immutable list builder

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>

kroki-collections/src/main/kotlin/io/github/kerubistan/kroki/collections/Collections.kt

@@ -24,3 +24,6 @@ fun <T : Comparable<T>> List<T>.immutableSorted(): List<T> =
 		is ImmutableArrayList -> this.sortedToImmutable()
 		else -> this.sorted()
 	}
+
+inline fun <reified T : Any> buildList(builder: ImmutableListBuilder<T>.() -> Unit) =
+	ImmutableListBuilder<T>().apply(builder).build()

kroki-collections/src/main/kotlin/io/github/kerubistan/kroki/collections/ImmutableListBuilder.kt

@@ -0,0 +1,26 @@
+package io.github.kerubistan.kroki.collections
+
+class ImmutableListBuilder<T : Any> {
+	private var increment : Int = 128
+	private var size : Int = 0
+	private var items : Array<Any?> = arrayOfNulls(increment)
+
+	fun add(item : T) {
+		if (size >= items.size) {
+			items = items.copyOf(size + increment)
+		}
+		items[size] = item
+		size ++
+	}
+
+	fun build() : List<T> =
+		when (size) {
+			0 -> emptyList()
+			items.size -> {
+				ImmutableArrayList(items as Array<T>)
+			}
+			else -> {
+				ImmutableArrayList((items.copyOf(size)) as Array<T>)
+			}
+		}
+}

kroki-collections/src/test/kotlin/io/github/kerubistan/kroki/collections/CollectionsKtTest.kt

@@ -1,13 +1,8 @@
 package io.github.kerubistan.kroki.collections
 
-import io.kotest.matchers.collections.shouldBeEmpty
-import io.kotest.matchers.collections.shouldBeSingleton
-import io.kotest.matchers.collections.shouldContainAll
-import io.kotest.matchers.collections.shouldEndWith
-import io.kotest.matchers.collections.shouldNotBeEmpty
-import io.kotest.matchers.collections.shouldNotContainAll
-import io.kotest.matchers.collections.shouldStartWith
+import io.kotest.matchers.collections.*
 import io.kotest.matchers.should
+import io.kotest.matchers.shouldBe
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 
@@ -28,4 +23,20 @@ class CollectionsKtTest {
 		assertEquals(listOf("A"), listOf("A").toImmutableList())
 		assertEquals(listOf("A", "B"), listOf("A", "B").toImmutableList())
 	}
+
+	@Test
+	fun buildList() {
+		buildList<String> { } shouldBe emptyList<String>()
+		buildList<String> { add("A") } shouldBe listOf("A")
+		buildList<String> {
+			add("A")
+			add("B")
+			add("C")
+		} shouldBe listOf("A", "B", "C")
+		buildList<String> { repeat(1000) { add(it.toString()) } }.let {
+			it shouldStartWith "0"
+			it shouldEndWith "999"
+			it shouldHaveSize 1000
+		}
+	}
 }

kroki-jmh/src/main/kotlin/io/github/kerubistan/kroki/benchmark/collections/ListBuilderBenchmark.kt

@@ -0,0 +1,38 @@
+package io.github.kerubistan.kroki.benchmark.collections
+
+import com.google.common.collect.ImmutableList
+import org.openjdk.jmh.annotations.Benchmark
+import org.openjdk.jmh.annotations.Param
+import org.openjdk.jmh.annotations.Scope
+import org.openjdk.jmh.annotations.State
+import org.openjdk.jmh.infra.Blackhole
+
+@State(Scope.Benchmark)
+open class ListBuilderBenchmark {
+
+	@Param("0", "1", "2", "16", "1024", "4096")
+	var size = 0
+
+	@Benchmark
+	fun buildImmutableArrayList(blackhole: Blackhole) {
+		blackhole.consume(
+			io.github.kerubistan.kroki.collections.buildList {
+				repeat(size) {
+					add(it)
+				}
+			}
+		)
+	}
+
+	@Benchmark
+	fun buildGuavaImmutableList(blackhole: Blackhole) {
+		blackhole.consume(
+			ImmutableList.builder<Int>().apply {
+				repeat(size) {
+					add(it)
+				}
+			}.build()
+		)
+	}
+
+}