like they may incur performance hits, mutate the underlying object, or both.

data class Player(val name: String, val score: Int = 0)

val players: List<Player> = ...

// Do I do this?
val updatedPlayers1 = players.mapIndexed { i, player ->
    if (i == 2) player.copy(score = 100)
    else player
}

// Or this?
val updatedPlayer = players[2].copy(score = 100)
val mutable = players.toMutableList()
mutable.set(2, updatedPlayer)
val updatedPlayers2 = mutable.toList()

If there is no performant way to do this, is there a more appropriate data structure in the Kotlin stdlib or other library? Kotlin doesn't seem to have vectors.

Answer

Ruslan picture Ruslan · Oct 24, 2016

For me obvious that second way should be faster, but how much?

So I wrote some benchmarks here

@State(Scope.Thread)
open class ModifyingImmutableList {

    @Param("10", "100", "10000", "1000000")
    var size: Int = 0

    lateinit var players: List<Player>

    @Setup
    fun setup() {
        players = generatePlayers(size)
    }

    @Benchmark fun iterative(): List<Player> {
        return players.mapIndexed { i, player ->
            if (i == 2) player.copy(score = 100)
            else player
        }
    }

    @Benchmark fun toMutable(): List<Player> {
        val updatedPlayer = players[2].copy(score = 100)
        val mutable = players.toMutableList()
        mutable.set(2, updatedPlayer)
        return mutable.toList()
    }

    @Benchmark fun toArrayList(): List<Player> {
        val updatedPlayer = players[2].copy(score = 100)
        return players.set(2, updatedPlayer)
    }
}

And got following results:

$ java -jar target/benchmarks.jar -f 5 -wi 5 ModifyingImmutableList
Benchmark                            (size)   Mode  Cnt         Score        Error  Units
ModifyingImmutableList.iterative         10  thrpt  100   6885018.769 ± 189148.764  ops/s
ModifyingImmutableList.iterative        100  thrpt  100    877403.066 ±  20792.117  ops/s
ModifyingImmutableList.iterative      10000  thrpt  100     10456.272 ±    382.177  ops/s
ModifyingImmutableList.iterative    1000000  thrpt  100       108.167 ±      3.506  ops/s
ModifyingImmutableList.toArrayList       10  thrpt  100  33278431.127 ± 560577.516  ops/s
ModifyingImmutableList.toArrayList      100  thrpt  100  11009646.095 ± 180549.177  ops/s
ModifyingImmutableList.toArrayList    10000  thrpt  100    129167.033 ±   2532.945  ops/s
ModifyingImmutableList.toArrayList  1000000  thrpt  100       528.502 ±     16.451  ops/s
ModifyingImmutableList.toMutable         10  thrpt  100  19679357.039 ± 338925.701  ops/s
ModifyingImmutableList.toMutable        100  thrpt  100   5504388.388 ± 102757.671  ops/s
ModifyingImmutableList.toMutable      10000  thrpt  100     62809.131 ±   1070.111  ops/s
ModifyingImmutableList.toMutable    1000000  thrpt  100       258.013 ±      8.076  ops/s

So this tests shows that iterating over collection about 3~6 times slower, that copying. Also I provide my implementation: toArray, which looks like more performant.

On 10 element, toArray method has throughput 33278431.127 ± 560577.516 operations per second. Is it slow? Or it's extremely fast? I write "baseline" test, which shows cost of copying Players and mutating array. Results interesting:

@Benchmark fun baseline(): List<Player> {
    val updatedPlayer = players[2].copy(score = 100)
    mutable[2] = updatedPlayer;
    return mutable
}

Where mutable - just MutableList, which is ArrayList.

$ java -jar target/benchmarks.jar -f 5 -wi 5 ModifyingImmutableList
Benchmark                            (size)   Mode  Cnt         Score         Error  Units
ModifyingImmutableList.baseline          10  thrpt  100  81026110.043 ± 1076989.958  ops/s
ModifyingImmutableList.baseline         100  thrpt  100  81299168.496 ±  910200.124  ops/s
ModifyingImmutableList.baseline       10000  thrpt  100  81854190.779 ± 1010264.620  ops/s
ModifyingImmutableList.baseline     1000000  thrpt  100  83906022.547 ±  615205.008  ops/s
ModifyingImmutableList.toArrayList       10  thrpt  100  33090236.757 ±  518459.863  ops/s
ModifyingImmutableList.toArrayList      100  thrpt  100  11074338.763 ±  138272.711  ops/s
ModifyingImmutableList.toArrayList    10000  thrpt  100    131486.634 ±    1188.045  ops/s
ModifyingImmutableList.toArrayList  1000000  thrpt  100       531.425 ±      18.513  ops/s

On 10 elements we have 2x regression, and on 1 million about 150000x!

So looks like ArrayList not the best choice for immutable data structures. But there are a lot other collections, one of them is pcollections. Let's see what they got in our scenario:

@Benchmark fun pcollections(): List<Player> {
    val updatedPlayer = players[2].copy(score = 100)
    return pvector.with(2, updatedPlayer)
}

Where pvector is pvector:PVector<Player> = TreePVector.from(players).

$ java -jar target/benchmarks.jar -f 5 -wi 5 ModifyingImmutableList
Benchmark                             (size)   Mode  Cnt         Score         Error  Units
ModifyingImmutableList.baseline           10  thrpt  100  79462416.691 ± 1391446.159  ops/s
ModifyingImmutableList.baseline          100  thrpt  100  79991447.499 ± 1328008.619  ops/s
ModifyingImmutableList.baseline        10000  thrpt  100  80017095.482 ± 1385143.058  ops/s
ModifyingImmutableList.baseline      1000000  thrpt  100  81358696.411 ± 1308714.098  ops/s
ModifyingImmutableList.pcollections       10  thrpt  100  15665979.142 ±  371910.991  ops/s
ModifyingImmutableList.pcollections      100  thrpt  100   9419433.113 ±  161562.675  ops/s
ModifyingImmutableList.pcollections    10000  thrpt  100   4747628.815 ±   81192.752  ops/s
ModifyingImmutableList.pcollections  1000000  thrpt  100   3011819.457 ±   45548.403  ops/s

Nice results! On 1 million case we have only 27x slower execution, which is pretty cool, but on small collections pcollections little bit slower that ArrayList implementation.

Update: as @mfulton26 mentioned, in toMutable benchmark toList is unnecessary, so I deleted it and reran tests. Also I added benchmark on cost of creation TreePVector from existing array:

$ java -jar target/benchmarks.jar  ModifyingImmutableList
Benchmark                                 (size)   Mode  Cnt         Score         Error  Units
ModifyingImmutableList.baseline               10  thrpt  200  77639718.988 ± 1384171.128  ops/s
ModifyingImmutableList.baseline              100  thrpt  200  75978576.147 ± 1528533.332  ops/s
ModifyingImmutableList.baseline            10000  thrpt  200  79041238.378 ± 1137107.301  ops/s
ModifyingImmutableList.baseline          1000000  thrpt  200  84739641.265 ±  557334.317  ops/s

ModifyingImmutableList.iterative              10  thrpt  200   7389762.016 ±   72981.918  ops/s
ModifyingImmutableList.iterative             100  thrpt  200    956362.269 ±   11642.808  ops/s
ModifyingImmutableList.iterative           10000  thrpt  200     10953.451 ±     121.175  ops/s
ModifyingImmutableList.iterative         1000000  thrpt  200       115.379 ±       1.301  ops/s

ModifyingImmutableList.pcollections           10  thrpt  200  15984856.119 ±  162075.427  ops/s
ModifyingImmutableList.pcollections          100  thrpt  200   9322011.769 ±  176301.745  ops/s
ModifyingImmutableList.pcollections        10000  thrpt  200   4854742.140 ±   69066.751  ops/s
ModifyingImmutableList.pcollections      1000000  thrpt  200   3064251.812 ±   35972.244  ops/s

ModifyingImmutableList.pcollectionsFrom       10  thrpt  200   1585762.689 ±   20972.881  ops/s
ModifyingImmutableList.pcollectionsFrom      100  thrpt  200     67107.504 ±     808.308  ops/s
ModifyingImmutableList.pcollectionsFrom    10000  thrpt  200       268.268 ±       2.901  ops/s
ModifyingImmutableList.pcollectionsFrom  1000000  thrpt  200         1.406 ±       0.015  ops/s

ModifyingImmutableList.toArrayList            10  thrpt  200  34567833.775 ±  423910.463  ops/s
ModifyingImmutableList.toArrayList           100  thrpt  200  11395084.257 ±   76689.517  ops/s
ModifyingImmutableList.toArrayList         10000  thrpt  200    134299.055 ±     602.848  ops/s
ModifyingImmutableList.toArrayList       1000000  thrpt  200       549.064 ±      15.317  ops/s

ModifyingImmutableList.toMutable              10  thrpt  200  32441627.735 ±  391890.514  ops/s
ModifyingImmutableList.toMutable             100  thrpt  200  11505955.564 ±   71394.457  ops/s
ModifyingImmutableList.toMutable           10000  thrpt  200    134819.741 ±     526.830  ops/s
ModifyingImmutableList.toMutable         1000000  thrpt  200       561.031 ±       8.117  ops/s