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Coroutines support libraries for Kotlin

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/*
 * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
 */

package kotlinx.coroutines.internal

internal open class ArrayQueue {
    private var elements = arrayOfNulls(16)
    private var head = 0
    private var tail = 0

    val isEmpty: Boolean get() = head == tail

    public fun addLast(element: T) {
        elements[tail] = element
        tail = (tail + 1) and elements.size - 1
        if (tail == head) ensureCapacity()
    }

    @Suppress("UNCHECKED_CAST")
    public fun removeFirstOrNull(): T? {
        if (head == tail) return null
        val element = elements[head]
        elements[head] = null
        head = (head + 1) and elements.size - 1
        return element as T
    }

    public fun clear() {
        head = 0
        tail = 0
        elements = arrayOfNulls(elements.size)
    }

    private fun ensureCapacity() {
        val currentSize = elements.size
        val newCapacity = currentSize shl 1
        val newElements = arrayOfNulls(newCapacity)
        elements.copyInto(
            destination = newElements,
            startIndex = head
        )
        elements.copyInto(
            destination = newElements,
            destinationOffset = elements.size - head,
            endIndex = head
        )
        elements = newElements
        head = 0
        tail = currentSize
    }
}