common/kotlinx-coroutines-core-common/src/channels/ConflatedChannel.kt

/*
 * Copyright 2016-2017 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package kotlinx.coroutines.experimental.channels

import kotlinx.coroutines.experimental.selects.ALREADY_SELECTED
import kotlinx.coroutines.experimental.selects.SelectInstance

/**
 * 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.
 */
public open class ConflatedChannel<E> : AbstractChannel<E>() {
    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

    /**
     * This implementation conflates last sent item when channel is closed.
     * @suppress **This is unstable API and it is subject to change.**
     */
    override fun onClosed(closed: Closed<E>) {
        conflatePreviousSendBuffered(closed)
    }

    // 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
                    val sendResult = sendConflated(element)
                    when (sendResult) {
                        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")
            }
        }
    }
}