#include "kpl.h"

void kpl_task_queue_init(kpl_task_queue *task_queue) {
    __atomic_store_n(&task_queue->dummy.next, NULL, __ATOMIC_RELAXED);
    __atomic_store_n(&task_queue->head, &task_queue->dummy, __ATOMIC_RELAXED);
    task_queue->tail = (kpl_task*) &task_queue->dummy;
}

void kpl_task_queue_add(kpl_task_queue *task_queue, kpl_task *task) {
    __atomic_store_n(&task->next, NULL, __ATOMIC_RELAXED);
    kpl_task *head = __atomic_exchange_n(&task_queue->head, task, __ATOMIC_SEQ_CST);
    __atomic_store_n(&head->next, task, __ATOMIC_RELEASE);
}

kpl_task *kpl_task_queue_next(kpl_task_queue *task_queue) {
    kpl_task *task = NULL;
    for (;;) {
        kpl_task *tail = task_queue->tail, *next = __atomic_load_n(&tail->next, __ATOMIC_ACQUIRE);
        if (tail == (kpl_task*) &task_queue->dummy) {
            if (!next)
                break;
            tail = task_queue->tail = next;
            next = __atomic_load_n(&tail->next, __ATOMIC_ACQUIRE);
        }
        if (!next) {
            kpl_task *head = __atomic_load_n(&task_queue->head, __ATOMIC_ACQUIRE);
            if (tail != head)
                continue;
            kpl_task_queue_add(task_queue, (kpl_task*) &task_queue->dummy);
            next = __atomic_load_n(&tail->next, __ATOMIC_ACQUIRE);
        }
        if (next) {
            task_queue->tail = next;
            task = tail;
            break;
        }
    }
    if (task)
        __atomic_store_n(&task->next, NULL, __ATOMIC_RELAXED);
    return task;
}