#include <xen/config.h>
#include <xen/init.h>
#include <xen/lib.h>
#include <xen/sched.h>
#include <xen/domain.h>
#include <xen/delay.h>
#include <xen/event.h>
#include <xen/time.h>
#include <xen/perfc.h>
#include <xen/sched-if.h>
#include <xen/softirq.h>
#include <asm/atomic.h>
#include <xen/errno.h>

/* New VCPUs will be appended to the below linked list. The VCPU on the head of the list
will be scheduled and then moved to the end of the list.*/

static struct vcpu *vcpu_list_head = NULL;
static struct vcpu *vcpu_list_tail = NULL;
static struct vcpu *head;
unsigned int vcpus = 0;



/* Move the head VCPU to the end */

static inline void increment_run_queue(void){
    (struct vcpu) (vcpu_list_tail)->sched_priv = vcpu_list_head;
    vcpu_list_tail = vcpu_list_head;
    vcpu_list_head = VCPU_NEXT(vcpu_list_tail);
    (vcpu_list_tail)->sched_priv = NULL;
}
/* Add a VCPU */
int rrobin_vcpu_init(struct vcpu *v){
    if(vcpu_list_head == NULL){
        vcpu_list_tail = v;
        vcpu_list_head = vcpu_list_tail;
    }
    else{
        vcpu_list_tail = v;
        (struct vcpu) vcpu_list_tail->sched_priv = vcpu_list_tail;
    }
    v->sched_priv = NULL;
    return 0;
}

/*Remove a VCPU*/

void rrobin_vcpu_destroy(struct vcpu *v){
    if(v == vcpu_list_head){
        vcpu_list_head = VCPU_NEXT(v);
    }
    else{
        struct vcpu *last = NULL;
        struct vcpu *current = vcpu_list_head;
        while((current != v) && (current != NULL)){
            last = current;
            current = VCPU_NEXT(current);
        }
        if(current != NULL){
            last->sched_priv = VCPU_NEXT(current);
        }
    }
}

/* Pick a VCPU from the list */

struct task_slice rrobin_schedule(s_time_t now){
    struct task_slice ret;
    ret.time = MILLISECS(10); /* Time quantum to scan the VCPU list to find a runnable VCPU */
    head = vcpu_list_head; /* Since its round robin, we pick the head of the list for scheduling and later move it back */
    do{
        /* Find a runnable CPU in the list */
        increment_run_queue();

        if(vcpu_runnable(vcpu_list_head)){
            ret.task = vcpu_list_head;
        }
    }while(head != vcpu_list_head);
/* If no runnable VCPU is found, return the idle task */
    ret.task = ((struct vcpu) __get_per_cpu(schedule_data).idle);
    return ret;

}
struct scheduler sched_rrobin_def = {
    .name           = "Round robin Scheduler",
    .opt_name       = "rrobin",
    .sched_id       = XEN_SCHEDULER_RR,

    .init_vcpu      = rrobin_vcpu_init,
    .destroy_vcpu   = rrobin_vcpu_destroy,

    .do_schedule    = rrobin_schedule,

};