/*
 * File     : lock_free_queue.c
 * Author   : Jérémy Zurcher  <jeremy@asynk.ch>
 * Date     : 01/11/09 
 * License  : stolen from http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf
 */

#include "stdlib.h"
#include "lock_free_queue.h"
#ifdef DEBUG
#include "stdio.h"
#endif

/* CMPXCHG8B m64   Compare EDX:EAX with m64. If equal, set ZF and load ECX:EBX into m64. Else, clear ZF and load m64 into EDX:EAX. */
static inline unsigned int compare_and_swap(volatile unsigned long long *mem,
                                            volatile unsigned long long old,
                                            volatile unsigned long long new) {
    char result;
    __asm__ __volatile__("lock; cmpxchg8b %0; setz %1;"
            : "=m"(*mem), "=q"(result)
            : "m"(*mem), "d" ((unsigned long)(old>>32)), "a" ((unsigned long)old),
            "c" ((unsigned long)(new>>32)), "b" ((unsigned long)new)
            : "memory");
    return (int)result;
}

void init( lfq_t *q ) {
    node_t *node = (node_t*)malloc(sizeof(node_t));
    node->next.split.ptr = NULL;
    node->next.split.count = 0;
    q->head.split.ptr = q->tail.split.ptr = node;
    q->head.split.count = q->tail.split.count = 0;
}

void enqueue( lfq_t *q, void *data ) {
    node_t *node;
    pointer_t tail;
    pointer_t next;
    pointer_t tmp;

    node = (node_t*)malloc(sizeof(node_t));
    node->data = data;
    node->next.split.ptr = NULL;
    /* node->next.split.count = 0; */
    for(;;) {
        tail.concat = q->tail.concat;                   /* copy tail pointer */
        next.concat = tail.split.ptr->next.concat;      /* copy next pointer */
        if (tail.concat == q->tail.concat) {            /* tail is still consistent */
            if (next.split.ptr == NULL) {               /* next is still the last node */
                /* if tail->next is the same as next, link node at the end of the list */
                tmp.split.ptr = node;
                tmp.split.count = next.split.count+1;
                if ( compare_and_swap( &tail.split.ptr->next.concat, next.concat, tmp.concat ) ) break;
            } else {
                /* try to swing tail to the next node, if q-> tail is still tail => next is ok */
                tmp.split.ptr = next.split.ptr;
                tmp.split.count = tail.split.count+1;
                compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
            }
        }
    }
    /* try to swing tail to the next node, may have been done by a concurrent push */
    tmp.split.ptr = node;
    tmp.split.count = tail.split.count+1;
    compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
}

void* dequeue( lfq_t *q ) {
    void *data;
    pointer_t head;
    pointer_t tail;
    node_t *next;
    pointer_t tmp;
    
    for(;;) {
        head.concat = q->head.concat;
        tail.concat = q->tail.concat;
        next = (node_t *)head.split.ptr;
        if (head.concat == q->head.concat) {
            if (head.split.ptr == tail.split.ptr) {     /* still consistent */
                if (next->next.split.ptr == NULL) {     /* queue empty */
                    return NULL;
                }
                /* new node has been linked, but tail is behind, should advance it */
                tmp.split.ptr = next->next.split.ptr;
                tmp.split.count = tail.split.count+1;
                compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
            } else {
                data = next->data;
                /* try to swing head to the next node */
                tmp.split.ptr = next->next.split.ptr;
                tmp.split.count = head.split.count+1;
                if( compare_and_swap( &q->head.concat, head.concat, tmp.concat ) ) break;
            }
        }
    }
    free((void*)head.split.ptr);
    return data;
}