fun removeFirstOrNull()

in kotlinx-coroutines-core/common/src/internal/LockFreeTaskQueue.kt [160:192]

• 25 lines of code
• 9 McCabe index (conditional complexity)

    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
                val element = array[head and mask].value
                if (element == null) {
                    // If queue is Single-Consumer, then element == null only when add has not finished yet
                    if (singleConsumer) return null // consider it not added yet
                    // retry (spin) until consumer adds it
                    return@loop
                }
                // element == Placeholder can only be when add has not finished yet
                if (element is Placeholder) return null // 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 could have been copied by another thread and it is perfectly fine, since only elements
                    // between head and tail were copied and there are no extra steps we should take here
                    array[head and mask].value = null // now can safely put null (state was updated)
                    return element // successfully removed in fast-path
                }
                // Multi-Consumer queue must retry this loop on CAS failure (another consumer might have removed element)
                if (!singleConsumer) return@loop
                // Single-consumer queue goes to slow-path for remove in case of interference
                var cur = this
                while (true) {
                    @Suppress("UNUSED_VALUE")
                    cur = cur.removeSlowPath(head, newHead) ?: return element
                }
            }
        }
    }