public fun removeFirstOrNull()

in ktor-utils/posix/src/io/ktor/util/collections/LockFreeMPSCQueueNative.kt [146:168]

• 20 lines of code
• 6 McCabe index (conditional complexity)

    public fun removeFirstOrNull(): Any? {
        _state.loop { state ->
            if (state and FROZEN_MASK != 0L) return REMOVE_FROZEN // frozen -- cannot modify
            state.withState { head, tail ->
                if ((tail and mask) == (head and mask)) return null // empty
                // because queue is Single Consumer, then element == null|Placeholder can only be when add has not finished yet
                val element = array[head and mask].value ?: return null
                if (element is Placeholder) return null // same story -- consider it not added yet
                // we cannot put null into array here, because copying thread could replace it with Placeholder and that is a disaster
                val newHead = (head + 1) and MAX_CAPACITY_MASK
                if (_state.compareAndSet(state, state.updateHead(newHead))) {
                    array[head and mask].value = null // now can safely put null (state was updated)
                    return element // successfully removed in fast-path
                }
                // Slow-path for remove in case of interference
                var cur = this
                while (true) {
                    @Suppress("UNUSED_VALUE")
                    cur = cur.removeSlowPath(head, newHead) ?: return element
                }
            }
        }
    }