loom sequence

testLoomSeq.kt

@file:Suppress("JAVA_MODULE_DOES_NOT_EXPORT_PACKAGE")

import jdk.internal.vm.Continuation
import jdk.internal.vm.ContinuationScope

internal class Wrapper<T>(val value: T)

class SequenceScope<T>(
    private val seq: Sequence<T>
) {
    fun emit(value: T) {
        seq.buffer = Wrapper(value)
        Continuation.yield(seq.continuationScope)
    }
}

class Sequence<T>(private val generator: SequenceScope<T>.() -> Unit) {
    internal val continuationScope = ContinuationScope("loom action")
    private var finished = false
    private val continuation = Continuation(continuationScope) {
        scope.generator()
        finished = true
    }
    private val scope = SequenceScope(this)

    internal var buffer: Wrapper<T>? = null

    fun hasNext(): Boolean {
        if (buffer != null) {
            return true
        }
        if (finished) {
            return false
        }
        generateNext()
        return buffer != null
    }

    fun next(): T {
        if (!hasNext()) {
            throw NoSuchElementException()
        }
        val next = buffer!!.value
        buffer = null
        return next
    }

    private fun generateNext() {
        continuation.run()
    }
}



fun main() {
    val fib = Sequence {
        var prev = 0L
        var curr = 1L
        while (true) {
            emit(curr)
            val tmp = curr
            curr += prev
            prev = tmp
        }
    }

    for (i in 0..10) {
        println(fib.next())
    }
}

Run with --enable-preview --add-exports java.base/jdk.internal.vm=ALL-UNNAMED JVM arguments