commonMain.channels.ConflatedChannel.kt Maven / Gradle / Ivy

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/*
 * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
 */

package kotlinx.coroutines.channels

import kotlinx.coroutines.selects.*
import kotlinx.coroutines.internal.*

/**
 * Channel that buffers at most one element and conflates all subsequent `send` and `offer` invocations,
 * so that the receiver always gets the most recently sent element.
 * Back-to-send sent elements are _conflated_ -- only the the most recently sent element is received,
 * while previously sent elements **are lost**.
 * Sender to this channel never suspends and [offer] always returns `true`.
 *
 * This channel is created by `Channel(Channel.CONFLATED)` factory function invocation.
 *
 * This implementation is fully lock-free.
 */
internal open class ConflatedChannel : AbstractChannel() {
    protected final override val isBufferAlwaysEmpty: Boolean get() = true
    protected final override val isBufferEmpty: Boolean get() = true
    protected final override val isBufferAlwaysFull: Boolean get() = false
    protected final override val isBufferFull: Boolean get() = false

    override fun onClosedIdempotent(closed: LockFreeLinkedListNode) {
        @Suppress("UNCHECKED_CAST")
        (closed.prevNode as? SendBuffered)?.let { lastBuffered ->
            conflatePreviousSendBuffered(lastBuffered)
        }
    }

    /**
     * Queues conflated element, returns null on success or
     * returns node reference if it was already closed or is waiting for receive.
     */
    private fun sendConflated(element: E): ReceiveOrClosed<*>? {
        val node = SendBuffered(element)
        queue.addLastIfPrev(node) { prev ->
            if (prev is ReceiveOrClosed<*>) return@sendConflated prev
            true
        }
        conflatePreviousSendBuffered(node)
        return null
    }

    private fun conflatePreviousSendBuffered(node: SendBuffered) {
        // Conflate all previous SendBuffered, helping other sends to conflate
        var prev = node.prevNode
        while (prev is SendBuffered<*>) {
            if (!prev.remove()) {
                prev.helpRemove()
            }
            prev = prev.prevNode
        }
    }

    // result is always `OFFER_SUCCESS | Closed`
    protected override fun offerInternal(element: E): Any {
        while (true) {
            val result = super.offerInternal(element)
            when {
                result === OFFER_SUCCESS -> return OFFER_SUCCESS
                result === OFFER_FAILED -> { // try to buffer
                    when (val sendResult = sendConflated(element)) {
                        null -> return OFFER_SUCCESS
                        is Closed<*> -> return sendResult
                    }
                    // otherwise there was receiver in queue, retry super.offerInternal
                }
                result is Closed<*> -> return result
                else -> error("Invalid offerInternal result $result")
            }
        }
    }

    // result is always `ALREADY_SELECTED | OFFER_SUCCESS | Closed`.
    protected override fun offerSelectInternal(element: E, select: SelectInstance<*>): Any {
        while (true) {
            val result = if (hasReceiveOrClosed)
                super.offerSelectInternal(element, select) else
                (select.performAtomicTrySelect(describeSendConflated(element)) ?: OFFER_SUCCESS)
            when {
                result === ALREADY_SELECTED -> return ALREADY_SELECTED
                result === OFFER_SUCCESS -> return OFFER_SUCCESS
                result === OFFER_FAILED -> {} // retry
                result is Closed<*> -> return result
                else -> error("Invalid result $result")
            }
        }
    }
}